11institutetext: I. Physikalisches Institut, Universität zu Köln, Zülpicher
Str. 77, 50937 Köln, Germany
11email<EMAIL_ADDRESS>22institutetext: Institute of Graduate Studies in
Science, Program of Astronomy and Space Sciences, Istanbul University,
Istanbul, Turkey 33institutetext: Kapteyn Astronomical Institute, Landleven
12, 9747AD, Groningen, The Netherlands 44institutetext: SRON Netherlands
Institute for Space Research, Landleven 12, 9747AD, Groningen, The Netherlands
55institutetext: Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
66institutetext: Institut de Radioastronomie Milimétrique (IRAM), 300 rue de
la Piscine, F-38406 Saint-Martin-d’Hères, France 77institutetext: INAF,
Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125, Florence,
Italy

# A search for radio jets from massive young stellar objects

Association of radio jets with H2O and CH3OH masers

Ü. Kavak 11 2 2 3 3 4 4 Á. Sánchez-Monge 11 A. López-Sepulcre 5566 R. Cesaroni
77 F. F. S. van der Tak 3344 L. Moscadelli 77 M. T. Beltrán 77 P. Schilke 11

(Received February 4, 2020; accepted ??)

###### Abstract

Context. Recent theoretical and observational studies debate the similarities
between the formation process of high-mass ($>8$ $M_{\odot}$) and low-mass
stars. The formation of low-mass star formation is directly associated with
the presence of disks and jets. Theoretical models predict that stars with
masses up to 140 $M_{\odot}$ can be formed through disk-mediated accretion in
disk-jet systems. According to this scenario, radio jets are expected to be
common in high-mass star-forming regions.

Aims. We aim to increase the number of known radio jets in high-mass star
forming regions by searching for radio jet candidates at radio continuum
wavelengths.

Methods. We have used the Karl G. Jansky Very Large Array (VLA) to observe 18
high-mass star-forming regions in the C band (6 cm,
$\approx$1$\aas@@fstack{\prime\prime}$0 resolution) and K band (1.3 cm,
$\approx$0$\aas@@fstack{\prime\prime}$3 resolution). We have searched for
radio jet candidates by studying the association of radio continuum sources
with shock activity signposts (e.g., molecular outflows, extended green
objects, and maser emission). Our VLA observations also target the 22 GHz H2O
and 6.7 GHz CH3OH maser lines.

Results. We have identified 146 radio continuum sources, with 40 of them being
located within the field of view of both images (C and K band maps). For these
sources we have derived their spectral index, which is consistent with thermal
emission (between $-0.1$ and $+2.0$) for 73% of them. Based on the association
with shock activity signposts, we have identified 28 radio jet candidates. Out
these, we have identified 7 as the most probable radio jets. The radio
luminosity of the radio jet candidates is correlated with the bolometric
luminosity and the outflow momentum rate. About 7–36% of the radio jet
candidates are associated with non-thermal emission. The radio jet candidates
associated with 6.7 GHz CH3OH maser emission are preferentially thermal winds
and jets, while a considerable fraction of radio jet candidates associated
with H2O masers show non-thermal emission, likely due to strong shocks.

Conclusions. About 60% of the radio continuum sources detected within the
field of view of our VLA images are potential radio jets. The remaining
sources could be compact H ii regions in their early stages of development, or
radio jets for which we do not have yet further evidence of shock activity.
Our sample of 18 regions is divided in 8 less evolved, infrared-dark regions
and 10 more evolved, infrared-bright regions. We have found that $\approx$71%
of the identified radio jet candidates are located in the more evolved
regions. Similarly, 25% of the less evolved regions harbor one of the most
probable radio jets, while up to 50% of the more evolved regions contain one
of these radio jet candidates. This suggests that the detection of radio jets
in high-mass star forming regions is larger in slightly more evolved regions.

###### Key Words.:

Stars: formation – Stars: massive – ISM: jets and outflows – Radio continuum:
ISM – (ISM:) HII Regions

##  1 Introduction

High-mass stars (O and B-type stars with masses $\geq 8\ \mathrm{M_{\sun}}$)
play a crucial role in the chemical and physical composition of their host
galaxies throughout their lifetimes, by injecting energy and material on
different scales through energetic outflows, intense UV radiation, powerful
stellar winds, and supernova explosions. Despite their importance, the
formation process of massive stars is still not well understood due to
observational and theoretical challenges (e.g., massive stars form in crowded
environments and are located at far distances, see reviews by Tan et al. 2014;
Motte et al. 2018). On the other hand, the formation of low-mass stars is
better understood and is explained with a model based on accretion via a
circumstellar disk and a collimated jet/outflow that removes angular momentum
and enables accretion to proceed (e.g., Larson 1969; Andre et al. 2000).
Circumstellar disks have been indeed observed around low-mass protostars
(-e.g., Williams & Cieza 2011; Luhman 2012), while ejection of material has
been mainly observed by large-scale, collimated jets and outflows (e.g.,
Bachiller 1996; Bally 2016). For high-mass stars, it remains to be understood
the role that (accretion) disks and jets/outflows play in their formation, as
well as how their properties vary with the mass of the forming star and the
environment. As far as observations are concerned, some studies have
concentrated on disks and jets/outflows in selected high-mass star-forming
regions (see e.g., Beuther et al. 2002a; Arce et al. 2007; López-Sepulcre et
al. 2009; Bally 2016). The advent of facilities such as the Atacama Large
Millimeter/Submillimeter Array (ALMA) or the upgraded Karl G. Jansky Very
Large Array (VLA) provides the high spatial resolution and sensitivity
necessary to fully resolve the structure of disks and jets/outflows in high-
mass star-forming regions. While disks are bright at millimeter wavelengths
and constitute perfect targets for ALMA observations (e.g., Sánchez-Monge et
al. 2013b, 2014; Beltrán et al. 2014; Johnston et al. 2015; Cesaroni et al.
2017; Maud et al. 2019), jets are found to be bright at centimeter wavelengths
observable with the VLA (e.g., Carrasco-González et al. 2010, 2015; Moscadelli
et al. 2013, 2016).

Surveys of low-mass star-forming regions with the VLA (e.g., Anglada 1996;
Anglada et al. 1998; Beltrán et al. 2001) revealed radio-continuum sources
elongated in the direction of the large-scale molecular outflows. These
sources are called thermal radio jets, because their emission is interpreted
as thermal (free-free) emission of ionised, collimated jets at the base of
larger-scale optical jets and molecular outflows (e.g., Curiel et al. 1987,
1989; Rodriguez 1995). Due to the high spatial resolution that can be achieved
at radio wavelengths with interferometers such as the VLA, thermal radio jets
constitute strong evidence of collimated outflows on small scales ($\sim$100
au; Torrelles et al. 1985; Anglada 1996) and permit to pinpoint the location
of the star that is forming and powering the jet/outflow seen on larger
scales. Although the emission of jets at radio-wavelengths is mainly thermal,
some jets show a contribution from a non-thermal component (e.g., Reid et al.
1995; Carrasco-González et al. 2010; Moscadelli et al. 2013, 2016).

Following the strategy used in the study of low-mass star-forming regions, we
aim to search for radio jets associated with high-mass star-forming regions in
a large sample of sources. Until recently, only a limited number of regions
harboring high-mass stellar objects were known to be associated with radio
jets (e.g., HH80/81: Marti et al. 1993; Carrasco-González et al. 2010,
CepAHW2: Rodriguez et al. 1994, IRAS 16547$-$4247: Rodríguez et al. 2008, IRAS
16562$-$1732: Guzmán et al. 2010, G35.20$-$0.74 N: Beltrán et al. 2016). In
the last years, progress has been made in increasing the number of known jets
associated with high-mass young stellar objects (e.g., Moscadelli et al. 2016;
Rosero et al. 2016; Sanna et al. 2018; Purser et al. 2018). In this work, we
used the VLA in two different frequency bands to search for radio jets in a
sample of 18 high-mass star-forming regions associated with molecular outflow
emission.

This paper is structured as follows. In Section 2, we present both the sample
and the details of the observations. The results of the observations of the
radio continuum (and maser) emission are presented in Section 3. The analysis
of the properties of the discovered sources is presented in Section 4, while
Appendix A describes the properties of each region in more detail. In Section
5, we discuss the implications of our results in the context of high-mass star
formation, and in Section 6 we summarize the most important conclusions.

##  2 Observations

###  2.1 Selected sample

We have selected 18 high-mass star-forming regions from the samples of López-
Sepulcre et al. (2010, 2011) and Sánchez-Monge et al. (2013d), using the
following criteria: (i) clump mass $>100$ M☉, to exclude regions forming
mainly low-mass stars, (ii) distance $<4$ kpc, to resolve spatial scales
$<4000$ AU when observed with interferometers at a resolution of 1″, (iii)
declination $>-15$°, to be observable from northern telescopes, (iv)
association with an HCO+ bipolar outflow and SiO emission with line widths
broader than $>20$ km s-1 (López-Sepulcre et al. 2011; Sánchez-Monge et al.
2013d), and (v) absence of bright centimeter continuum emission, to exclude
developed H ii regions.

We used the NRAO VLA Sky Survey (NVSS; Condon et al. 1998), the MAGPIS
(Helfand et al. 2006), CORNISH(Hoare et al. 2012; Purcell et al. 2013), and
RMS (Urquhart et al. 2008; Lumsden et al. 2013) surveys to eliminate star-
forming regions with developed H ii regions that would hinder the detection of
faint radio jets. Our final sample of 18 high-mass star-forming regions is
listed in Table 1.

Table 1: High-mass star-forming regions observed with the Very Large Array in
this work

| R.A. (J2000) | Dec. (J2000) |  $d$aa$a$Distances ($d$) and clump masses ($M$) from López-Sepulcre et al. (2011) and Sánchez-Monge et al. (2013d). |  $M$aa$a$Distances ($d$) and clump masses ($M$) from López-Sepulcre et al. (2011) and Sánchez-Monge et al. (2013d). | C bandbb$b$Synthesized beam ($\theta_{\mathrm{beam}}$) major and minor axis in arcsecond, and position angle (PA) in degrees. The rms noise level is given in units of $\mu$Jy beam-1. Regions marked with † and ‡ in the first column indicate IR-loud and IR-dark sources, respectively, based on the classification of López-Sepulcre et al. (2010). |  | K bandbb$b$Synthesized beam ($\theta_{\mathrm{beam}}$) major and minor axis in arcsecond, and position angle (PA) in degrees. The rms noise level is given in units of $\mu$Jy beam-1. Regions marked with † and ‡ in the first column indicate IR-loud and IR-dark sources, respectively, based on the classification of López-Sepulcre et al. (2010).
---|---|---|---|---|---|---|---
Region | (h:m:s) | (°:′:″) | (kpc) | (M☉) |  $\theta_{\mathrm{beam}}$, PA | rms |  |  $\theta_{\mathrm{beam}}$, PA | rms
IRAS 05358$+$3543† | 05:39:12.2 |  $+$35:45:52.0 | 1.8 |  0127 |  $1.27\times 1.23$, $+$61 |  08.1 |  | …cc$c$Region not observed in the K band. | …
G189.78$+$0.34† | 06:08:34.5 |  $+$20:38:51.0 | 1.8 |  0150 |  $1.28\times 1.09$, $+$23 | 16.0 |  | …cc$c$Region not observed in the K band. | …
G192.58$-$0.04† | 06:12:52.9 |  $+$18:00:34.9 | 2.6 |  0500 |  $1.40\times 1.19$, $+$21 | 23.6 |  | …cc$c$Region not observed in the K band. | …
G192.60$-$0.05† | 06:12:54.0 |  $+$17:59:23.0 | 2.6 |  0460 |  $1.36\times 1.14$, $+$20 | 26.5 |  | …cc$c$Region not observed in the K band. | …
G18.18$-$0.30† | 18:25:07.3 |  $-$13:14:22.9 | 2.6 |  0110 |  $1.74\times 1.05$, $-$16 | 10.0 |  |  $0.54\times 0.31$, $+$25 | 16.7
IRAS 18223$-$1243† | 18:25:10.9 |  $-$12:42:27.0 | 3.7 |  0980 |  $1.89\times 1.14$, $-$14 | 24.5 |  |  $0.58\times 0.34$, $+$37 | 15.0
IRAS 18228$-$1312† | 18:25:42.3 |  $-$13:10:18.0 | 3.0 |  0740 |  $1.88\times 1.16$, $-$14 | 35.0 |  |  $0.73\times 0.67$, $-$07 | 59.1
G19.27$+$0.1M2‡ | 18:25:52.6 |  $-$12:04:47.9 | 2.4 |  0114 |  $2.08\times 1.14$, $-$14 |  09.8 |  |  $0.50\times 0.32$, $-$26 | 20.7
G19.27$+$0.1M1‡ | 18:25:58.5 |  $-$12:03:58.9 | 2.4 |  0113 |  $1.95\times 1.19$, $-$16 |  09.6 |  |  $0.79\times 0.42$, $+$49 | 20.0
IRAS 18236$-$1205† | 18:26:25.4 |  $-$12:03:50.9 | 2.7 |  0780 |  $1.99\times 1.11$, $-$18 | 10.2 |  |  $0.51\times 0.32$, $-$26 | 16.9
G23.60$+$0.0M1‡ | 18:34:11.6 |  $-$08:19:05.9 | 2.5 |  0365 |  $1.85\times 1.13$, $-$22 |  08.7 |  |  $0.54\times 0.30$, $+$32 | 36.1
IRAS 18316$-$0602† | 18:34:20.5 |  $-$05:59:30.0 | 3.1 | 1000 |  $1.71\times 1.09$, $-$21 |  08.4 |  |  $0.53\times 0.30$, $+$35 | 40.0
G24.08$+$0.0M2‡ | 18:34:51.1 |  $-$07:45:32.0 | 2.5 |  0201 |  $1.80\times 1.13$, $-$21 | 17.0 |  |  $0.53\times 0.30$, $+$32 | 37.0
G24.33$+$0.1M1‡ | 18:35:07.8 |  $-$07:35:04.0 | 3.8 | 1759 |  $1.69\times 1.25$, $-$13 | 21.0 |  |  $0.49\times 0.31$, $-$31 | 26.1
G24.60$+$0.1M2‡ | 18:35:35.7 |  $-$07:18:08.9 | 3.7 |  0483 |  $1.66\times 1.03$, $-$20 | 18.2 |  |  $0.79\times 0.41$, $+$51 | 21.8
G24.60$+$0.1M1‡ | 18:35:40.2 |  $-$07:18:37.0 | 3.7 |  0192 |  $1.67\times 1.22$, $-$10 | 12.7 |  |  $0.75\times 0.38$, $+$57 | 22.5
G34.43$+$0.2M3‡ | 18:53:20.3 |  $+$01:28:23.0 | 2.5 |  0301 |  $1.57\times 1.46$, $-$59 | 17.9 |  |  $0.53\times 0.29$, $+$40 | 19.0
IRAS 19095$+$0930† | 19:11:54.0 |  $+$09:35:52.0 | 3.0 |  0500 |  $1.61\times 1.37$, $-$83 | 17.0 |  |  $0.53\times 0.29$, $+$44 | 63.4

111

###  2.2 VLA observations

We used the VLA of the NRAO222The Very Large Array (VLA) is operated by the
National Radio Astronomy Observatory (NRAO), a facility of the National
Science Foundation operated under cooperative agreement by Associated
Universities, Inc. to observe the 18 selected regions (see Table 1). The
observations were conducted between June and August 2012 (project number
12A-099), when the array was transitioning to its current upgraded phase and
was known as ‘expanded VLA’ (EVLA). During the observations, the array was in
its B configuration, which provides a maximum baseline of 11 km. We observed
the frequency bands C (4–8 GHz) and K (18–26.5 GHz) with 16 spectral windows
of 128 MHz each, covering a total bandwidth of 2048 MHz in each band. Each
spectral window has 128 channels, with a channel width of 1 MHz. The time
spent per source is $\sim$20 minutes and $\sim$30 minutes at 6 cm (C band) and
1.3 cm (K band), respectively. Flux calibration was achieved by observing the
quasars 3C286 ($F_{\mathrm{1.3\leavevmode\nobreak\ cm}}$=2.59 mJy,
$F_{\mathrm{6\leavevmode\nobreak\ cm}}$=7.47 mJy) and 3C48
($F_{\mathrm{1.3\leavevmode\nobreak\ cm}}$=1.13 mJy,
$F_{\mathrm{6\leavevmode\nobreak\ cm}}$=5.48 mJy). The amplitude and phase
were calibrated by monitoring the quasars J0555$+$3948, J0559$+$2353,
J1832$-$1035, and J1851$+$0035\. We used the standard guidelines for the
calibration of VLA data. The data were processed using the Common Astronomy
Software Applications (CASA; McMullin et al. 2007) software.

