# THE POPULATION OF COMPACT RADIO SOURCES IN M 17

Vanessa Yanza11affiliation: Departamento de Astronomía, Universidad de
Guanajuato, Apdo. Postal 144, 36000 Guanajuato, México 22affiliation:
Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de
México, Morelia 58089, México , Josep M. Masqué11affiliation: Departamento de
Astronomía, Universidad de Guanajuato, Apdo. Postal 144, 36000 Guanajuato,
México , Sergio A. Dzib33affiliation: Max Planck Institut für Radioastronomie,
Auf dem Hügel 69, D-53121 Bonn, Germany ,Luis F. Rodríguez22affiliation:
Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de
México, Morelia 58089, México , S.-N.X. Medina33affiliation: Max Planck
Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany , Stan
Kurtz22affiliation: Instituto de Radioastronomía y Astrofísica, Universidad
Nacional Autónoma de México, Morelia 58089, México , Laurent
Loinard22affiliation: Instituto de Radioastronomía y Astrofísica, Universidad
Nacional Autónoma de México, Morelia 58089, México , Miguel A.
Trinidad11affiliation: Departamento de Astronomía, Universidad de Guanajuato,
Apdo. Postal 144, 36000 Guanajuato, México , Karl M. Menten33affiliation: Max
Planck Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany ,
Carlos A. Rodríguez-Rico11affiliation: Departamento de Astronomía, Universidad
de Guanajuato, Apdo. Postal 144, 36000 Guanajuato, México

###### Abstract

We present a catalog of radio sources of the M 17 region based on deep X band
radio observations centered at 10 GHz obtained with the Jansky Very Large
Array in the A configuration. We detect a total of 194 radio sources, 12 of
them extended and 182 compact. We find that a significant fraction (at least
40% in our catalog) have suspected gyrosynchrotron emission associated with
stellar coronal emission. By comparing the radio luminosities of our sources
with their X ray counterparts, when available, we find that they are
underluminous in X rays with respect to the Güdel Benz relation, but a
correlation with a similar slope is obtained provided that only sources with
evident non thermal nature are selected from the sample compiled for the Orion
Nebula Cluster (ONC) and M 17. The comparison of M 17 with the ONC and NGC
6334D-F leads to a similar luminosity function for the three regions, at least
for the more luminous sources. However, the radio sources in M 17 are three
times more numerous compared to the other regions at a given luminosity and
their spatial distribution differs from that of Orion. Moreover, an arc-shaped
structure of 40$\arcsec$ in extent is observed in our map, identified
previously as an ionizing front, with the cometary Hyper Compact source UC1 at
its focus. Archival 1 mm ALMA data reveals compact emission coincident with
the radio wavelength peak, possibly associated with a protostellar disk of the
massive star exciting UC1.

Radio Source Catalog - Star Forming Regions - Young Stellar Objects (YSO)

##  1 Introduction

The Messier 17 (M 17) region (the Omega Nebula, Horseshoe Nebula, Swan Nebula,
Checkmark Nebula, W 38, S 45 or NGC 6618) is a bright HII region associated
with a giant molecular cloud located in the Sagittarius constellation at a
distance of $1.98_{-0.12}^{+0.14}$ kpc (Wu et al., 2014). The complex is
considered one of the most massive and luminous regions of recent star
formation in the Milky Way (Lada et al., 1974). It is composed of a main star-
forming region located between two bars situated to the North (N-bar) and to
the South (S-bar, Felli et al. 1980). This region has an associated open
stellar cluster (Beetz et al., 1976; Morales et al., 2013) whose most luminous
object is the massive multiple system CEN 1 (Chini et al., 1980). The main
components of this system, CEN 1a and CEN 1b (Hoffmeister et al., 2008) with
nearly equal masses, have been detected at several wavelengths and found to be
time variable in some of them (Broos et al., 2007; Rodríguez et al., 2009).

The cluster ionizes the surrounding interstellar medium (ISM) so that a large
arc-shaped structure is formed (Felli et al., 1980; Johnson et al., 1998) that
is associated with an ionization front. Beyond this limit the material of the
cloud is mostly molecular (Felli et al., 1980, 1984). At the focus of the arc
structure there is a very radio-bright Hyper-Compact HII (HCHII) region called
UC1 (Felli et al., 1980). Many studies dedicated to this source show that it
is associated with a disk-like structure and masers (Johnson et al., 1998;
Nielbock et al., 2007) among other features. Given the peculiar location of
UC1 with respect to the ionization front, Felli et al. (1984) and Johnson et
al. (1998) discuss the possibility of shock-induced star formation occurring
in this source.