Continuum images of each source were obtained after excluding channels with
line emission, corresponding to H2O and CH3OH maser lines. The images were
done using the ‘clean’ task with the Briggs weighting parameter set to 2,
which results in a typical synthesized beam of 1$\aas@@fstack{\prime\prime}$5
and 0$\aas@@fstack{\prime\prime}$4 for the C and K bands, respectively, and
typical rms noise levels of $\sim$22 $\mu$Jy beam-1 at 6 cm and $\sim$30
$\mu$Jy beam-1 at 1.3 cm (see Table 1).

The spectral resolution of the observations is limited (about 50 km s-1 and 13
km s-1 for the C and K bands, respectively) and insufficient to resolve
spectral features. Despite this limitation, we have produced image cubes of
spectral windows that cover the frequencies of the H2O maser line at
22235.0798 MHz and the CH3OH maser line at 6668.519 MHz. This allowed us to
search for maser features that can be associated with the continuum emission.
The rms noise levels of these cubes are $0.5$ mJy beam-1 and $0.3$ mJy beam-1
per channel of 13 and 50 km s-1 for the H2O and CH3OH images, respectively.

##  3 Results

###  3.1 Continuum emission

We have detected compact continuum emissions in all 18 observed high-mass
star-forming regions. A total of 146 compact sources are identified with
intensities above $3\sigma$ level, where $\sigma$ is the rms noise level of
each map (see Table 1). In Table LABEL:t:catalogue, we list the coordinates,
fluxes and source sizes.

Figure 1: Number of radio continuum sources detected in the K band images
(marked in blue and corresponding to 15 sources) and in the C band images
(marked in green and corresponding to 142 sources). Out of the 146 detections,
only four are detected only in the K band images. The vast majority (131) are
detected only in the C band images (see Sect. 3.1 for more details). The
bottom labels marked the sources located within the primary beams of the K
band and C band images. We find 38 sources located outside the C band primary
beam (oC), 68 sources located inside the C band primary beam but outside the K
band primary beam (iC/oK) and 40 sources located within the primary beam of
both images (iC/iK). See Sects. 3.1 and 3.2 for more details.

Most of the sources (a total of 131) are only detected in the C band image,
while four of them are only detected in the K band (see Fig. 1). Only eleven
sources are detected at both frequencies. The higher detection rate of sources
in the C band is due to several factors. First, four regions were only
observed in the C band (see Table 1). This results in 26 radio continuum
sources for which we have no access to K band images. Second, the field of
view of the C band images (primary beam $\approx$9′) is larger compared to the
K band primary beam ($\approx$2′). Only 40 sources are located within the K
band primary beam (identified as iC/iK, see Fig. 1). This number reduces to
only 24 when considering only the sources that have been observed in both
frequency bands. A total of 68 sources are inside the C band primary beam
(identified as iC), but outside the K band primary beam (identified as oK, see
Fig. 1). The remaining 38 sources are outside the primary beam of the C band
observations (marked oC; see also Table LABEL:t:catalogue). The sources that
are outside the primary beam are bright enough to be detected even when the
telescope’s sensitivity is highly reduced. Thirdly, the spatial filtering of
the interferometer is different at both frequencies. In the B-configuration,
the VLA recovers scales up to 11″ in the C band, and only up to 4″ in the K
band (see also Appendix A of Palau et al. 2010). Finally, one cannot exclude
the possibility that some of these sources are extragalactic objects that can
only be detected at low frequencies. We have followed the approach of Anglada
et al. (1998) to determine the possible contamination of background sources in
our catalog. The expected number of background sources $N_{\mathrm{bg}}$ is
given by

$\begin{split}N_{\mathrm{bg}}=&1.4\left\\{1-\exp\left[-0.0066\left(\frac{\theta_{F}}{\mathrm{arcmin}}\right)^{2}\left(\frac{\nu}{5\leavevmode\nobreak\
\mathrm{GHz}}\right)^{2}\right]\right\\}\\\
&\times\left(\frac{S_{0}}{\mathrm{mJy}}\right)^{-0.75}\,\left(\frac{\nu}{5\leavevmode\nobreak\
\mathrm{GHz}}\right)^{-2.52},\end{split}$ (1)

where $\theta_{F}$ is the area of the sky that has been observed (18 fields in
C band, and 14 fields in K band), $\nu$ is the frequency of the observations,
and $S_{0}$ is the detectable flux density threshold ($3\times$rms, with an
average rms of $22\ \mathrm{\mu Jy\leavevmode\nobreak\ beam^{-1}}$ in the C
band, and $30\ \mathrm{\mu Jy\leavevmode\nobreak\ beam^{-1}}$ in the K band).
This results in $N_{\mathrm{bg}}=11$ and $N_{\mathrm{bg}}=0.2$ for the C and K
band images, respectively. Less than 5% of the sources detected in the C band
might be background objects not related to the star-forming regions, while we
do not expect contamination in the K band images.

###  3.2 Spectral index analysis

The spectral index ($\alpha$) is defined as $S_{\nu}\propto\nu^{\alpha}$,
where Sν is the flux density and $\nu$ is the frequency. We have calculated
the spectral index for the continuum sources using the measured flux densities
at 1.3 cm (K band) and 6 cm (C band). For the sources without detection in one
of the bands, we assume an upper limit of flux density equal to $5\sigma$. It
should be noted here that the flux densities of the sources were corrected for
the primary beam response of the antennas. The sources far away from the phase
centers (listed in Table 1) have larger uncertainties in the correction
factors of the primary beam and thus, in the final (corrected) flux.
Therefore, the sources located within the primary beams in both frequency
bands (i.e., sources listed as ‘iC/iK’ in Table LABEL:t:catalogue) have more
accurate flux estimates. For the sources outside one of the primary beams
(i.e., oK or oC), we do not determine the spectral index because of the high
uncertainty involved in the fluxes. In the last column of Table
LABEL:t:catalogue we list the calculated spectral indices. For the sources
detected at both frequencies, we have improved the determination of the
spectral index by creating new images with the same $uv$ (visibility) coverage
(see Table 7). This ensures that the interferometer is sensitive to similar
spatial scales at both frequencies.

In Fig. 2, we present the spectral index against the flux density to intensity
peak ratio for the 24 continuum sources observed at both bands and located
within the primary beams. For the sources detected at 6 cm only, we derive an
upper limit333We note that the real spectral index may not be always an upper
limit if the source emission is completely filtered out in our K band images.
Further observations at different wavelengths, with a similar $uv$-sampling
and angular resolution are necessary to constrain the spectral index of the
sources detected only in the C band images. to the spectral index (see red
triangles pointing downwards), while we derive a lower limit for the spectral
index for the sources detected only at 1.3 cm, (see blue triangles pointing
upwards). The sources detected at both wavelengths (black dots) have a more
precise determination of the spectral index. For most sources, we derive
spectral indices consistent with thermal emission (i.e., in the range of
$-0.1$ to $+2$), and in agreement with observations of other radio jets (e.g.,
Anglada et al. 2018). Only a few sources show very negative spectral indices
(#48, #96 and #144). These sources are likely to be partially filtered out in
the K band images, which may result in lower limits for the actual value of
the spectral index. In particular, source #48 appears as three distinguishable
peaks, which we refer to as a, b, and c, surrounded by a more diffuse and
extended structure that is mainly visible in the C band image. It is worth
noting that large flux-to-intensity ratios indicate that the source is likely
extended and most likely partially filtered out in the K band images, which
may result in negative spectral indices.

Figure 2: Spectral index ($\alpha$, see Sect. 3.2) against the flux to
intensity ratio for the radio continuum sources detected in both frequency
bands and inside the primary beam of both images (sources listed as ‘iC/iK’ in
Table LABEL:t:catalogue). The grey-shaded region depicts the spectral index
regime associated with thermal free-free emission (i.e., in the range from
$-0.1$ to $+2$). Black dots correspond to sources detected in both bands (see
spectral indices in Table 3), blue up-pointing triangles correspond to sources
detected only in the K band (i.e., lower limits), and red down-pointing
triangles correspond to sources detected only in the C band (i.e., upper
limits). Table 2: H2O and CH3OH maser features

|  | R.A. (J2000) | Dec. (J2000) | $V_{\mathrm{maser}}^{\mathrm{a}}$ | $V_{\mathrm{LSR}}^{\mathrm{H^{13}CO^{+}}}$ | Intensity | Continuum
---|---|---|---|---|---|---|---
Region | Maser | (h:m:s) | (°:′:″) | (km s-1) | (km s-1) | (Jy beam-1) | source IDb
IRAS 05358$+$3543 | CH3OH | 05:39:13.071 |  $+$35:45:50.938 |  $-$304 |  0$-$15.8 |  00.028 | 2
G189.78$+$0.34 | CH3OH | 06:08:35.304 |  $+$20:39:06.405 |  0$-$13 |  00$+$9.2 |  00.014 | 14
G192.58$-$0.04 | CH3OH | 06:12:54.026 |  $+$17:59:23.060 |  0$-$14 |  00$+$9.1 |  00.720 | 22
G18.18$-$0.30 | H2O | 18:25:07.575 |  $-$13:14:31.487 |  00$-$3 |  0$+$50.0 |  00.570 | –
IRAS 18223$-$1243 | H2O | 18:25:10.804 |  $-$12:42:26.234 |  0$+$24 |  0$+$45.2 |  00.006 | –
IRAS 18228$-$1312 | H2O | 18:25:41.935 |  $-$13:10:19.591 |  0$+$24 |  0$+$33.1 |  00.022 | 48
IRAS 18236$-$1205 | H2O | 18:26:25.677 |  $-$12:03:48.402 |  0$+$28 |  0$+$26.5 |  00.010 | 63
| H2O | 18:26:25.575 |  $-$12:03:48.502 |  0$+$28 |  0$+$26.5 |  00.006 | 63
| H2O | 18:26:25.782 |  $-$12:03:53.263 |  0$+$15 |  0$+$26.5 |  00.010 | 64
| H2O | 18:26:27.149 |  $-$12:03:54.888 |  0$+$15 |  0$+$26.5 |  00.014 | –
| CH3OH | 18:26:25.788 |  $-$12:03:53.456 |  00$+$5 |  0$+$26.5 |  00.260 | 64
G19.27$+$0.1M1 | H2O | 18:25:58.546 |  $-$12:03:58.516 |  0$+$28 |  0$+$26.5 |  00.022 | –
G23.60$+$0.0M1 | H2O | 18:34:11.237 |  $-$08:19:07.680 |  $+$108 |  $+$106.5 |  00.440 | –
| H2O | 18:34:11.452 |  $-$08:19:07.138 |  $+$108 |  $+$106.5 |  00.100 | –
IRAS 18316$-$0602 | H2O | 18:34:20.918 |  $-$05:59:41.638 |  0$+$41 |  0$+$42.5 | 11.100 | 83
| CH3OH | 18:34:20.913 |  $-$05:59:42.087 |  $-$233 |  0$+$42.5 |  00.014 | 83
G24.33$+$0.1M1 | H2O | 18:35:08.123 |  $-$07:35:04.216 |  $+$108 |  $+$113.6 |  04.030 | 110
| CH3OH | 18:35:08.147 |  $-$07:35:04.260 |  $-$182 |  $+$113.6 |  00.010 | 110
G24.60$+$0.1M2 | H2O | 18:35:35.728 |  $-$07:18:08.796 |  $+$122 |  $+$115.3 |  00.031 | –
G24.60$+$0.1M2 | H2O | 18:35:40.120 |  $-$07:18:37.417 |  0$+$54 |  0$+$53.2 |  01.020 | 136
IRAS 19095$+$0930 | H2O | 19:11:53.975 |  $+$09:35:50.559 |  0$+$37 |  0$+$43.9 | 26.800 | 143
| H2O | 19:11:53.990 |  $+$09:35:49.848 |  0$+$37 |  0$+$43.9 | 10.300 | 143
| CH3OH | 19:11:53.993 |  $+$09:35:50.641 |  0$+$39 |  0$+$43.9 |  00.043 | 143

444 . The systemic velocities ($V_{\mathrm{LSR}}^{\mathrm{H^{13}CO^{+}}}$) are
reported in López-Sepulcre et al. (2011).

###  3.3 Maser Emission

H2O and CH3OH masers are excellent indicators of star formation activity
(e.g., Beuther et al. 2002b; Moscadelli et al. 2005; de Villiers et al. 2015).
We created image cubes of the H2O and CH3OH spectral lines and searched for
maser features by scanning the entire velocity range. Despite the limited
spectral resolution of our observation setup (see Sect. 2), we found maser
emission in 14 of the 18 regions. In Table 4, we list the coordinates of the
maser features detected in each region, together with the velocity at which
the feature is detected and its intensity. We also compare the velocities of
the maser features with the systemic velocities determined from H13CO+ (1–0)
observations (López-Sepulcre et al. 2011). We find that the H2O masers have
velocities that match the H13CO+ (1–0) velocities, while the CH3OH masers have
a larger discrepancy, probably due to the lower spectral resolution.

The low spectral resolution in our observations compared to the typical maser
line widths (a few km s-1; Elitzur 1982; Kalenskii & Kurtz 2016) leads to
smearing of the maser intensities. The intensities given in Table 4 should be
considered as lower limits. Despite this limitation, the high-angular
resolution of our observations can be used to spatially associate the H2O and
CH3OH masers to the detected continuum sources. If the angular separation
between the continuum source and the maser is smaller than the synthesized
beam size (listed in Table 1), we assume that the maser is associated with the
continuum source. In the last column of Table 4, we specify the identifier of
the continuum source (see Table LABEL:t:catalogue) with which the maser is
associated. We find 10 continuum sources associated with maser features (see
Section 4.4 for more details).

##  4 Analysis and discussion

In this section, we determine how many sources in our sample are potential
radio jets. For this purpose we study the nature of the detected radio
continuum emission, and investigate the association with molecular outflows,
masers and EGOs555The so-called EGOs (extended green objects) are sources with
bright emission in the Spitzer/IRAC 4.5 $\mu$m band and are usually found
associated with strong shocks and jets (e.g., Cyganowski et al. 2008, 2009).
By using these criteria, we identify the best radio jet candidates in our
sample and characterize their properties.