The large luminosity of M 17 ($\sim 10^{7}~{}L_{\odot}$) and its extended size
(up to 1° considering the molecular environment, see Elmegreen et al., 1977),
together with the wealth of structures found within, motivated many surveys of
the region at multiple wavelengths. At IR and optical bands, following the
early survey of Chini et al. (1980), the population of massive and
intermediate mass young objects came to light in the subsequent surveys (Chini
& Kruegel, 1985; Chini & Wargau, 1998; Hanson et al., 1997; Nielbock et al.,
2007). Moreover, deeper observations show that beyond the central cluster of
massive stars, additional generations of young stellar objects populating
other parts of the complex are present (Jiang et al., 2002; Chini et al.,
2004; Povich et al., 2009). This stellar richness was confirmed by Broos et
al. (2007) with X-ray observations, which detected 886 sources around M 17
covering a wide field. At radio wavelengths, Rodríguez et al. (2012, hereafter
RGM2012) detected 38 compact radio sources, 19 of them with stellar
counterparts. These 38 sources constitute the most extensive catalog of radio
sources in M 17 to date.

Because star forming regions are embedded in dense molecular clouds, radio
emission counterparts provide information on the most obscured objects
associated to the region. Furthermore, since the whole region is a strong
radio emitter due to the extended gas ionized by massive stars,
interferometric observations are crucial to isolate and explore the most
compact parts. Such observations reveal the presence of a rich variety of
types of radio sources, some of them individual and compact, suggesting that
they are associated with young objects embedded in the parental nebula; many
of them are not detected at any other wavelength.

The radio emission of these Compact Radio Sources (CRSs) can be thermal and/or
non-thermal. The former is produced by free-free emission from ionized gas
present in Hyper-Compact (HC)HII regions surrounding OB stars, ionized winds
of massive stars, externally ionized protoplanetary disks (proplyds) or jets
from low-mass protostars (Panagia & Felli, 1975; Shull, 1980; Avalos et al.,
2009; Sánchez-Monge et al., 2011; Zapata et al., 2004; Stecklum et al., 1998;
Reynolds, 1986; Anglada et al., 2018). On the other hand, non-thermal radio
emission is generated by relativistic charged particles moving in a magnetic
field. This emission can be produced by gyrosynchrotron processes occurring in
low-mass stars with superficial magnetic fields, or synchrotron processes due
to the collision between winds of massive binary stars, among other
possibilities (Feigelson & Montmerle, 1999; Dzib et al., 2015; Dougherty &
Williams, 2000; Blomme et al., 2013).

The extended family of possible radio emitting objects described above implies
a wide range of radio flux density expected for them. As a consequence, weak
sources found at the lower end of this range become potentially undetectable
and surveys might be biased to strong sources (Medina et al., 2019; Brunthaler
et al., 2021). This has been alleviated in recent years thanks to the upgraded
instrumentation installed on radio interferometers. This enables deep studies
of star forming regions, that have notably increased the number of detected
CRSs. Using one of the most important instruments, the Very Large Array (VLA),
several studies were conducted of different HII regions and very faint CRSs
were detected. For example, early surveys of the Orion Nebula reported $\sim
20$ radio sources of a few mJy of flux density (Churchwell et al., 1987; Garay
et al., 1987). Zapata et al. (2004) later increased the cataloged sources to
$77$ and found that a significant fraction of CRSs show time variability.
Recently, Forbrich et al. (2016) presented the deepest observations ever
performed on Orion using the same instrument, upgraded to the Karl G. Jasky
Very Large Array and detected $556$ radio sources of differing nature. The
Nebula NGC 6334, a string of star forming regions of different evolutionary
status, including several HII regions, was studied recently by Medina et al.
(2018) who cataloged $83$ CRSs; only a few were previously known. Despite the
larger distances of other regions as compared to Orion, in general,
sufficiently deep observations of star forming regions show a significant
increase in radio source detections compared to the old catalogs. This
suggests that a rich population of radio sources exists in many regions of
star formation; in most cases, this rich population remains to be uncovered.