###  4.1 Nature of the radio continuum emission

Figure 3: Scatter plot of bolometric luminosity ($L_{\mathrm{bol}}$) and
observed radio continuum luminosity ($L_{\mathrm{radio}}$) at 6 cm (C band).
Open black circles correspond to the continuum sources detected in our work
and located within the primary beam of the K band (1.3 cm) images (i.e.,
‘iC/iK’ in Table LABEL:t:catalogue). Blue and red symbols mark the sources
with positive and negative spectral indices, respectively, as listed in Table
LABEL:t:catalogue and shown in Fig. 2). The solid line represents the values
expected from Lyman continuum radiation for a zero-age main-sequence star of a
given luminosity (Thompson 1984). The dashed line is a least-squares fit to
the radio jets reported by Anglada et al. (2018, shown as grey squares),
corresponding to [$L_{\mathrm{radio}}$/mJy kpc2] = $10^{-1.90}$
[$L_{\mathrm{bol}}$/$L_{\odot}$]+0.59 (see their Eq. 28).

Usually two mechanisms are invoked to explain the origin of thermal free-free
radiation from ionized gas in star-forming regions: Photoionization and
ionization through shocks (e.g., Gordon & Sorochenko 2002; Kurtz 2005;
Sánchez-Monge et al. 2008, 2013c; Anglada et al. 2018). In the case of
photoionization, ultraviolet (UV) photons with energies above 13.6 eV are
emitted by massive stars and ionize the surrounding atomic hydrogen. In the
second scenario, the ionization is produced when ejected material associated
with outflows and jets interacts in a shock with neutral and dense material
surrounding the forming star (e.g., Curiel et al. 1987, 1989; Anglada et al.
1992).

Anglada (1995, 1996) show that the relation between the radio luminosity and
the bolometric luminosity of young stellar objects (YSOs) depends on the
origin of the ionization: stellar UV radiation or shocks (see also Anglada et
al. 2018). We use this relation to investigate the nature of our continuum
sources. The solid line in Fig. 3 shows the maximum radio luminosity that a
high-mass object of a given luminosity may have according to its UV radiation,
the so-called Lyman continuum limit usually associated with H ii regions. The
radio luminosity decreases fast with decreasing bolometric luminosity. In
contrast, the radio luminosity originated in shocks (i.e., radio jets) has a
less steep curve. The dotted line in Fig. 3 shows the least-squares fit to the
sample of radio jets studied in Anglada et al. (2018) and shown as grey
squares in the figure.

We have calculated the radio luminosity of our continuum sources as
$L_{\mathrm{radio}}=S_{\nu}\,d^{2}$, where $S_{\nu}$ is the observed flux
density in the C band (listed in Table LABEL:t:catalogue) and $d$ is the
distance to the source (listed in Table 1). The bolometric luminosity
($L_{\mathrm{bol}}$) of each source is uncertain due to the lack of high-
resolution data at far-infrared wavelengths. The bolometric luminosity of each
region is given in Table A.1 of López-Sepulcre et al. (2011) and provides an
upper limit to the actual luminosity. As a simple approach, we divide the
bolometric luminosity by the number of radio sources detected within the
primary beam to have an estimate of the expected average luminosity for the
continuum sources in the region. Circle symbols in Fig. 3 show the continuum
sources detected in our work and located inside the K band primary beam (i.e.,
with reliable flux measurements and listed as iC/iK in Table
LABEL:t:catalogue). Colored symbols correspond to those sources for which we
could derive the spectral index (see Fig. 2), with blue symbols corresponding
to positive spectral indices (i.e., $\alpha>+0.0$, mainly thermal emission)
and red symbols corresponding to negative spectral indices. In general, our
sources lie in between the two lines defining the radio jet and H ii region
regimes666Some sources lie above the solid curve depicting the Lyman continuum
limit. This is in agreement with other studies that report the existence of a
population of H ii regions with radio fluxes larger than the Lyman continuum
limit (see e.g., Sánchez-Monge et al. 2013a; Cesaroni et al. 2016)..
Interestingly, sources with positive spectral indices (blue symbols) seem to
preferentially follow, although with some dispersion, the relation found for
radio jets, while sources with negative spectral indices (red symbols) are
located closer to the Lyman continuum regime. This favours our previous
interpretation that sources with negative spectral indices may be slightly
extended H ii regions that are partially filtered out in the K band images.

###  4.2 Association with molecular outflows

We investigate the association of radio continuum sources with molecular
outflows by comparing the location of radio sources with respect to the
molecular outflow emission reported mainly by López-Sepulcre et al. (2010) and
Sánchez-Monge et al. (2013d). It is expected that the most promising radio jet
candidates will be in the center of the molecular outflow emission.

We find a total of twenty-four radio continuum sources that are spatially
associated with molecular outflow emission (see Table 3 and Fig. 4 for more
details). Out of these sources, eighteen (#2, #4, #13, #14, #15, #16, #22,
#23, #25, #48a, #48b, #48c, #74, #83, #95, #110, #137 and #143) are located at
or near the geometric center of the molecular outflow emission, while the
remaining six (#12, #63, #64, #65, #73, and #144) are located within the
outflow lobes. Although we cannot confirm that these six sources are at the
base of the outflows detected with single-dish telescopes (with angular
resolutions of 11–29″), we cannot exclude that they might drive molecular
outflows. Further observations of outflow tracers at higher angular resolution
are necessary to confirm and better associated the molecular outflows with the
radio continuum sources. In Table 3, we list the outflow momentum rates
reported in the literature (see López-Sepulcre et al. 2010; Sánchez-Monge et
al. 2013d). For #137, no outflow momentum rate has been reported (Hatchell et
al. 2001; Liu et al. 2013).

###  4.3 Association with EGOs

In this section, we investigate the association of radio continuum sources
with Spitzer/IRAC 4.5 $\mu$m emission tracing EGOs, which are considered
related to the shocked gas. For the association with EGOs we used the
catalogues of Cyganowski et al. (2008, 2009). In total, we found six sources
(#42, #63, #64, #119, #137 and #139) with an EGO counterpart (see Table 3).

We have also inspected the Spitzer/IRAC images of the different regions to
search for other possible EGOs not included in previous catalogues. We have
identified nine radio continuum sources in this category (see sources #48,
#65, #73, #74, #83, #110, and #143, marked with a ’?’ symbol in Table 3). The
association of these sources with bright 4.5 $\mu$m emission suggests their
association with strong shocks and favors the hypothesis of a radio jet origin
for the radio continuum emission of these objects. However, a more detailed
characterization of the infrared properties of the nine additional sources is
necessary to confirm whether these objects are EGOs or not.

Table 3: Properties of the radio jet candidates.

| Flux propertiesaa$a$Uncertainties in the reported maser velocities ($V_{\mathrm{maser}}$) are expected to be $\sim$50 km s-1 for the CH3OH masers and $\sim$13 km s-1 for the H2O masers (see Sect. 2) |  | Source size propertiesbb$b$Radio continuum source spatially associated with the maser feature and listed as identified in Table LABEL:t:catalogue. |  | Outflow/shock activitycc$c$Association of the radio continuum source with outflow and shock activity. The associations correspond to: (i) Molecular outflows, with the outflow momentum rate $\dot{P}_{\mathrm{out}}$ given in units of $M_{\sun}$ yr-1 km s-1 (from López-Sepulcre et al. 2010; Sánchez-Monge et al. 2013d), with the symbol † indicating those radio continuum sources located within the outflow lobes and not at the center of the outflow, (ii) EGOs (or extended green objects), based on the catalogue of Cyganowski et al. (2008, question mark symbols indicate the presence of bright Spitzer/IRAC 4.5 $\mu$m emission although without confirmation of the object being an EGO), and (iii) H2O and CH3OH masers, as listed in Table 4. Source #137, marked with a ‡ symbol, is associated with molecular outflow emission (Hatchell et al. 2001; Liu et al. 2013), but no outflow momentum rate has been reported.
---|---|---|---|---|---
ID | $S_{\mathrm{C\leavevmode\nobreak\ band}}$ | $S_{\mathrm{K\leavevmode\nobreak\ band}}$ | $\alpha$ |  | $\theta_{\mathrm{C\leavevmode\nobreak\ band}}$ | $\theta_{\mathrm{K\leavevmode\nobreak\ band}}$ | $\beta$ |  | log($\dot{P}_{\mathrm{out}}$) | EGOs | Masers
Radio jet candidates with signposts of outflow activity |  |
002dd$d$Sources not observed in the K band. For these sources we do not have information on the K band flux and presence of H2O masers. |  000.53$\pm$0.01 | — | — |  |  ¡0.75 | — | — |  | $-3.9\phantom{{}^{\dagger}}$ | n | CH3OH
004dd$d$Sources not observed in the K band. For these sources we do not have information on the K band flux and presence of H2O masers. |  000.33$\pm$0.01 | — | — |  |  ¡1.23 | — | — |  | $-3.9\phantom{{}^{\dagger}}$ | n | …
12dd$d$Sources not observed in the K band. For these sources we do not have information on the K band flux and presence of H2O masers. |  000.72$\pm$0.03 | — | — |  |  ¡0.97 | — | — |  | $-3.1^{\dagger}$ | n | …
13dd$d$Sources not observed in the K band. For these sources we do not have information on the K band flux and presence of H2O masers. |  001.24$\pm$0.05 | — | — |  |  ¡0.75 | — | — |  | $-3.1\phantom{{}^{\dagger}}$ | n | …
14dd$d$Sources not observed in the K band. For these sources we do not have information on the K band flux and presence of H2O masers. |  000.69$\pm$0.05 | — | — |  |  ¡1.08 | — | — |  | $-3.1\phantom{{}^{\dagger}}$ | n | CH3OH
15dd$d$Sources not observed in the K band. For these sources we do not have information on the K band flux and presence of H2O masers. |  000.94$\pm$0.04 | — | — |  |  ¡1.60 | — | — |  | $-3.1\phantom{{}^{\dagger}}$ | n | …
16dd$d$Sources not observed in the K band. For these sources we do not have information on the K band flux and presence of H2O masers. |  001.04$\pm$0.04 | — | — |  |  ¡1.16 | — | — |  | $-3.1\phantom{{}^{\dagger}}$ | n | …
22dd$d$Sources not observed in the K band. For these sources we do not have information on the K band flux and presence of H2O masers. |  010.27$\pm$0.22 | — | — |  |  ¡1.55 | — | — |  | $-3.3\phantom{{}^{\dagger}}$ | n | CH3OH
23dd$d$Sources not observed in the K band. For these sources we do not have information on the K band flux and presence of H2O masers. |  01.56 $\pm$0.05 | — | — |  | … | — | — |  | $-3.3\phantom{{}^{\dagger}}$ | n | …
25dd$d$Sources not observed in the K band. For these sources we do not have information on the K band flux and presence of H2O masers. |  00.49 $\pm$0.02 | — | — |  | … | — | — |  | $-3.3\phantom{{}^{\dagger}}$ | n | …
42 |  004.16$\pm$0.08 | … | … |  |  ¡0.81 | … | … |  | … | Y | …
48aee$e$Sources detected at both frequency bands and for which we have created new images using a common uv-range that allows us to sample similar spatial scales. Fluxes and source sizes for these sources are taken from Table 7. Fluxes for source #137 can not be primary beam corrected and are not usable for spectral index determination. |  055.32$\pm$3.50 |  015.19$\pm$1.51 | $-0.99\pm 0.09$ |  |  ¡2.70 |  0¡2.07 |  ¡$-0.20$ |  | $-2.9\phantom{{}^{\dagger}}$ | ? | H2O
48bee$e$Sources detected at both frequency bands and for which we have created new images using a common uv-range that allows us to sample similar spatial scales. Fluxes and source sizes for these sources are taken from Table 7. Fluxes for source #137 can not be primary beam corrected and are not usable for spectral index determination. |  075.41$\pm$9.50 |  010.53$\pm$1.30 | $-1.50\pm 0.14$ |  |  ¡3.84 |  0¡1.64 |  ¡$-0.65$ |  | $-2.9\phantom{{}^{\dagger}}$ | ? | …
48cee$e$Sources detected at both frequency bands and for which we have created new images using a common uv-range that allows us to sample similar spatial scales. Fluxes and source sizes for these sources are taken from Table 7. Fluxes for source #137 can not be primary beam corrected and are not usable for spectral index determination. | 129.52$\pm$8.30 |  054.41$\pm$3.51 | $-0.66\pm 0.07$ |  |  ¡2.28 |  0¡1.89 |  ¡$-0.14$ |  | $-2.9\phantom{{}^{\dagger}}$ | ? | …
63ee$e$Sources detected at both frequency bands and for which we have created new images using a common uv-range that allows us to sample similar spatial scales. Fluxes and source sizes for these sources are taken from Table 7. Fluxes for source #137 can not be primary beam corrected and are not usable for spectral index determination. |  000.99$\pm$0.06 |  000.75$\pm$0.13 | $-0.22\pm 0.07$ |  |  ¡1.19 |  0¡1.11 |  ¡$-0.05$ |  | $-2.6^{\dagger}$ | Y | H2O
64ee$e$Sources detected at both frequency bands and for which we have created new images using a common uv-range that allows us to sample similar spatial scales. Fluxes and source sizes for these sources are taken from Table 7. Fluxes for source #137 can not be primary beam corrected and are not usable for spectral index determination. |  000.28$\pm$0.02 |  000.35$\pm$0.08 | $+0.18\pm 0.04$ |  |  $<$1.38 |  0¡0.66 | $>-0.56$ |  | $-2.6^{\dagger}$ | Y | H2O, CH3OH
65ee$e$Sources detected at both frequency bands and for which we have created new images using a common uv-range that allows us to sample similar spatial scales. Fluxes and source sizes for these sources are taken from Table 7. Fluxes for source #137 can not be primary beam corrected and are not usable for spectral index determination. |  000.57$\pm$0.14 |  002.31$\pm$0.09 | $+1.08\pm 0.19$ |  |  $<$1.46 |  0$<$0.75 | … |  | $-2.6^{\dagger}$ | ? | …
73ee$e$Sources detected at both frequency bands and for which we have created new images using a common uv-range that allows us to sample similar spatial scales. Fluxes and source sizes for these sources are taken from Table 7. Fluxes for source #137 can not be primary beam corrected and are not usable for spectral index determination. |  000.24$\pm$0.15 |  000.43$\pm$0.09 | $+0.45\pm 0.50$ |  |  ¡0.38 |  0$<$0.74 | $<+0.51$ |  | $-2.4^{\dagger}$ | ? | …
74ee$e$Sources detected at both frequency bands and for which we have created new images using a common uv-range that allows us to sample similar spatial scales. Fluxes and source sizes for these sources are taken from Table 7. Fluxes for source #137 can not be primary beam corrected and are not usable for spectral index determination. |  000.35$\pm$0.03 |  000.49$\pm$0.13 | $+0.26\pm 0.21$ |  |  ¡0.53 |  0¡0.42 |  ¡$-0.18$ |  | $-2.4\phantom{{}^{\dagger}}$ | ? | …
83ee$e$Sources detected at both frequency bands and for which we have created new images using a common uv-range that allows us to sample similar spatial scales. Fluxes and source sizes for these sources are taken from Table 7. Fluxes for source #137 can not be primary beam corrected and are not usable for spectral index determination. |  003.35$\pm$0.21 |  003.73$\pm$0.29 | $+0.08\pm 0.08$ |  |  ¡0.87 |  0¡0.85 |  ¡$-0.02$ |  | $-1.8\phantom{{}^{\dagger}}$ | ? | H2O, CH3OH
95 |  $<$0.042 |  000.25$\pm$0.07 | $>+1.36$ |  | … |  0$<$0.42 | … |  | $-1.8\phantom{{}^{\dagger}}$ | n | …
110ee$e$Sources detected at both frequency bands and for which we have created new images using a common uv-range that allows us to sample similar spatial scales. Fluxes and source sizes for these sources are taken from Table 7. Fluxes for source #137 can not be primary beam corrected and are not usable for spectral index determination. |  000.46$\pm$0.21 |  001.20$\pm$0.06 | $+0.73\pm 0.35$ |  |  $<$1.39 |  0¡0.37 | $>-1.01$ |  | $-2.9\phantom{{}^{\dagger}}$ | ? | H2O, CH3OH
119 |  001.11$\pm$0.06 | $<0.022$ | … |  |  ¡2.28 | … | … |  | … | Y | …
136 |  $<$0.10 |  000.85$\pm$0.12 | $>+1.67$ |  | … |  0¡1.50 | … |  | … | n | H2O
137ee$e$Sources detected at both frequency bands and for which we have created new images using a common uv-range that allows us to sample similar spatial scales. Fluxes and source sizes for these sources are taken from Table 7. Fluxes for source #137 can not be primary beam corrected and are not usable for spectral index determination. |  014.40$\pm$1.20 |  002.21$\pm$0.20 | … |  |  ¡2.53 |  0¡1.60 |  ¡$-0.35$ |  |  …‡ | Y | …
139 |  000.73$\pm$0.03 | $<0.019$ | … |  |  ¡0.87 | … | … |  | … | Y | …
143ee$e$Sources detected at both frequency bands and for which we have created new images using a common uv-range that allows us to sample similar spatial scales. Fluxes and source sizes for these sources are taken from Table 7. Fluxes for source #137 can not be primary beam corrected and are not usable for spectral index determination. |  039.50$\pm$1.60 | 130.87$\pm$2.60 | $+1.12\pm 0.04$ |  |  ¡0.55 |  0¡0.28 |  ¡$-0.52$ |  | $-3.4\phantom{{}^{\dagger}}$ | ? | H2O, CH3OH
144 |  015.57$\pm$0.89 |  $<$0.32 | $<-2.97$ |  |  ¡2.44 | … |  |  | $-3.4^{\dagger}$ | n | …
Radio continuum sources consistent with positive spectral index, but with no signposts of outflow activity |  |
61 |  000.14$\pm$0.03 |  $<$2.10 | $<+2.07$ |  |  ¡1.79 | … | … |  | … | n | …
62 |  $<$0.05 |  00.24$\pm$0.09 | $>+1.23$ |  | … |  0$<$0.44 | … |  | … | n | …
86 |  000.35$\pm$0.01 |  $<$2.32 | $<+1.43$ |  |  $<$1.45 | … | … |  | … | n | …
109 |  000.08$\pm$0.03 |  $<$1.75 | $<+2.33$ |  |  $<$0.94 | … | … |  | … | n | …
113 |  000.28$\pm$0.01 |  $<$0.31 | $<+0.07$ |  |  ¡0.85 | … | … |  | … | n | …
126 |  000.11$\pm$0.01 |  $<$0.37 | $<+0.96$ |  |  $<$1.37 | … | … |  | … | n | …
129 |  000.16$\pm$0.01 |  $<$0.20 | $<+0.18$ |  |  ¡1.23 | … | … |  | … | n | …
145 |  002.84$\pm$0.02 |  $<$4.34 | $<+0.32$ |  |  $<$1.48 | … | … |  | … | n | …