Following previous radio observations of the M 17 region (RGM2012), we carried
out a study toward this region with the upgraded VLA. We achieve a
significantly better sensitivity in our maps than in previous observations.
This allows us to very significantly expand the radio source catalog of the M
17 region by exploring the weakest radio components yet detected in the
region. Furthermore, we analyze the nature of the population of radio sources
using methods developed in other regions. In Section 2, we describe the
observations and present the final map after the self-calibration process. In
Section 3, we present the catalog of CRSs extracted from the final map. An
analysis of the properties of CRSs extracted from the map is presented in
Section 4. In Section 5, we constrain the nature of the CRSs and discuss their
implications for the region. Finally, in Section 6 we draw our conclusions.

##  2 Observations

The VLA data were taken using the most extended A configuration in the X band
(8-12 GHz) toward the M 17 star forming region. The pointing center was
$\alpha$(J2000) = 18h20m30s and $\delta$(J2000) =
$-16^{\circ}10^{\prime}45^{\prime\prime}$ and the Full Width Half Maximum
(FWHM) of the primary beam at the center of the band was 5′ (though it varies
significantly over the 4 GHz bandwidth). The observations were made in three
epochs with a total observation time of four hours (one hour on 2018 May 3,
one hour on May 4 and two hours on May 8). The data were taken using the 3-bit
sampling mode with an integration time set to 2 seconds. The correlator was
configured to 4 GHz of continuous bandwidth coverage in full polarization
mode, divided into 32 contiguous spectral windows, each with 64 channels of 2
MHz. The quasar 3C 286 was used as flux reference and bandpass calibrator, and
J1832-1032 was used for the gain calibration. The theoretical noise of the
combined observations is $2.6~{}\mu$Jy beam-1 and the expected synthesized
beam is $\sim 0\hbox to0.0pt{.\hss}^{\prime\prime}2$.

The data were edited, calibrated and imaged using the Common Astronomy
Software Applications package, CASA (McMullin et al., 2007). An initial
calibration and basic flagging were done following standard procedures. We
combined the data of the three epochs into a single initial map using a
weighting of robust$=-0.5$. This provided a map with a beam size of $\sim
0\hbox to0.0pt{.\hss}^{\prime\prime}2$ and an rms noise of $27~{}\mu{\rm
Jy~{}beam}^{-1}$. As this noise level was still far from the expected
theoretical noise, we self-calibrated the combined data using the brightest
source in the region, the UC1 source, assuming that this source does not vary
significantly over the three observing days (RGM2012). In this process we
applied two calibrations in phase, followed by one in amplitude and, finally,
one more in phase. This improved the noise level of the map to $5~{}\mu$Jy
beam-1. This final map, which was primary beam-corrected, is shown in Fig. 1.
Although the different primary beam function between the 9 and 11 GHz maps
must bias the source detection toward sources with negative spectral indices,
this effect is specially notable toward the outskirts of the map. Therefore,
we adopted the FWHM of the primary beam at 9 GHz as the limiting size of the
region where the spectral indices are trustable, which includes the majority
of our sources. Moreover, most sources are gathered around the map center
(i.e. in the HII region) where this effect is probably not significant.

Figure 1: _Top panel:_ VLA radio continuum map of the M 17 region (color
scale) using the A configuration and X band ($8-12$ GHz). The noise at its
center is $5~{}\mu$Jy beam-1. The synthesized beam is $0\hbox
to0.0pt{.\hss}^{\prime\prime}285\times 0\hbox
to0.0pt{.\hss}^{\prime\prime}152$, P.A. = $-28\hbox to0.0pt{.\hss}^{\circ}77$.
White circles represent the position of the compact radio sources detected.
Orange squares reveal the position of the peak of the extended sources. The
magenta circle represents the position of the brightest source, UC1. The
increased noise at the outer edge of the image is due to the correction for
the primary beam response. There is also increased noise in the surroundings
of UC1. The dashed red box represents the area shown enlarged in the bottom
panel. The full FITS image is available upon request to<EMAIL_ADDRESS>_Bottom panel:_ Zoom-in image of the central area of the observed field.
Symbols are the same as in the top panel. The beam is the same as in the top
panel and shown in the bottom left corner.

##  3 Results

###  3.1 Source Extraction

We extracted sources from the final map using the BLOBCAT software (Hales et
al., 2012). This software has been successfully used to do source extractions
from radio images of other star forming regions (e.g., Medina et al., 2018)
and from radio surveys covering significantly larger areas of the sky (e.g.,
Bihr et al., 2016; Medina et al., 2019). We follow a similar method to those
used in these previous works which we briefly describe below.

BLOBCAT extracts information of islands of pixels using the flood fill
algorithm in 2D astronomical images. It applies bias corrections to Gaussian
and non-Gaussian sources to obtain accurate measurements. This allows us to
perform an automatic extraction of radio sources and determine some of their
observational properties. To correctly run the extraction, the Graphical
Astronomy and Imaging Analysis Tool (GAIA, based on SEXtractor; Bertin &
Arnouts, 1996; Holwerda, 2005) software was used to generate an $rms$ noise 2D
map of the data. To obtain the background noise map, the chosen Photometry
type and Checkimage were “Background RMS” and “Full res. noise background”,
respectively with a minimum island size of 5 pixels and a mesh size of 70 (see
Hales et al., 2012). This noise map was then used with a signal to noise ratio
of 5 (i.e., setting the $dSNR$ parameter of BLOBCAT to $5\sigma$) for the
processing over our M 17 map. With these parameters BLOBCAT extracts 200
preliminary radio sources.

###  3.2 Extragalactic sources and false detections

The expected number of background extragalactic sources in the imaged area was
obtained using the equation below taken from Anglada et al. (1998). In this
expression, $\theta_{F}$ is the angular diameter of the observed field, $\nu$
is the central frequency and $S_{o}$ is the minimum detected flux density at
the field center. These values were set to 7.2′, 10 GHz, and 0.025 mJy,
respectively.

$\footnotesize\centering\begin{split}\langle
N\rangle~{}=1.4~{}\left\\{1-exp\left[-0.0066\left(\frac{\theta_{F}}{arcmin}\right)^{2}\left(\frac{\nu}{5~{}GHz}\right)^{2}\right]\right\\}\\\
\left(\frac{S_{o}}{mJy}\right)^{-0.75}\left(\frac{\nu}{5~{}GHz}\right)^{-2.52}\end{split}\@add@centering$
(1)

This equation gives the expected number of extragalactic sources of $\langle
N\rangle=3\pm 2$. In addition, using the statistical equation 1 from Medina et
al. (2018), the maximum expected number of false detections is $5$. This
suggests that a maximum of about 10 sources from the BLOBCAT outcome are
artifacts or extragalactic sources. In consequence, almost all of the sources
we catalog are real and potentially belong to the M 17 region.

###  3.3 Final catalog

We visually inspected the BLOBCAT detections, defining them as compact or
extended sources. We classify as extended sources those that have an irregular
resolved morphology, and as compact those with an approximately Gaussian shape
and a single well-defined peak. Typical sizes for compact radio sources (CRSs)
are not larger than a fraction of an arcsec. In some cases BLOBCAT detected
several contiguous sources that are part of a single large structure.
Specifically, 4 extended sources were comprised of 9 sources extracted by
BLOBCAT. For these extended sources we re-measured the flux density using CASA
tools, considering each one as a single source. Moreover, we rejected 2 likely
spurious sources by v̱isual inspection, e.g. sources at the edge of the map
where the rms noise level is high enough to create unreal sources. As a
result, we retained 12 extended sources and 182 compact radio sources. These
results are summarized in Table 1.

Table 1: Summary of the source selection process to obtain the final catalog. Description | Number of sources
---|---
\- Total of sources extracted with BLOBCAT | 200
Rejected BLOBCAT sourcesaaWe interpreted these sources as parts of extended ones that are counted in the 4th row (see text). | 9
Extended sources | 8
Manually added extended sources | 4
Rejected BLOBCAT CRSs | 2
Manually added CRSs | 1
Number of total extended sources | 12
Number of CRSs in the final catalog | 182

Table 2 shows the properties of the extended sources, namely their position
(centered in the source emitting area), the peak flux, integrated flux density
and angular area of each source measured by IMSTAT from CASA using contours of
$5\sigma$. Sources 6 and 10 have been detected before by RGM2012 with their C
band observations and are labeled 11 and 18 in their catalog, respectively.
Their reported sizes are $0\hbox to0.0pt{.\hss}^{\prime\prime}4$ and $0\hbox
to0.0pt{.\hss}^{\prime\prime}6$. Our measured sizes for these sources are
approximately $1\hbox to0.0pt{.\hss}^{\prime\prime}3\times 0\hbox
to0.0pt{.\hss}^{\prime\prime}4$ and $3\hbox
to0.0pt{.\hss}^{\prime\prime}7\times 2\hbox to0.0pt{.\hss}^{\prime\prime}2$.
This difference is likely due to the different sensitivity between the
observations.