777

###  4.4 Association with masers

Our VLA observations (see Sect. 2.2) allow us to search for H2O and CH3OH
maser spots associated with radio continuum sources. As shown in Table 4, we
have found a total of sixteen H2O and seven CH3OH maser spots.

We find ten radio continuum sources associated with maser features, of which
three (#2, #14 and #22) are associated with CH3OH masers only, three sources
(#48, #63, and #136) are associated only with H2O masers, and four sources
(#64, #83, #110 and #143) are associated with both types of masers (see Table
3 and Fig. 4). It is worth noting that the three sources associated only with
CH3OH masers correspond to regions observed only in the C band. Future
observations of these sources in the K band together with observations of the
H2O maser line could confirm that all sources associated with CH3OH maser are
also associated with H2O maser features.

The observed Class II 6.7 GHz CH3OH masers are ideal signposts for embedded
young stellar objects and mark the location of deeply embedded massive
protostars (e.g., Breen et al. 2013). On the other hand, 22 GHz H2O masers
have been found associated with outflow activity (e.g., Torrelles et al. 2011)
as well as tracing disk-like structures around young stellar objects (e.g.,
Moscadelli et al. 2019). Our maser observations have therefore enabled us to
identify at least seven potential candidates for a radio jet (i.e., sources
associated with outflow activity).

Figure 4: Diagrams summarizing the outflow-activity associations of the radio jet candidates studied in this work. The integer numbers indicate the number radio jet candidates in a specific group (see Table 3). The top diagram summarizes the association of the radio jet candidates with molecular outflows, masers and EGOs (see Sect. 4, for EGOs we only consider association if the source is labeled as ‘Y’ in Table 3). The bottom-row diagrams summarize the results regarding the thermal (spectral index $>-0.1$) and non-thermal (spectral index $<-0.1$) properties of the radio continuum emission. The number of sources for which we could not derive the spectral index is also indicated. Table 4: Number of the sources with thermal and non-thermal radio continuum emission associated with different outflow activity signatures. | Non-thermal Sources | Thermal Sources
---|---|---
Outflows | 5/5 (100%) | 7/8 (88%)
EGOs | 1/5 (20%) | 1/8 (13%)
Masers (all) | 2/5 (40%) | 5/8 (63%)
H2O | 2/5 (40%) | 5/8 (63%)
CH3OH | 0/5 (0%) | 4/8 (50%)

###  4.5 Radio jet candidates

Out of the 146 radio continuum sources detected in our study, we have
identified twenty-eight sources (see list at the beginning of Table 3) as
possible radio jet candidates, based on their association with outflow and
shock activity. We find twenty-four of these sources associated with molecular
outflow emission, six of them with EGOs, and ten with masers. In the sketch
presented in Fig. 4, we summarize these findings.

In addition to these twenty-eight sources, we have also identified eight
radio-continuum sources with spectral indices consistent with thermal emission
(see bottom list in Table 3). Based on the results shown in Fig. 3, these
sources could also be radio jet candidates, despite their lack of association
with tracers of outflow and shock activity. In the following, we build on the
properties of the identified radio jet candidates.

####  4.5.1 Radio continuum properties

Reynolds (1986) describe radio jets with a model that assumes a jet of varying
temperature, velocity, and ionization fraction. In case of constant
temperature, the relations of the flux density ($S_{\nu}$) and source size
($\theta_{\nu}$) with frequency are given by

$S_{\nu}\propto\nu^{\alpha}=\nu^{1.3-0.7/\epsilon}\\\ \mathrm{and}\\\
\theta_{\nu}\propto\nu^{\beta}=\nu^{-0.7/\epsilon},$ (2)

where $\epsilon$ depends only on the geometry of the jet and is the power-law
index that describes how the width of the jet varies with the distance from
the central object. In this model, the spectral index $\alpha$ is always
smaller than 1.3 and drops to values $<0.6$ for confined jets ($\epsilon<1$;
Anglada et al. 1998).

In Table 3, we list the spectral index ($\alpha$) and the source size index
($\beta$) for our radio jet candidates. The latter only for the sources
detected at both frequencies. Nine of the radio jet candidates associated with
outflow/shock activity have spectral indices consistent with thermal emission
($>-0.1$), with six showing clear positive ($>+0.4$) spectral indices. These
values are consistent with the model of Reynolds (1986) for values of
$\epsilon>0.6$. For such geometries of the jet, the source size index
($\beta$) is expected to be about $-1$. The source size indices reported in
Table 3 are mainly in the range $-0.1$ to $-1.0$, in agreement with the model
of Reynolds (1986) for radio jets.

Despite most of our radio jet candidates have spectral indices consistent with
thermal emission (64% of the sample, see Table 3), we find some sources
(accounting for 36% of the sample, five sources888As discussed in Sect. 4.5.3,
four of these five sources are most likely H ii regions. This would reduce the
number of non-thermal radio jets to only one out of 14 (7% of our sample).)
that show negative spectral indices. This finding is in agreement with some
recent works. For example, Moscadelli et al. (2016) find about 20% of their
sample of 15 radio continuum sources to be associated with non-thermal
emission. The presence of non-thermal emission is explained in terms of
synchrotron emission from relativistic electrons accelerated in strong shocks
within the jets, and a number of cases have been studied in more detail (e.g.,
Carrasco-González et al. 2010; Sanna et al. 2019). Further detailed
observations towards these new four non-thermal radio jet candidates, can
provide further constraints to understand the characteristics of this kind of
objects.

We have searched for possible relations between the presence of thermal and
non-thermal radio jets and different outflow/shock activity signposts (i.e.,
outflows, masers and EGOs). We summarize our findings in the bottom panels of
Fig. 4). We do not find a preferred relation between thermal and non-thermal
radio jets with the outflow activity signposts, since we find similar
percentages (see Table 4) for the association with outflows (88% and 100%,
respectively), EGOs (13% and 20%), and masers (55% and 40%). The low number of
objects included in our analysis prevents us from deriving further
conclusions, and we indicate that a larger sample of radio jets needs to be
studied to better understand the properties and differences between thermal
and non-thermal radio jets. It is also worth noting that all the four objects
associated with both CH3OH and H2O masers are thermal radio jet candidates
(see Table 3), while only one of the three objects associated with only H2O
masers shows thermal emission. This might suggest that radio jets associated
with CH3OH masers tend to have positive spectral indices (i.e., thermal
emission), while radio jets associated with only H2O masers might
preferentially have negative spectral indices (i.e., non-thermal emission).
However, the low number of sources studied in our sample prevents from
deriving further conclusions. One should also note that the different levels
of association of the radio continuum sources with maser emission may be
affected by the variability of masers (Felli et al. 2007; Sugiyama et al.
2017; Ashimbaeva et al. 2017). Moreover, we can not discard that the poor
spectral resolution of our observations, which may smear out the intensity of
the maser lines making some of them undetectable with our sensitivity limits,
may also affect our detectability limits. Despite these limitations, our
results are in agreement with the 6.7 GHz CH3OH masers tracing the actual
location of the newly-born YSOs usually associated with thermal winds/jets,
while H2O masers may be originated in strong shocks where non-thermal
synchrotron emission can be relevant (see e.g., Moscadelli et al. 2013, 2016).

Figure 5: Relation between radio luminosity ($L_{\mathrm{radio}}$) and outflow
momentum rate ($\dot{P}_{\mathrm{out}}$). Open boxes show data from (Anglada
et al. 2018, see also Anglada et al. 1992). The dashed line is a least-squares
fit to the radio jets reported by Anglada et al. (2018), corresponding to
[$L_{\mathrm{radio}}$/mJy kpc2] = 10+2.97 [$\dot{P}$/$M_{\odot}$ yr-1 km
s-1]+1.02 (see their Eq. 31).

####  4.5.2 Jet-outflow connection

It has been found that the radio luminosity ($L_{\mathrm{rad}}=S_{\nu}d^{2}$)
of thermal radio jets is correlated with the energetics of the associated
molecular outflows. The relationship between radio luminosity and momentum
rate in the outflow ($\dot{P}_{\mathrm{out}}$) is empirically derived by
Anglada et al. (1992, see also ). In Fig. 5, we compare our radio jet
candidates (see Table 3) with the radio jets studied by Anglada et al. (2018).
As reported by Anglada et al. (2018), there is a tight correlation between the
radio luminosity of the jet and the outflow momentum rate. This relationship
is interpreted as proof that shocks are the ionization mechanism of radio jets
(see e.g., Rodríguez et al. 2008; Anglada et al. 2018). Most of the radio jet
candidates investigated in this work, with the exception of only a few
sources, follow this relationship, suggesting a radio jet origin for the
detected radio continuum emission. The exceptions are mainly the sources #48a,
#48b, #48c, #143 and #144, which have a much larger radio luminosity compared
to the associated outflow momentum rate. This excess suggests that another
mechanism could be responsible for a large fraction of the observed radio
continuum emission. Based on the location of these sources in the diagram
shown in Fig. 3, these sources may correspond to more evolved and extended H
ii regions, instead of radio jets, thus explaining the discrepancy between the
observed radio luminosity and outflow momentum rate. In this case, we could be
facing two possible scenarios. The first is that the sources are indeed radio
jets transitioning into a more evolved H ii region phase (similar to what has
been proposed for G35.20$-$0.74N, Beltrán et al. 2016). The second scenario is
that the radio continuum sources that we are detecting are only associated
with an H ii region, and the spatial coincidence with the molecular outflow
emission is due to the presence of another (lower-mass) object powering the
outflow but with non-detectable radio continuum emission in our maps. Higher
angular resolution observations of the molecular outflow can better establish
the location of the powering source and its association with the detected
radio continuum sources.

Following Eq. 8 of Anglada et al. (2018, see also ), we estimate the ionized
mass-loss rate ($\dot{M}_{\mathrm{ion}}$) of our radio jet candidates as

$\begin{split}\left(\frac{\dot{M}_{\mathrm{ion}}}{10^{-6}\leavevmode\nobreak\
M_{\sun}\leavevmode\nobreak\
\mathrm{yr}^{-1}}\right)=&0.108\left(\frac{d}{\mathrm{kpc}}\right)^{1.5}\left[\frac{\left(2-\alpha\right)\left(0.1+\alpha\right)}{1.3-\alpha}\right]^{0.75}\\\
&\times\left(\frac{T}{10^{4}\leavevmode\nobreak\
\mathrm{K}}\right)^{-0.075}\left[\left(\frac{S_{\nu}}{\mathrm{mJy}}\right)\left(\frac{\nu}{10\leavevmode\nobreak\
\mathrm{GHz}}\right)^{-\alpha}\right]^{0.75}\\\
&\times\left(\frac{V_{\mathrm{jet}}}{200\leavevmode\nobreak\
\mathrm{km}\leavevmode\nobreak\
\mathrm{s}^{-1}}\right)\left(\frac{\nu_{m}}{10\leavevmode\nobreak\
\mathrm{GHz}}\right)^{0.75\alpha-0.45}\\\
&\times\left(\frac{\theta_{0}}{\mathrm{rad}}\right)^{0.75}(\sin\leavevmode\nobreak\
i)^{-0.25},\end{split}$ (3)

where $\alpha$ is the spectral index and $S_{\nu}$ is the radio continuum
flux, both listed in Table 3, and $d$ is the distance to the source. The
opening angle of the jet $\theta_{0}$ can be approximated as
$2\arctan(\theta_{\mathrm{min}}/\theta_{\mathrm{maj}})$ (Beltrán et al. 2001;
Anglada et al. 2018). We assume a value of 0.5 for the ratio of the minor and
major axis of the jet. We also assume that the velocity of the jet
($V_{\mathrm{jet}}$) is 500 km s-1 and that it lies in the plane of the sky
(i.e., $\sin\leavevmode\nobreak\ i=1$). For a random orientation of the jet on
the celestial plane, the value of $\sin\leavevmode\nobreak\ i$ is on average
$\pi/4$ (e.g., Beltrán et al. 2001). Usually, a value of $T=10^{4}$ K is
adopted for ionized gas. For the turnover frequency $\nu_{m}$, we assume a
value of 40 GHz (see discussion in Anglada et al. 2018). In Fig. 6, we show
the relationship between the mass loss rates of the ionized and the molecular
outflow for the thermal radio jet candidates listed in Table 3 and associated
with the molecular outflows. Major uncertainties in the determination of
$\dot{M}_{\mathrm{ion}}$ may arise from parameters such as the jet velocity,
the turnover frequency or the aspect ratio of the jet, as they cannot be
determined from our observational data. However, their effects are almost
negligible and variations within reasonable ranges result in variations of the
ionized mass loss rate of less than a factor of 10. Our derived
$\dot{M}_{\mathrm{ion}}$ are mainly in the range of $10^{-7}$ to $10^{-5}$
$M_{\odot}$ yr-1, consistent with the values reported for low-mass radio jets
($\approx 10^{-10}$ $M_{\odot}$ yr-1) and high-mass radio jets ($\approx
10^{-5}$ $M_{\odot}$ yr-1, see Rodriguez et al. 1994; Beltrán et al. 2001;
Guzmán et al. 2010, 2012). The dashed lines in Fig. 6 indicate different
degrees of ionization for the mass loss rate. Most of our radio jet
candidates, with the exception of source #143, which is probably associated
with an already developed H ii region (see Fig. 5 and discussion above), have
ionization levels of $\dot{M}_{\mathrm{ion}}$ = 10-3 $\times$
$\dot{M}_{\mathrm{out}}$. These values are about one to two orders of
magnitude smaller than those reported in previous studies (see e.g., Rodriguez
et al. 1990; Hartigan et al. 1994; Bacciotti et al. 1995; Anglada et al.
2018). This difference may be due to uncertainties in the assumed parameters
of Eq. 3, as well as to the fact that our molecular outflow emission is
studied with a single-dish (sensitive to all scale structures), while the
radio jet observations were carried out with a large interferometric
configuration likely resolving out part of the radio jet emission.