Table 2: Extended sources in the M 17 region from the final map. ID |  $\alpha$[${}^{\rm h}\,{}^{\rm m}\,{}^{\rm s}$] |  $\delta$[${}^{\circ}\,{}^{\prime}\,{}^{\prime\prime}$] | $S_{\rm peak}$ | $S_{\rm int}$ | Area
---|---|---|---|---|---
# | (J2000) | (J2000) | ($\mu{\rm Jy~{}beam}^{-1}$) | ($\mu$Jy) | (arcsec2)
1 | 18 20 24.931 | –16 11 28.29 | 560 $\pm$ 30 | 101425 $\pm$ 5433 | 40.3
2 | 18 20 29.950 | –16 12 46.44 | 117 $\pm$ 19 | 12341 $\pm$ 2004 | 20.5
3 | 18 20 26.264 | –16 10 53.71 | 158 $\pm$ 45 | 25439 $\pm$ 7245 | 18.9
4 | 18 20 30.042 | –16 08 40.72 | 76 $\pm$ 15 | 12770 $\pm$ 639 | 6.5
5 | 18 20 30.869 | –16 08 48.88 | 160 $\pm$ 15 | 9770 $\pm$ 489 | 6.1
6 | 18 20 27.588 | –16 08 31.24 | 103 $\pm$ 18 | 8533 $\pm$ 427 | 3.9
7 | 18 20 26.698 | –16 13 14.56 | 244 $\pm$ 25 | 9800 $\pm$ 490 | 3.7
8 | 18 20 29.128 | –16 08 58.76 | 251 $\pm$ 16 | 4837 $\pm$ 242 | 3.4
9 | 18 20 28.387 | –16 11 19.52 | 81 $\pm$ 10 | 2448 $\pm$ 123 | 2.2
10 | 18 20 30.008 | –16 11 01.08 | 57 $\pm$ 7 | 887 $\pm$ 109 | 2.1
11 | 18 20 26.736 | –16 07 21.72 | 309 $\pm$ 43 | 1707 $\pm$ 237 | 0.5
12 | 18 20 26.813 | –16 07 30.76 | 231 $\pm$ 37 | 1903 $\pm$ 304 | 0.4

###  3.4 Counterparts at other wavelengths

We searched for counterparts at other wavelengths cross-matching our CRSs
using the SIMBAD database. The maximum discrepancy tolerance in position
between our CRSs and the X-ray counterparts was $0\hbox
to0.0pt{.\hss}^{\prime\prime}8$. For IR, optical and radio wavelengths it was
$0\hbox to0.0pt{.\hss}^{\prime\prime}75$. These limits include the astrometric
uncertainty our VLA map and our fit, and the approximate position errors in
the counterpart. In Table LABEL:tab:CRS (see Appendix A) we indicate the
counterparts of our CRSs and their respective detected band.

We searched the Gaia catalog for counterparts to the radio sources identified
in our survey. Table 3 lists the sources with Gaia counterparts, their
parallax and distances. The errors on the distances were calculated using
standard error propagation. We find that six of the sources have distances
discrepant from that of M 17 (d = 1.98 kpc) by more than two sigmas. These are
sources with number 8, 27, 40, 110, 128 and 170. These sources are likely to
be foreground objects, rather than sources associated with M 17 itself. In the
following, we exclude these sources our analysis.

Table 3: Distances of the Gaia counterparts of the CRSs ID | Parallax | Distance
---|---|---
# | (mas) | (kpc)
6 | 0.57 $\pm$ 0.32 | 1.75 $\pm$ 0.98
8 | 1.01 $\pm$ 0.19 | 0.99 $\pm$ 0.19
27 | 0.92 $\pm$ 0.21 | 1.09 $\pm$ 0.25
29 | 0.63 $\pm$ 0.10 | 1.59 $\pm$ 0.25
40 | 0.72 $\pm$ 0.12 | 1.39 $\pm$ 0.23
46 | 0.70 $\pm$ 0.24 | 1.43 $\pm$ 0.49
59 | 0.61 $\pm$ 0.16 | 1.64 $\pm$ 0.43
110 | 0.77 $\pm$ 0.155 | 1.30 $\pm$ 0.26
124 | 0.96 $\pm$ 0.55 | 1.04 $\pm$ 0.60
128 | 0.90 $\pm$ 0.15 | 1.11 $\pm$ 0.19
163 | 0.52 $\pm$ 0.11 | 1.92 $\pm$ 0.40
170 | 0.84 $\pm$ 0.20 | 1.19 $\pm$ 0.28