Figure 6:  Relation between ionized ($\dot{M}_{\mathrm{ion}}$) and molecular
outflow ($\dot{M}_{\mathrm{out}}$) mass loss rates for the radio jet
candidates listed in Table 3. The ionized mass loss rate is derived for the
thermal radio jets using Eq. 3, while the molecular outflow mass loss rate is
provided in López-Sepulcre et al. (2010) and Sánchez-Monge et al. (2013d). The
dashed lines indicate different ionization levels given by the ratio
$\dot{M}_{\mathrm{ion}}$ to $\dot{M}_{\mathrm{out}}$.

####  4.5.3 Best radio jet candidates

In previous sections, we have analyzed the properties of the 146 detected
radio continuum sources and built a sample of possible radio jet candidates
based on their association with outflow activity: molecular outflows, EGOs,
and masers (see Table 3). In Sect. 4.5.1 and 4.5.2, we have investigated in
more detail the possible nature of the radio continuum emission and its
relation to the outflow activity tracers, in particular, the outflow momentum
rate. The results presented in Figs. 3 and 5 allow us to identify sources with
properties that differ from those expected from radio jets, therefore
suggesting that these sources may actually not be radio jets. From this
analysis and the individual description of selected sources (see Appendix A),
we discuss in this section which objects are most likely to be radio jets.

Out of the 28 sources listed in Table 3, five of them have radio luminosities
similar to those expected for H ii regions: #48a, #48b, #48c, #143 and #144
(see Fig. 3). In addition, all of these sources, with the exception of #143,
have negative spectral indices. These negative values could be due to the
sources being slightly extended (as expected for H ii regions) and partially
filtered out in the K band images. Moreover, these five sources also exhibit
large radio luminosities compared to their associated outflow momentum rates
(see Fig. 5), which supports the interpretation that there may be an excess of
radio continuum emission not necessarily associated with a radio jet, but with
an H ii region. In the absence of further evidence, we are not in a position
to interpret further and we can not consider these sources to be among the
best radio jet candidates. Further observations, sensitive to extended
emission, can provide the necessary information to better characterize these
sources in terms of their spatial extend and the nature of the emission. It is
also worth noting that sources with negative spectral indices could correspond
to background sources with synchrotron radiation since we expect about 11
objects in our sample to have this possible origin (see Sect. 3.1). Source #22
also shows an excess of radio-continuum emission compared to its outflow
momentum rate, which suggests that this is also a dubious radio jet candidate.
From the individual source descriptions presented in Appendix A, sources #42
and #137 seem to be radio continuum sources with most of their emission
dominated by cometary/ultracompact H ii regions, which makes it difficult to
identify a radio jet in our data.

Out of the remaining sources listed in Table 3, we can identify seven of them
having a high probability to be radio jets. These are sources #2, #14, #22,
#64, #74, #83 and #110, which are associated with additional outflow/shock
activity such as masers and EGOs. One should note that source #74 is adjacent
to two H2O maser features, which are only 2″ away and coincident with extended
4.5 $\mu$m emission (see Fig. 13). The remaining sources do still classify as
radio jet candidates, since we do not have clear evidence against that. Some
of them are located at the center of molecular outflows (e.g., #95) but are
not associated with additional outflow/shock signposts. This could be related
to the variability of H2O masers (see Sect. 4.5.1). Others are located within
molecular outflow lobes (e.g., #12, #63, #64, #65, #73, and #144), and for
which higher angular resolution observations of outflow tracers are necessary
to confirm if they are powering some of the molecular outflows. Other sources,
despite not being associated with molecular outflows, show other shock
activity signposts such as the presence of EGOs (e.g., #119, #136, #139).
Further observational constrains are therefore needed to fully confirm or
discard these objects as radio jets. The results acquired so far, allow us to
classify them as radio jet candidates.

##  5 Implications for high-mass star formation

Recently, Rosero et al. (2019) studied the properties of 70 radio continuum
sources associated with the earliest stages of high-mass star formation. They
find that $\approx$30–50% of their sample are ionized jets. This fraction is
in agreement with our findings. Out of the 146 radio continuum sources
detected in our study, we identify 28 possible radio jets (i.e., about 19% of
our sample). However, if we focus on the sources for which we have more
accurate information (i.e., sources classified as iC/iK in Table
LABEL:t:catalogue, see also Sect. 3.1), we have 24 out of 40 sources being
potential radio jets. Therefore, the percentage of radio continuum sources
being radio jets increases up 60%. This suggests that about half of the radio
continuum sources found in star-forming regions at early evolutionary stages
may indeed be radio jets powered by young stars. The remaining $\approx$50% of
objects could still be radio jets for which we have not yet identified shock
activity signposts, or they could represent extremely compact H ii regions in
their early stages of development. These objects could be powered by early
B-type stars and not necessarily by the most massive stars, and could be an
intermediate stage between radio jets and already developed H ii regions (see
e.g., Beltrán et al. 2016; Rivera-Soto et al. 2020).

López-Sepulcre et al. (2010) classified the regions studied in our work as
infrared dark (IRDC, infrared dark cloud) and infrared bright (HMSFR, high-
mass star-forming region) based on their detectable infrared emission. Our
sample, therefore, consists of two sub-classes: IRDC (8 regions) and HMSFR (10
regions; see Table 1). We assume that these two types belong to different
evolutionary phases of massive star formation, with the IRDC regions being
less evolved than the HMSRF regions. Considering the 40 sources located within
the primary beam of our images (i.e., sources classified as iC/iK), we find
$\approx$2.8 radio continuum sources per HMSFR region, and $\approx$1.8 radio
continuum sources per IRDC region. This suggests that it is more probable to
detect compact radio continuum emission in more evolved regions. Regarding the
presence of radio jets in these two evolutionary stages, we find 21 out of the
28 radio jet candidates listed in Table 3 in HMSFR regions (corresponding to
75%), while we only find 7 (corresponding to 25%) in the less evolved IRDC
regions. If we consider only the best radio jet candidates (see Sect. 4.5.3),
we find five radio jets (#2, #14, #22, #64 and #83; corresponding to 71%) in
HMSRF regions and two radio jets (#74 and #110; corresponding to 29%) in IRDC
regions. This shows a preference of radio jets to be found in more evolved
clouds. Complementary to this, we can determine the fraction of IRDC and HMSFR
regions that harbor radio jets. Out of the 8 IRDC regions studied in this
work, only 2 (corresponding to 25%) harbor one of the best radio jet
candidates. This increases up to 50% (5 out of 10) for the HMSFR regions.
Therefore, the frequency of radio jets in IRDC regions is lower than in HMSFR
regions. One possible explanation is that the jets may become larger and
brighter with time. Our limited data do not show that IRDC jets are smaller or
fainter than HMSFR jets, but future work on larger source samples may provide
further insight.

##  6 Summary

In this work, we have used of the VLA in two different bands (C and K band,
corresponding to 6 and 1.3 cm wavelengths) to search for radio jets powered by
high-mass YSOs. We have studied a sample of 18 high-mass star-forming regions
with signposts of SiO and HCO+ outflow activity. In the following we summarize
our main results.

  * •

We have identified 146 radio continuum sources in the 18 high-mass star
forming regions, with 40 of the radio continuum sources located within the
primary beams of our images (i.e., labeled as iC/iK and with reliable flux
measurements). Out of these sources, 131 (27 iC/iKs) are only detected in the
C band, 4 (3 iC/iKs) are only detected in the K band, and 11 (10 iC/iKs) are
detected in both bands. This different detection level is likely due to
different factors: (i) four regions were not observed in the K band, (ii) the
C band images have a larger field of view, and (iii) the K band images are
affected by a larger interferometric spatial filtering. In addition to the
continuum emission, we have detected 23 maser features in the 6.7 GHz CH3OH
and 22 GHz H2O lines.

  * •

Out of the 146 continuum sources, only 40 sources are located within the field
of view of both images allowing for an accurate characterization of their
radio properties. For these sources we have derived the spectral index, which
we find to be consistent with thermal emission (i.e., in the range $-$0.1 to
$+$2.0) for most of the objects (73%).

  * •

We have investigated the nature of the radio continuum emission by comparing
the radio luminosity to the bolometric luminosity. We find that most sources
with positive spectral indices (i.e., thermal emission) follow the trend
expected for radio jets, while sources with large negative spectral indices
seem to follow the relation expected for H ii regions. These large negative
spectral indices are likely due to the emission in the K band images being
partially filtered out.

  * •

Based on the association of the radio continuum sources with shock activity
signposts (i.e., association with molecular outflows, EGOs or masers), we have
compiled a list of 28 radio jet candidates. This corresponds to $\approx$60%
of the radio continuum sources located within the field of view of both VLA
images. Out of these sample of radio jet candidates, we have identified 7
objects (#2, #14, #22, #64, #74, #83, and #110) as the most probable radio
jets. The remaining 21 require additional observations, either at different
radio frequency bands or of molecular outflow tracers at higher resolution, to
confirm or discard them as radio jets.

  * •

We find about 7–36% of the possible radio jet candidates to show non-thermal
radio continuum emission. This is consistent with previous studies reporting
$\approx$20% of non-thermal radio jets. We do not find a clear association of
the non-thermal emission with the presence of outflows, EGOs or masers.
However, and despite the low statistics, we find that radio jet candidates
associated with CH3OH masers have thermal emission, while those radio jet
candidates associated with only H2O masers tend to have non-thermal emission.
This is in agreement with the 6.7 GHz CH3OH masers tracing the actual location
of newly-born YSOs powering thermal winds and jets, while the H2O masers may
be originated in strong shocks where non-thermal emission becomes relevant.

  * •

As previously found in other works, we find a correlation between the radio
luminosity of our radio jet candidates and their associated outflow momentum
rates. We derive an ionized mass loss rate in the range $10^{-7}$ to $10^{-5}$
$M_{\odot}$ yr-1, which results in ionization levels of
$\dot{M}_{\mathrm{ion}}=10^{-3}\leavevmode\nobreak\ \dot{M}_{\mathrm{out}}$
(i.e., $\approx$0.1% of the outflow mass being ionized).

  * •

The 18 high-mass star-forming regions studied in this work are classified in
two different evolutionary stages: 8 less evolved IRDC and 10 more evolved
HMSFR. We find more radio continuum sources ($\approx$2.8 sources per region)
in the more evolved HMSFR compared to the IRDC ($\approx$1.8). Regarding radio
jets, we find about 71% of the radio jet candidates to be located in HMSFR
regions, and only 29% in IRDC regions. Complemenary to this, 25% of the IRDC
regions harbor one of the most probable radio jet candidates, while this
percentage increases up to 50% for the HMSFR regions. This suggests that the
frequency of radio jets in the less evolved IRDC regions is lower compared to
the more evolved HMSFR regions.

###### Acknowledgements.

The authors thank the referee for his/her review and greatly appreciate the
comments and suggestions that have contributed significantly to improve the
quality of the publication. ÜK also thanks Jonathan Tan for useful
discussions. This work has been partially supported by the Scientific and
Technological Research Council of Turkey (TÜBİTAK), project number: 116F003.
Part of this work was supported by the Research Fund of Istanbul University,
project number: 44659. ASM research is carried out within the Collaborative
Research Centre 956, sub-project A6, funded by the Deutsche
Forschungsgemeinschaft (DFG; project ID 184018867). ÜK would like to thank
William Pearson for checking the language of the paper and Kyle Oman for
helping with Python issues.

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##  Appendix A Comments on individual sources

In the following, we comment on different aspects of selected sources for
which additional literature information is available.

### IRAS 05358$+$3543 (#2 and #4)

In region IRAS 05358$+$3543, we have identified two radio continuum sources
that can be potential radio jets (see Fig. 7). Sources #2 and #4 were observed
only in the C band, and therefore we cannot determine a spectral index for
these sources. Despite this limitation, both sources are located at the center
of the molecular outflow reported by López-Sepulcre et al. (2010).
Furthermore, source #2 is associated with a 6.7 GHz CH3OH maser emission,
suggesting that this marks the position of a massive YSO. Of the two radio
continuum sources, source #2 is likely the main object powering the molecular
outflow for which its centimeter emission traces a radio jet. Further
observations at different frequency bands are necessary to better constrain
its properties.

### G189.78$+$0.34 (#12, #13, #14, #15 and #16)

In region G189.78+0.34, we have identified five radio continuum sources
(sources #12 to #16) associated with the molecular outflow reported by López-
Sepulcre et al. (2010). All of them are located at the center of the outflow,
with source #12 slightly offset from the rest (see Fig. 8). All these sources
were observed only in the C band and no spectral index can be derived. Out of
the five sources, source #14 is associated with CH3OH maser emission (see also
Caswell et al. 2010) suggesting that this marks the location of a massive YSO.
The radio continuum sources are found in an elongated chain extending from the
south-east to the north-west. This direction is consistent with the
orientation of the molecular outflow (López-Sepulcre et al. 2010). Overall, we
consider that source #14 is the powering source and most likely the main
component of the radio jet. The other sources could correspond to different
radio continuum knots located along the jet, as seen in other sources (e.g.,
HH 80-81: Carrasco-González et al. 2010, and G35.20$-$0.74 N: Beltrán et al.
2016), where the radio continuum knots usually show non-thermal spectral
indices. Observations at different frequency bands are necessary to gather
information on the spectral index and thermal/non-thermal nature of the
different radio continuum sources in the region.

---
Figure 7: VLA C band (6 cm) continuum emission map of the radio jet candidates
#2 and #4 located in the region IRAS 05358$+$3543\. The pink ellipse is the
beam size of the C band. The orange star marks the location of the CH3OH maser
(see Table 4). The grey double-headed arrow indicates the direction of the
outflows.
---
Figure 8: VLA C band (6 cm) continuum emission map of the radio jet candidates
#12, #13, #14, #15 and #16 located in the region G189.78$+$0.34. The pink
ellipse is the beam size of the C band. The orange star marks the location of
the CH3OH maser (see Table 4). The grey double-headed arrow indicates the
direction of the outflows.

### G192.58$-$0.04 (#22, #23 and #25)

In region G192.58$-$0.04, we have identified sources #22, #23, and #25 as
potential radio jets (see Fig. 9). These sources were observed only in the C
band and no spectral index can be derived. Out of the three sources, source
#22 is associated with CH3OH maser emission suggesting that this source may
mark the location of a massive YSO. The sources are located at the center of
the molecular outflow reported by López-Sepulcre et al. (2010). Out of the
three sources, we consider that source #22 is the most probable radio jet. The
comparison between the radio continuum luminosity with the outflow momentum
rate (see Fig. 5), also confirms this possibility, although there seems to be
a slightly excess of radio continuum emission, suggesting that there can be an
additional contribution to the radio continuum source (e.g., from an early-
stage H ii region).

---
Figure 9: VLA C band (6 cm) continuum emission map of the radio jet candidates
#22, #23 and #25 located in the region G192.58$-$0.04. The pink ellipse is the
beam size of the C band. The orange star marks the location of the CH3OH maser
(see Table 4). The grey double-headed arrow indicates the direction of the
outflows.

### IRAS 18223$-$1243 (#42)

In the region IRAS 18223$-$1243, we identified the radio continuum source #42
(see Fig. 10) as adjacent to the one reported in Cyganowski et al. (2011) EGO
F G18.67$+$0.03$-$CM1. This is the only signposts of shock activity, since no
molecular outflow or maser emission are found for this object. In addition,
Cyganowski et al. (2012) report the existence of a massive protocluster
consisting of a hot molecular core and an ultracompact H ii region. Our source
seems to be located at the same position of the ultracompact H ii region,
which makes us to doubt if this can be a radio jet candidate.

---
Figure 10: VLA C band (6 cm) continuum emission map of the radio jet candidate
#42 located in the region IRAS 18223$-$1243\. The pink ellipse is the beam
size of the C band. The green circle with a radius of $\sim$5″ marks the EGO F
G18.67$+$0.03-CM1 reported by Cyganowski et al. (2011).

### IRAS 18228$-$1312 (#48)

Radio continuum source #48 is observed as a group of three compact sources
(#48a, #48b and #48c) surrounded by an extended and more diffuse structure.
One of these compact sources (#48a) is clearly associated with H2O maser
emission (see Fig. 11). The Spitzer/IRAC 4.5 $\mu$m emission is extended and
spatially coincident with the radio continuum extended emission. The spectral
indices for these three compact sources are in the range $-0.6$ to $-1.5$,
most likely due to additional filtering of the emission in the K band image.
Previous studies have classified this extended source as a region containing
hypercompact (HC) and ultracompact (UC) H ii regions (e.g., Chini et al. 1987;
Lockman 1989; Kurtz et al. 1994; Kuchar & Clark 1997; Leto et al. 2009), which
is consistent with our derived radio continuum luminosity (see Figs. 3 and 5).
It is worth noting that the the three sources are spatially located at the
center of a molecular outflow (e.g., López-Sepulcre et al. 2010). This may
suggest that one of the compact sources may be powering the molecular outflow.
In this case, this object would be in a evolutionary stage where the radio jet
still exists but a young H ii region has already developed, similar to the
high-mass young stellar object G35.20$-$0.74 N (e.g, Sánchez-Monge et al.
2013b, 2014; Beltrán et al. 2016).

---
Figure 11: VLA C band (6 cm) continuum emission map of source #48 located in
the region IRAS 18228$-$1312\. Three bright peaks are visible and labelled as
#48a, #48b and #48c. The green contour levels of the K band (1.3 cm) continuum
emission are 3, 5 and 9 times 0.7 mJy beam-1. The magenta contours show the
Spitzer/GLIMPSE 4.5 $\mu$m emission. The pink and green ellipses are the beam
sizes of the C and K bands, respectively. The white star marks the location of
the H2O maser (see Table 4).

### IRAS 18236$-$1205 (#63, #64 and #65)

We have identified nine radio continuum sources in the IRAS 18236$-$1205
region (also referred to in the literature as G19.36$-$0.03), three of which
have been classified as radio jet candidates: Sources #63, #64 and #65 with
spectral indices of $-0.22\pm 0.07$, $+0.18\pm 0.04$ and $+1.08\pm 0.19$. We
have identified four H2O maser features near these sources (see Fig. 12). Two
maser features are associated with source #63, one maser feature is associated
with source #64 (which also spatially coincides with a CH3OH maser), and the
last maser feature is located in the center of the red-shifted outflow lobe
where no radio continuum emission is detected. The molecular outflow in this
region has been mapped in the lines SiO (2–1) and HCO+ (1–0) by (Sánchez-Monge
et al. 2013d).

Figure 12: VLA C band (6 cm) continuum emission map of the radio jet
candidates #63, #64 and #65 located in the region IRAS 18236$-$1205\. A close-
up view of the three radio sources is shown in the bottom and right panels.
The green contour levels of the K band (1.3 cm) continuum emission are 3, 5
and 7 times 30 $\mu$Jy beam-1. The magenta contours show the Spitzer/GLIMPSE
4.5 $\mu$m emission. The blue- and red-shifted outflow lobed of SiO (2$-$1)
are shown as blue-solid and red-dashed contours, respectively (see Sánchez-
Monge et al. 2013d). The pink and green ellipses are the beam sizes of the C
and K bands, respectively. The white stars mark the location of the H2O masers
(see Table 4).

Sources #63 and #64 are associated with EGOs (see Cyganowski et al. 2009),
indicating the presence of strong shock activity in these two sources. Their
location near the center of the molecular outflow together with their
association with H2O maser emission and EGOs, suggests that these two sources
could be candidates for radio jets. The source #64 is spatially more
coincident with the geometric center of the outflow, and its association with
6.7 GHz CH3OH maser emission, suggests that a massive YSO exists at this
position. This massive YSO could be the driving source of the molecular
outflow seen on larger scales. The third candidate (source #65) is located
$\sim$28$\arcsec$ away from the source #64 and the center of the outflow, and
has been studied by (Cyganowski et al. 2011, G19.36-0.03-CM2). This source is
associated with an emission of 4.5 $\mu$m, although it is unclear whether it
can be convincingly classified as an EGO (Cyganowski et al. 2009). The
positive spectral index indicates the presence of thermal emission, which
could come from a radio jet. However, there is no clear evidence of outflow or
shock activity. The source #65 is also located in the vicinity of a dense core
(18236$-$1205 P8) identified by Lu et al. (2014) in the VLA NH3 maps, which
supports the interpretation of this source as an embedded young stellar
object. Overall, the source #65 could be a YSO-powered radio jet in the
vicinity of the more massive object (sources #63 and #64) in IRAS
18236$-$1205.

### G23.60$+$0.0M1 (#73 and #74)

The G23.60$+$0.01M1 star-forming region has been studied in the literature by
various authors (e.g., Rathborne et al. 2006; Battersby et al. 2010; Ginsburg
et al. 2013), who reported the presence of two massive dense clumps with
masses of 100 $M_{\odot}$ and 120 $M_{\odot}$. The two candidate radio jets
(#73 and #74) have positive spectral indices consistent with thermal emission
of radio jets. In particular, the source #74 is located at the center of the
molecular outflow reported by Sánchez-Monge et al. (2013d) and is associated
with a strong 4.5 $\mu$m emission (see Fig. 13). The two H2O maser detected in
the region are slightly displaced from the radio continuum source but coincide
with the extended 4.5 $\mu$m emission (see Fig. 13-bottom). The association of
molecular outflow emission, bright and extended 4.5 $\mu$m emission, and close
H2O maser features favor the interpretation of this source is a good radio jet
candidate.

The second radio continuum source (#73) is located relatively close to the
center of the outflow. However, no maser or EGOs are found in connection with
the source. Although we cannot reject this source as a radio jet, we prefer
the source #74 as the main object driving the outflow observed in the region.

---
Figure 13: VLA C band (6 cm) continuum emission map of the radio jet
candidates #73 and #74 located in the region G23.60$+$0.0M1. A close-up view
of the two radio sources is shown in the bottom panel. The green contour
levels of the K band (1.3 cm) continuum emission are 3, 5, 7, 9 and 11 times
20 $\mu$Jy beam-1. The magenta contours show the Spitzer/GLIMPSE 4.5 $\mu$m
emission. The blue- and red-shifted outflow lobed of SiO (2$-$1) are shown as
blue-solid and red-dashed contours, respectively (see Sánchez-Monge et al.
2013d). The pink and green ellipses are the beam sizes of the C and K bands,
respectively. The white stars mark the location of the H2O masers (see Table
4).

### IRAS 18316$-$0602 (#83 and #95)

We have identified thirteen radio continuum sources in the IRAS 18316$-$0602
region, two of which have been classified as radio jet candidates: Sources #83
and #95. The source #83 has an almost flat spectral index ($0.08\pm 0.08$) and
shows a weak extension to the south, which is better resolved in the K band
image. Source #95 is fainter, appears located about 3″to the south-east of
source #83, and is visible only in the K band (some faint emission slightly
above the noise level is visible in the C band image, see Fig. 14). As for the
maser emission, we have identified both a H2O and a CH3OH maser feature
associated with the brightest source #83. This source has been studied in
previous works (e.g., Roueff et al. 2006; Cutri & et al. 2012; Azatyan et al.
2016; Stecklum et al. 2017), in some of them named RAFGL 7009S. The source is
detected in the near-infrared together with two other objects separated by
about 10″, and surrounded by a diffuse and extended structure (see e.g.,
Stecklum et al. 2017).

López-Sepulcre et al. (2010) and Sánchez-Monge et al. (2013d) report on
molecular outflow emission in the region. The blue and red contours in Fig. 14
show the SiO (2–1) blue-shifted and red-shifted outflow emission. Both radio
continuum sources (#83 and #95) are located close to the center of the
outflow. Although this region was not included in the Cyganowski et al. (2008,
2009, 2011) surveys, we have identified a bright 4.5 $\mu$m source in the
Spitzer/GLIMPSE data. However, the source is located at the edge of the area
surveyed by Spitzer, which prevents a detailed characterization of its
infrared emission. The association of the source #83 with a 6.7 GHz CH3OH
maser emission suggests that this source harbors a massive YSO together with
its central location within the outflow and its association with a H2O maser
feature hints towards this source is the radio jet that drives the outflow.
The almost flat spectral index may indicate that this is a fully ionized radio
jet. However, further observations in different frequency bands may help to
better narrow down the spectral index and the status of radio continuum
emission.

---
Figure 14: VLA C band (6 cm) continuum emission map of the radio jet
candidates #83 and #95 located in the region IRAS 18316$-$1602\. A close-up
view of the two radio sources is shown in the bottom panel. The green contour
levels of the K band (1.3 cm) continuum emission are 3, 5, 9 and 11 times 27
$\mu$Jy beam-1. The magenta contours show the Spitzer/GLIMPSE 4.5 $\mu$m
emission (note that half of the region was not covered in the mapped area).
The blue- and red-shifted outflow lobed of SiO (2$-$1) are shown as blue-solid
and red-dashed contours, respectively (see Sánchez-Monge et al. 2013d). The
pink and green ellipses are the beam sizes of the C and K bands, respectively.
The white and orange stars mark the location of the H2O and CH3OH masers,
respectively (see Table 4).

### G24.08$+$0.0 M2 (#96)

We have identified fourteen radio continuum sources in the region G24.08$+$0.0
M2, one of which, source #96, is detected in both frequency bands and has a
negative spectral index ($-0.84\pm+0.08$, see Fig. 15). The outflow activity
in the region has been studied by López-Sepulcre et al. (2011) and Sánchez-
Monge et al. (2013d) who found molecular outflow in different tracers.
However, this outflow is not spatially related to any of the radio continuum
sources identified in this work. Also, this source could be a background
source.

---
Figure 15: VLA C band (6 cm) continuum emission map of the radio source #96
located in the region G24.08$+$0.0M2. The green contour levels of the K band
(1.3 cm) continuum emission are 3, 5 and 9 times 20 $\mu$Jy beam-1. The pink
and green ellipses are the beam sizes of the C and K bands, respectively.

### G24.33$+$0.1 M1 (#110)

In the region G24.33$+$0.1 M1 we identified a radio continuum source (#110)
located in the center of our field of view and detected in both frequency
bands (see Fig. 16). This source has a positive spectral index of $+0.73\pm
0.35$, which is consistent with thermal emission. We have also detected maser
features of both H2O and CH3OH associated with the continuum source. In
addition, other authors have reported the presence of OH masers towards this
object (see e.g., Caswell & Green 2011). Rathborne et al. (2007) studied this
source in the millimeter regime, and identified a singular compact source with
a rich chemistry characteristic of hot molecular cores. Sánchez-Monge et al.
(2013d) reported molecular outflow activity, with the source #110 at its
geometric center. Overall, this source is one of the best candidates for a
thermal radio jet.

---
Figure 16: VLA C band (6 cm) continuum emission map of the radio jet candidate
#110 located in the region G24.33+0.1 M1. A close-up view of the radio source
is shown in the bottom panel. The green contour levels of the K band (1.3 cm)
continuum emission are 3, 5 and 9 times 7 $\mu$Jy beam-1. The blue- and red-
shifted outflow lobed of SiO (2$-$1) are shown as blue-solid and red-dashed
contours, respectively (see Sánchez-Monge et al. 2013d). The pink and green
ellipses are the beam sizes of the C and K bands, respectively. The white star
marks the location of the H2O (see Table 4).

### G24.60$+$0.1M1 (#119)

As for #119, the only information we have is that it is associated with an
EGO. The nearest studied object is a extended H2 emission that is 19″ away
from #119 (Froebrich et al. 2015). We can only say that we cannot determine
the spectral index because the source is outside the primary beam of the K
band images, but we propose that #119 is a radio jet.

### G24.60$+$0.1M2 (#136)

Source #136 is detected only in the K band, resulting in a spectral index
limit ($>+1.67$) that is consistent with thermal emission. The source is
associated with H2O maser emission. This source, although not directly
associated with one EGO, is located in the vicinity of G24.63$+$0.15 reported
by Cyganowski et al. (2008, see green circle in Fig. 17). Rathborne et al.
(2007) suggest that the main dense condensation, hosting source #136, may
contain several condensations, referred to as G024.60$+$00.08 MM1 (A, B, and
C). Our radio continuum source appears to be related to component C which is
an IRDC condensation. Further observations of this object are necessary to
confirm its possible nature as a radio jet.

---
Figure 17: VLA K band (1.3 cm) continuum emission map of the radio jet
candidate #136 located in the region G24.60$+$0.1 M2. The green circle with a
radius of $\sim$4″ marks EGO G24.63$+$0.15, reported by Cyganowski et al.
(2008). The pink ellipse is the beam size of the K band. The white star marks
the location of the H2O (see Table 4).

### G34.43$+$0.2M3 (#137 and #139)

Our radio continuum observations towards the region G34.43$+$0.2 M3 have led
to the discovery of six radio continuum sources, although most of them are
located far from the central region studied in Sánchez-Monge et al. (2013d).
The brightest source is #137, which is about 13′ from the phase center of our
observations, i.e., the source is outside the primary beam responses of the
VLA antennas on both bands. This object is so bright that it is detected in
both the C and K bands. At 6 cm, the source appears as a comet-like structure
(see Fig. 18), which resembles cometary H ii regions. The 1.3 cm continuum
emission also shows an arc-shaped structure shifted to the east, probably
tracing the head of the cometary object. This source, referred to in the
literature as G34.26$+$0.15, has been studied by other authors who report the
presence of two hyper-compact H ii regions (A and B) and one ultra-compact H
ii region (C), all marked in the Figure (see also Reid & Ho 1985; Gaume et al.
1994; Sewiło et al. 2011). Various studies (e.g., Hatchell et al. 2001; Liu et
al. 2013) have reported the presence of outflow activity in this region, but
no information on the outflow energetics such as the outflow momentum rate is
reported. Despite this source is associated with an EGO (Cyganowski et al.
2008) and with a molecular outflow, the bright emission together with previous
studies suggests that a major fraction of the radio continuum emission we have
detected is originated in a H ii region rather than in a radio jet. Source
#139 in the region, is also classified as a radio jet candidate in Table 3.
However, the only information we have this object is its association with an
EGO (i.e., G34.41$+$0.24, Cyganowski et al. 2008). Shepherd et al. (2004)
suggest that the embedded object (G34.4 MM) appears to be a massive B2
protostar at an early stage of evolution. This region is also associated with
the H2O maser activity (Cyganowski et al. 2013), which may favor a radio jet
origin for the detected radio continuum emission. Further observations are
needed to better constrain its properties.

---
Figure 18: VLA C band (6 cm) continuum emission map of the radio jet candidate
#137 located in the region G34.43$+$0.2 M3. The green contour levels of the K
band (1.3 cm) continuum emission are 3, 5 and 7 times 20 $\mu$Jy beam-1. The
magenta contours show the Spitzer/GLIMPSE 4.5 $\mu$m emission. The pink and
green ellipses are the beam sizes of the C and K bands, respectively. The
white circles (A, B, and C) mark the position of 2 mm continuum sources
reported by Gasiprong et al. (2002).

### IRAS 19095$+$0930 (#143 and #144)

We have identified four radio continuum sources in the region IRAS
19095$+$0930, also known in the literature as G43.80$-$0.13). Two of these
radio sources, #143 and #144, are located close to each other and in the
center of a molecular outflow (Sánchez-Monge et al. 2013d, see also Figure
19). The source #143 has a brighter flux, is also clearly visible in the K
band image, and is associated with H2O maser emission features.

This region has been studied in the past by different authors at different
wavelengths (Kurtz et al. 1994; Lekht 2000; De Buizer et al. 2005). De Buizer
et al. (2005) report a kidney-bean shape structure at mid-infrared wavelengths
that match the radio continuum sources reported by Kurtz et al. (1994) refers
to as a H ii region. The object is also associated with OH masers. We have not
found this source in the EGO catalogues (Cyganowski et al. 2008, 2009), but we
have identified a 4.5 $\mu$m source associated with #143. No 4.5 $\mu$m
infrared source appears to be associated with the eastern source #144.

We derive a spectral index of $+1.12\pm 0.04$ for #143, which is consistent
with thermal emission. This, together with its location at the geometrical
center of the outflow, and its association with masers may suggest that #143
is a good radio jet candidate. However, the large radio continuum flux of this
source seem to not be consistent with the typical properties of other radio
jets (see Figs. 3 and 5). This might mean that this source is in a transition
phase from a radio jet to an H ii region. However, this requires further
investigation.

---
Figure 19: VLA C band (6 cm) continuum emission map of the radio sources #143
and #144 located in the region IRAS 19095$+$0930\. The green contour levels of
the K band (1.3 cm) continuum emission are 3, 5 and 9 times 2 mJy beam-1. The
blue- and red-shifted outflow lobed of SiO (2$-$1) are shown as blue-solid and
red-dashed contours, respectively (see Sánchez-Monge et al. 2013d). The
magenta contours show the Spitzer/GLIMPSE 4.5 $\mu$m emission. The pink and
green ellipses are the beam sizes of the C and K bands, respectively. The
white and orange stars mark the of the H2O and CH3OH masers (see Table 4).

##  Appendix B Catalog of the continuum sources

In the following tables and figures, we provide information on the properties
of the radio continuum sources detected in the VLA observations presented in
this work. In Table LABEL:t:catalogue, we list the coordinates of the 146
radio continuum sources together with their flux density, intensity peak and
deconvolved size at 6 cm (C band) and 1.3 cm (K band). The fluxes and
intensities are corrected by the primary beam response of the VLA antennas.
For sources outside the C band primary beam (listed as ”oC” in column (11) of
Table LABEL:t:catalogue) the primary beam correction is not reliable and the
flux has to be taken with caution. Similarly, for sources located outside the
K band primary beam (labelled as ”oK” in the Table), the K band flux has to be
taken with caution. The last column of the Table lists the spectral index,
$\alpha$. For sources with no reliable flux estimate at one of the bands, we
do not determine the spectral index. For sources detected at both frequency
bands (C and K bands), the spectral index has been determined using the fluxes
determined after creating images with a common uv-range (see Sect. 3.2 for
more details). In Table LABEL:t:sizes, we list the observed and deconvolved
source sizes of all the detected sources. The source sizes are determined as
$\sqrt{\theta_{\mathrm{major}}\times\theta_{\mathrm{minor}}}$, where
$\theta_{\mathrm{major}}$ and $\theta_{\mathrm{minor}}$ are listed in Table
LABEL:t:sizes. We transform the angular size of each source into astronomical
unit (au) using the distances listed in Table 1. We give the source size in
Table LABEL:t:catalogue. Finally, in Table 7, we list the intensities, flux
densities and sizes determined from the images generated using a common uv-
range at both C and K bands. In Figs. 20 to 35, we present close-up views of
the C and K band emission for the 146 detected continuum sources.

Table 5:  Catalogue of radio continuum sources detected with the VLA. |  | Coordinates |  |  C bandaa$a$Primary beam corrected fluxes in mJy as listed in Table LABEL:t:catalogue. For sources #42, #119, #137 and #139 it was not possible primary beam correct the fluxes at both bands (see Sect 3), resulting in not usable spectral indices. The spectral index $\alpha$ is defined in Eq. 2. |  |  K bandbb$b$Source sizes in arcsec determined as $\sqrt{\theta_{\mathrm{major}}\times\theta_{\mathrm{minor}}}$, with $\theta_{\mathrm{major}}$ and $\theta_{\mathrm{minor}}$ listed in Table LABEL:t:sizes. Upper limits corresponds to sources for which we could not determine a deconvolved source size. The source size index $\beta$ is defined in Eq. 2. |  |
---|---|---|---|---|---|---|---|---
|  | R.A.(J2000) | Dec.(J2000) |  | Intensity | Flux | Size |  | Intensity | Flux | Size |  | Spectral
ID | Region | (h:m:s) | (°:′:″) |  |  (mJy beam-1) | (mJy) | (au) |  |  (mJy beam-1) | (mJy) | (au) |  |  indexcc$c$Spectral index determined from the fluxes at 6 cm (C band) and 1.3 cm (K band). For the sources detected in one band, we use a 5$\sigma$ upper limit for the non-detected band flux. Only for sources located with the primary beam of both images (sources listed as ‘iC/iK’) we can derive reliable fluxes and therefore spectral indices. Sources marked with † have been re-imaged using the common uv-range (see Table 7). More accurate spectral indices, derived using these new images, are listed in Table 3.
1 |  IRAS 05358$+$3543 | 05:39:25.838 |  $+$35:45:53.861 |  |  37.733$\pm$0.272 |  038.134$\pm$0.272 | ¡2200 |  | — | — | — | iC/oK | —
2 |  | 05:39:13.064 |  $+$35:45:51.125 |  |  00.387$\pm$0.010 |  000.534$\pm$0.014 |  ¡1300 |  | — | — | — | iC/iK | —
3 |  | 05:39:10.056 |  $+$35:46:07.731 |  |  00.105$\pm$0.003 |  000.075$\pm$0.002 | ¡1800 |  | — | — | — | iC/iK | —
4 |  | 05:39:12.839 |  $+$35:45:50.854 |  |  00.147$\pm$0.006 |  000.332$\pm$0.015 |  ¡2200 |  | — | — | — | iC/iK | —
5 |  | 05:39:09.938 |  $+$35:45:17.196 |  |  00.267$\pm$0.008 |  000.403$\pm$0.013 |  ¡1500 |  | — | — | — | iC/iK | —
6 |  | 05:39:33.539 |  $+$35:48:03.740 |  |  00.282$\pm$0.006 |  000.345$\pm$0.007 |  ¡1000 |  | — | — | — | iC/oK | —
7 |  | 05:39:14.367 |  $+$35:44:29.634 |  |  00.191$\pm$0.006 |  000.266$\pm$0.009 |  ¡1300 |  | — | — | — | iC/iK | —
8 |  | 05:39:12.120 |  $+$35:44:02.155 |  |  00.238$\pm$0.006 |  000.262$\pm$0.007 |  0¡500 |  | — | — | — | iC/iK | —
9 |  | 05:38:54.931 |  $+$35:44:41.720 |  |  00.521$\pm$0.004 |  000.571$\pm$0.005 |  0¡600 |  | — | — | — | iC/oK | —
10 |  | 05:38:39.158 |  $+$35:49:43.533 |  |  00.886$\pm$0.060 |  001.640$\pm$0.110 |  ¡2000 |  | — | — | — | iC/oK | —
11 |  | 05:39:37.247 |  $+$35:46:45.155 |  |  00.265$\pm$0.014 |  000.197$\pm$0.010 | ¡1900 |  | — | — | — | iC/oK | —
12 |  G189.78$+$0.34 | 06:08:35.111 |  $+$20:39:12.570 |  |  00.429$\pm$0.016 |  000.722$\pm$0.027 |  ¡1700 |  | — | — | — | iC/iK | —
13 |  | 06:08:35.273 |  $+$20:39:09.010 |  |  00.874$\pm$0.032 |  001.241$\pm$0.046 |  ¡1300 |  | — | — | — | iC/iK | —
14 |  | 06:08:35.308 |  $+$20:39:06.771 |  |  00.372$\pm$0.024 |  000.687$\pm$0.045 |  ¡1900 |  | — | — | — | iC/iK | —
15 |  | 06:08:35.380 |  $+$20:39:04.423 |  |  00.313$\pm$0.013 |  000.938$\pm$0.040 |  ¡2800 |  | — | — | — | iC/iK | —
16 |  | 06:08:35.449 |  $+$20:39:03.118 |  |  00.503$\pm$0.021 |  001.040$\pm$0.044 |  ¡2000 |  | — | — | — | iC/iK | —
17 |  | 06:08:43.600 |  $+$20:41:08.591 |  |  00.183$\pm$0.008 |  000.119$\pm$0.005 | ¡1700 |  | — | — | — | iC/oK | —
18 |  | 06:08:45.199 |  $+$20:38:16.647 |  |  00.114$\pm$0.008 |  000.164$\pm$0.011 |  ¡1300 |  | — | — | — | iC/oK | —
19 |  | 06:08:36.598 |  $+$20:43:14.366 |  |  00.185$\pm$0.015 |  000.300$\pm$0.024 |  0¡800 |  | — | — | — | iC/oK | —
20 |  | 06:08:44.256 |  $+$20:33:06.465 |  |  00.595$\pm$0.038 |  000.569$\pm$0.036 | ¡2000 |  | — | — | — | iC/oK | —
21 |  G192.58$-$0.04 | 06:12:53.607 |  $+$18:00:26.360 |  |  12.029$\pm$0.460 |  018.208$\pm$0.701 |  ¡2400 |  | — | — | — | iC/iK | —
22 |  | 06:12:54.013 |  $+$17:59:23.202 |  |  03.998$\pm$0.087 |  010.274$\pm$0.220 |  ¡4000 |  | — | — | — | iC/iK | —
23 |  | 06:12:53.842 |  $+$17:59:22.168 |  |  01.540$\pm$0.047 |  001.558$\pm$0.047 | ¡3300 |  | — | — | — | iC/iK | —
24 |  | 06:12:54.299 |  $+$17:59:33.802 |  |  00.637$\pm$0.033 |  000.494$\pm$0.025 | ¡2900 |  | — | — | — | iC/iK | —
25 |  | 06:12:54.335 |  $+$17:59:24.274 |  |  00.303$\pm$0.011 |  000.470$\pm$0.049 |  ¡5800 |  | — | — | — | iC/iK | —
26 |  | 06:12:49.910 |  $+$18:03:05.366 |  |  00.313$\pm$0.009 |  000.408$\pm$0.012 |  ¡1500 |  | — | — | — | iC/oK | —
27 |  G18.18$-$0.30 | 18:25:01.066 |  $-$13:15:39.515 |  |  04.240$\pm$0.262 |  021.202$\pm$1.367 |  ¡6600 |  | … | … | … | iC/oK | …
28 |  | 18:25:07.460 |  $-$13:17:58.837 |  |  02.412$\pm$0.089 |  003.863$\pm$0.144 |  ¡2200 |  | … | … | … | iC/oK | …
29 |  | 18:25:06.979 |  $-$13:18:10.638 |  |  01.745$\pm$0.034 |  003.390$\pm$0.069 |  ¡3000 |  | … | … | … | iC/oK | …
30 |  | 18:25:01.186 |  $-$13:15:45.881 |  |  01.303$\pm$0.020 |  001.989$\pm$0.032 |  ¡2400 |  | … | … | … | iC/oK | …
31 |  | 18:25:00.956 |  $-$13:15:35.726 |  |  01.298$\pm$0.039 |  006.719$\pm$0.204 |  ¡7000 |  | … | … | … | iC/oK | …
32 |  | 18:24:44.852 |  $-$13:14:45.806 |  |  02.647$\pm$0.036 |  004.447$\pm$0.064 |  ¡2400 |  | … | … | … | iC/oK | …
33 |  | 18:24:53.825 |  $-$13:12:52.490 |  |  00.713$\pm$0.007 |  000.670$\pm$0.007 | ¡3400 |  | … | … | … | iC/oK | …
34 |  | 18:25:06.706 |  $-$13:12:28.027 |  |  00.307$\pm$0.008 |  000.331$\pm$0.009 | ¡3600 |  | … | … | … | iC/oK | …
35 |  | 18:25:42.356 |  $-$13:10:21.217 |  |  00.361$\pm$0.023 |  001.177$\pm$0.075 |  ¡4500 |  | … | … | … | oC | …
36 |  | 18:25:18.546 |  $-$13:15:33.062 |  |  00.182$\pm$0.006 |  000.259$\pm$0.009 |  ¡2100 |  | … | … | … | iC/oK | …
37 |  | 18:24:55.788 |  $-$13:10:16.391 |  |  00.404$\pm$0.023 |  001.782$\pm$0.101 |  ¡6400 |  | … | … | … | iC/oK | …
38 |  | 18:24:55.941 |  $-$13:10:17.447 |  |  00.291$\pm$0.012 |  001.226$\pm$0.053 |  ¡6100 |  | … | … | … | iC/oK | …
39 |  IRAS 18223$-$1243 | 18:25:31.473 |  $-$12:41:24.031 |  |  54.190$\pm$1.078 |  105.645$\pm$2.077 |  ¡4500 |  | … | … | … | iC/oK | …
40 |  | 18:25:04.143 |  $-$12:37:44.910 |  |  48.631$\pm$1.238 |  075.337$\pm$1.932 |  ¡3800 |  | … | … | … | iC/oK | …
41 |  | 18:25:26.731 |  $-$12:40:33.237 |  |  01.694$\pm$0.063 |  002.477$\pm$0.094 |  ¡3600 |  | … | … | … | iC/oK | …
42 |  | 18:24:52.605 |  $-$12:39:19.804 |  |  02.989$\pm$0.054 |  004.158$\pm$0.075 |  ¡2900 |  | … | … | … | iC/oK | …
43 |  | 18:24:36.306 |  $-$12:51:03.315 |  |  01.762$\pm$0.091 |  006.550$\pm$0.340 |  ¡8800 |  | … | … | … | oC | …
44 |  | 18:24:34.617 |  $-$12:52:03.911 |  |  00.374$\pm$0.016 |  001.583$\pm$0.069 |  ¡9600 |  | … | … | … | oC | …
45 |  | 18:26:04.962 |  $-$12:34:35.177 |  |  00.250$\pm$0.009 |  000.577$\pm$0.023 |  ¡6000 |  | … | … | … | oC | …
46 |  | 18:26:05.248 |  $-$12:34:35.621 |  |  00.289$\pm$0.039 |  000.579$\pm$0.039 | ¡8200 |  | … | … | … | oC | …
47 |  | 18:24:34.692 |  $-$12:50:59.457 |  |  00.771$\pm$0.083 |  001.542$\pm$0.083 |  ¡168000 |  | … | … | … | oC | …
48a |  IRAS 18228$-$1312 | 18:25:41.925 |  $-$13:10:17.630 |  |  21.200$\pm$0.450 |  219.960$\pm$5.101 |  ¡135000 |  |  06.992$\pm$0.835 |  13.782$\pm$2.314 |  $<$7500 | iC/iK |  $-$2.11$\pm$0.13†
48b |  | 18:25:42.059 |  $-$13:10:16.545 |  |  19.373$\pm$0.720 |  383.051$\pm$9.900 |  ¡191000 |  |  06.218$\pm$1.203 |  14.341$\pm$3.711 |  ¡5400 | iC/iK |  $-$2.51$\pm$0.20†
48c |  | 18:25:42.407 |  $-$13:10:22.149 |  |  38.507$\pm$2.700 |  297.053$\pm$9.900 |  ¡112000 |  |  25.098$\pm$1.807 |  64.552$\pm$6.425 |  ¡6500 | iC/iK |  $-$1.17$\pm$0.10†
49 |  | 18:26:26.274 |  $-$13:05:48.537 |  |  00.468$\pm$0.010 |  000.445$\pm$0.009 | ¡4300 |  | … | … | … | oC | …
50 |  | 18:25:00.730 |  $-$13:15:35.808 |  |  00.312$\pm$0.019 |  013.170$\pm$0.800 |  ¡283000 |  | … | … | … | oC | …
51 |  G19.27$+$0.1M1 | 18:27:37.962 |  $-$11:56:33.896 |  |  00.348$\pm$0.023 |  002.140$\pm$0.140 |  ¡8200 |  | … | … | … | oC | …
52 |  | 18:26:20.973 |  $-$12:05:33.129 |  |  00.797$\pm$0.033 |  000.878$\pm$0.037 |  ¡1100 |  | … | … | … | iC/oK | …
53 |  | 18:26:03.677 |  $-$12:04:37.085 |  |  01.130$\pm$0.019 |  001.198$\pm$0.020 |  0¡800 |  | … | … | … | iC/iK |  $<-$0.57
54 |  | 18:25:45.520 |  $-$12:05:18.590 |  |  00.265$\pm$0.005 |  000.326$\pm$0.007 |  ¡1400 |  | … | … | … | iC/oK | …
55 |  | 18:25:54.272 |  $-$11:52:19.022 |  |  00.186$\pm$0.012 |  001.138$\pm$0.071 |  ¡3600 |  | … | … | … | oC | …
56 |  | 18:26:04.228 |  $-$11:52:32.053 |  |  00.299$\pm$0.041 |  007.300$\pm$1.000 |  ¡176000 |  | … | … | … | oC | …
57 |  | 18:26:48.638 |  $-$12:26:17.050 |  |  00.199$\pm$0.009 |  000.599$\pm$0.030 |  ¡5000 |  | … | … | … | oC | …
58 |  | 18:26:34.300 |  $-$11:57:59.910 |  |  00.284$\pm$0.008 |  000.518$\pm$0.016 |  ¡3100 |  | … | … | … | oC | …
59 |  | 18:26:13.780 |  $-$12:01:19.596 |  |  00.258$\pm$0.007 |  000.249$\pm$0.006 | ¡3500 |  | … | … | … | iC/oK | …
60 |  | 18:27:37.166 |  $-$11:56:26.273 |  |  00.168$\pm$0.008 |  000.409$\pm$0.021 |  ¡2200 |  | … | … | … | oC | …
61 |  | 18:26:05.577 |  $-$12:04:33.534 |  |  00.123$\pm$0.003 |  000.139$\pm$0.003 |  ¡4300 |  | … | … | … | iC/iK |  $<+$2.07
62 |  | 18:25:51.964 |  $-$12:05:13.440 |  | … | … | … |  |  00.203$\pm$0.006 |  000.244$\pm$0.092 |  $<$1100 | iC/iK |  $>+$1.23
63 |  IRAS 18236$-$1205 | 18:26:25.064 |  $-$12:03:48.923 |  |  00.070$\pm$0.004 |  000.099$\pm$0.006 | ¡1100 |  |  00.281$\pm$0.010 |  000.140$\pm$0.010 |  ¡2800 | iC/iK |  $+$0.26$\pm$0.21†
64 |  | 18:26:25.782 |  $-$12:03:53.203 |  |  00.331$\pm$0.013 |  000.383$\pm$0.015 | ¡4300 |  |  00.182$\pm$0.006 |  000.280$\pm$0.010 |  ¡0930 | iC/iK |  $-$0.24$\pm$0.12†
65 |  | 18:26:26.380 |  $-$12:04:19.782 |  |  00.558$\pm$0.004 |  000.570$\pm$0.004 | ¡4000 |  |  01.774$\pm$0.004 |  001.960$\pm$0.006 |  ¡0310 | iC/iK |  $+$0.94$\pm$0.02†
66 |  | 18:26:35.471 |  $-$12:01:13.807 |  |  00.125$\pm$0.002 |  000.113$\pm$0.002 | ¡3800 |  | … | … | … | iC/oK | …
67 |  | 18:26:22.554 |  $-$12:05:58.888 |  |  00.224$\pm$0.035 |  000.708$\pm$0.060 | ¡7400 |  | … | … | … | iC/oK | …
68 |  | 18:26:22.308 |  $-$12:05:58.065 |  |  00.135$\pm$0.005 |  000.270$\pm$0.020 | ¡6000 |  | … | … | … | iC/oK | …
69 |  | 18:26:22.051 |  $-$12:07:28.996 |  |  00.092$\pm$0.003 |  000.449$\pm$0.034 | ¡9700 |  | … | … | … | iC/oK | …
70 |  | 18:26:21.849 |  $-$12:07:31.842 |  |  00.070$\pm$0.001 |  000.143$\pm$0.019 | ¡4300 |  | … | … | … | iC/oK | …
71 |  | 18:26:21:659 |  $-$12:07:35.409 |  |  00.198$\pm$0.001 |  000.394$\pm$0.031 | ¡6500 |  | … | … | … | iC/oK | …
72 |  G23.60$+$0.0M1 | 18:33:53.474 |  $-$08:07:12.198 |  |  00.817$\pm$0.110 |  009.900$\pm$1.300 |  ¡119000 |  | … | … | … | oC | …
73 |  | 18:34:12.339 |  $-$08:19:01.195 |  |  00.191$\pm$0.003 |  000.190$\pm$0.003 | ¡3600 |  |  00.357$\pm$0.042 |  000.480$\pm$0.054 |  $<$1200 | iC/iK |  $+$0.70$\pm$0.26†
74 |  | 18:34:11.574 |  $-$08:19:06.420 |  |  00.310$\pm$0.004 |  000.328$\pm$0.004 | ¡3700 |  |  00.109$\pm$0.045 |  001.040$\pm$0.430 |  ¡1100 | iC/iK |  $+$0.87$\pm$0.94†
75 |  | 18:34:21.063 |  $-$08:18:12.357 |  |  00.420$\pm$0.006 |  000.452$\pm$0.006 |  0¡800 |  | … | … | … | iC/oK | …
76 |  | 18:34:33.022 |  $-$08:15:26.757 |  |  11.325$\pm$0.194 |  012.813$\pm$0.219 |  ¡1200 |  | … | … | … | iC/oK | …
77 |  | 18:34:44.839 |  $-$08:31:05.331 |  |  00.210$\pm$0.003 |  000.272$\pm$0.003 |  ¡1700 |  | … | … | … | oC | …
78 |  | 18:33:44.014 |  $-$08:21:22.952 |  |  01.234$\pm$0.038 |  002.500$\pm$0.077 |  ¡3500 |  | … | … | … | iC/oK | …
79 |  | 18:33:47.803 |  $-$08:23:34.272 |  |  00.763$\pm$0.049 |  001.718$\pm$0.109 |  ¡3900 |  | … | … | … | iC/oK | …
80 |  | 18:34:06.126 |  $-$08:24:38.779 |  |  00.276$\pm$0.012 |  000.336$\pm$0.015 | ¡3900 |  | … | … | … | iC/oK | …
81 |  | 18:34:17.741 |  $-$08:21:08.236 |  |  00.096$\pm$0.004 |  000.119$\pm$0.005 | ¡4000 |  | … | … | … | iC/oK | …
82 |  | 18:34:14.297 |  $-$08:24:10.308 |  |  00.351$\pm$0.010 |  000.390$\pm$0.011 | ¡3800 |  | … | … | … | iC/oK | …
83 |  IRAS 18316$-$0602 | 18:34:20.904 |  $-$05:59:41.968 |  |  02.275$\pm$0.081 |  003.345$\pm$0.120 |  ¡2500 |  |  00.606$\pm$0.062 |  002.621$\pm$0.259 |  ¡2000 | iC/iK |  $-$0.19$\pm$0.24†
84 |  | 18:34:33.930 |  $-$06:02:21.993 |  |  01.618$\pm$0.025 |  001.863$\pm$0.029 |  ¡1200 |  | … | … | … | iC/oK | …
85 |  | 18:34:27.632 |  $-$06:05:09.136 |  |  02.103$\pm$0.026 |  002.493$\pm$0.030 |  ¡1300 |  | … | … | … | iC/oK | …
86 |  | 18:34:14.921 |  $-$06:00:23.582 |  |  00.313$\pm$0.005 |  000.354$\pm$0.006 | ¡4400 |  | … | … | … | iC/iK |  $<+$1.43
87 |  | 18:34:26.746 |  $-$05:57:21.183 |  |  00.907$\pm$0.016 |  000.998$\pm$0.019 | ¡4400 |  | … | … | … | iC/oK | …
88 |  | 18:34:16.533 |  $-$05:45:48.502 |  |  00.109$\pm$0.006 |  000.742$\pm$0.045 |  ¡9900 |  | … | … | … | oC | …
89 |  | 18:32:42.034 |  $-$06:10:19.838 |  |  00.146$\pm$0.011 |  000.301$\pm$0.022 |  ¡3700 |  | … | … | … | oC | …
90 |  | 18:34:08.906 |  $-$05:52:55.202 |  |  01.454$\pm$0.064 |  001.658$\pm$0.073 | ¡4500 |  | … | … | … | iC/oK | …
91 |  | 18:34:32.327 |  $-$06:00:15.075 |  |  00.130$\pm$0.007 |  000.147$\pm$0.008 | ¡4400 |  | … | … | … | iC/oK | …
92 |  | 18:34:13.821 |  $-$05:53:01.189 |  |  00.405$\pm$0.013 |  000.971$\pm$0.031 |  ¡3400 |  | … | … | … | iC/oK | …
93 |  | 18:35:10.900 |  $-$06:02:32.533 |  |  00.067$\pm$0.001 |  000.136$\pm$0.008 |  ¡4000 |  | … | … | … | oC | …
94 |  | 18:34:17.698 |  $-$06:05:05.398 |  | … | … | … |  |  00.222$\pm$0.006 |  000.311$\pm$0.009 |  ¡2100 | iC/oK | …
95 |  | 18:34:20.819 |  $-$05:59:42.997 |  | … | … | … |  |  00.227$\pm$0.037 |  000.251$\pm$0.069 |  $<$1300 | iC/iK |  $>+$1.36
96 |  G24.08$+$0.0M2 | 18:34:48.710 |  $-$07:46:41.510 |  |  16.982$\pm$0.338 |  021.761$\pm$0.433 |  ¡1600 |  |  04.098$\pm$0.166 |  005.300$\pm$0.212 |  $<$1100 | iC/iK |  $-$1.08$\pm$0.10†
97 |  | 18:34:41.391 |  $-$07:43:55.450 |  |  07.986$\pm$0.172 |  010.670$\pm$0.239 |  ¡1600 |  | … | … | … | iC/oK | …
98 |  | 18:34:41.452 |  $-$07:43:47.693 |  |  01.411$\pm$0.017 |  002.558$\pm$0.032 |  ¡3000 |  | … | … | … | iC/oK | …
99 |  | 18:34:57.182 |  $-$07:43:26.184 |  |  03.122$\pm$0.052 |  004.364$\pm$0.073 |  ¡2000 |  | … | … | … | iC/oK | …
100 |  | 18:34:59.593 |  $-$07:43:00.526 |  |  03.762$\pm$0.104 |  007.750$\pm$0.217 |  ¡3500 |  | … | … | … | iC/oK | …
101 |  | 18:34:59.600 |  $-$07:42:57.062 |  |  01.208$\pm$0.021 |  002.517$\pm$0.089 |  ¡2800 |  | … | … | … | iC/oK | …
102 |  | 18:34:57.132 |  $-$07:45:22.485 |  |  01.141$\pm$0.014 |  001.454$\pm$0.017 |  ¡1700 |  | … | … | … | iC/iK |  $<-$0.06
103 |  | 18:34:12.138 |  $-$07:52:54.065 |  |  00.136$\pm$0.004 |  000.202$\pm$0.005 |  ¡2400 |  | … | … | … | oC | …
104 |  | 18:34:11.271 |  $-$07:53:07.879 |  |  00.088$\pm$0.004 |  000.146$\pm$0.006 |  ¡2800 |  | … | … | … | oC | …
105 |  | 18:33:59.500 |  $-$07:52:36.635 |  |  00.304$\pm$0.005 |  000.608$\pm$0.056 | ¡7600 |  | … | … | … | oC | …
106 |  | 18:34:25.403 |  $-$07:54:46.135 |  |  00.239$\pm$0.074 |  059.000$\pm$18.00 |  ¡557000 |  | … | … | … | oC | …
107 |  | 18:34:51.288 |  $-$07:42:14.421 |  |  00.094$\pm$0.005 |  000.195$\pm$0.033 | ¡2900 |  | … | … | … | iC/oK | …
108 |  | 18:35:23.922 |  $-$07:37:38.200 |  |  01.510$\pm$0.150 |  002.080$\pm$0.020 | ¡3500 |  | … | … | … | oC | …
109 |  | 18:34:52.966 |  $-$07:47:03.052 |  |  00.041$\pm$0.003 |  000.082$\pm$0.032 | ¡2300 |  | … | … | … | iC/iK |  $<+$2.33