##  4 Analysis

In order to characterize the nature of the CRSs, we explored basic physical
properties, namely spectral index, variability and polarization. The spectral
index is a valuable preliminary diagnostic to assess whether the radio
emission from a given source is of thermal or non thermal nature. Spectral
indices below –0.1 indicate undoubtedly non-thermal nature (Rodríguez et al.,
1993). On the other hand, spectral indices above this limit are expected for
thermal sources with the caveat that gyrosynchrotron emission can result in
spectral indices up to 2.5. This latter emission implies non-thermal processes
occurring usually on the stellar surfaces of young objects (e.g., magnetic
reconnection events, Dulk 1985; Feigelson & Montmerle 1999). Besides,
extragalactic sources are usually optically-thin emitters and their spectral
indices can be mimicked by gyrosynchrotron sources (e.g., Fleishman &
Melnikov, 2003; Forbrich et al., 2016; Wang et al., 2018). However, as seen in
the previous section, the number of extragalactic sources in our catalog of
CRS is not statistically significant. As we discuss in Sect. 5, two important
properties are associated with gyrosynchrotron emission: rapid variability and
circular polarization.

###  4.1 Spectral index

We estimated spectral indices of our CRSs dividing our data into two frequency
intervals of 2 GHz and imaged each one in independent maps of 16 spectral
windows. The maps have central frequencies of 9 GHz and 11 GHz, and noise
levels of $11\,\mu$Jy beam-1 and $9\,\mu$Jy beam-1, respectively. Using
BLOBCAT again, we determined the flux density of the CRSs belonging to each
map and calculated their spectral indices. Most of the spectral indices were
not well constrained due to the narrow frequency separation, causing large
errors. Hence, in Table 4 we list the sources whose spectral indices have
errors $\leq 0.5$, as well as their flux density at 9 and 11 GHz. We note that
negative spectral indices are dominant. Given that most sources are found at
the center of the map (see Fig. 1), this can not be an effect of the primary
beam difference between the 9 and 11 GHz maps.

Table 4: Spectral indices of the CRSsa

ID | $S_{9GHz}$ | $S_{11GHz}$ | Spectral
---|---|---|---
# | $(\mu Jy)$ | $(\mu Jy)$ | Index
22 | 92486 $\pm$ 4626 | 111808 $\pm$ 5591 | 0.9 $\pm$ 0.3
30 | 867 $\pm$ 48 | 573 $\pm$ 31 | –2.1 $\pm$ 0.4
31 | 405 $\pm$ 29 | 499 $\pm$ 27 | 1.0 $\pm$ 0.5
32 | 949 $\pm$ 55 | 791 $\pm$ 44 | –0.9 $\pm$ 0.4
38 | 291 $\pm$ 21 | 222 $\pm$ 14 | –1.4 $\pm$ 0.5
43 | 1526 $\pm$ 80 | 1814 $\pm$ 93 | 0.9 $\pm$ 0.4
47 | 254 $\pm$ 18 | 183 $\pm$ 11 | –1.6 $\pm$ 0.5
69 | 656 $\pm$ 34 | 648 $\pm$ 33 | –0.1 $\pm$ 0.4
79 | 885 $\pm$ 45 | 841 $\pm$ 43 | –0.3 $\pm$ 0.4
84 | 250 $\pm$ 17 | 191 $\pm$ 12 | –1.3 $\pm$ 0.5
86 | 519 $\pm$ 30 | 623 $\pm$ 34 | 0.9 $\pm$ 0.4
94 | 632 $\pm$ 33 | 581 $\pm$ 30 | –0.4 $\pm$ 0.3
99 | 439 $\pm$ 24 | 395 $\pm$ 21 | –0.5 $\pm$ 0.4
105 | 241 $\pm$ 16 | 186 $\pm$ 11 | –1.3 $\pm$ 0.4
108 | 304 $\pm$ 19 | 208 $\pm$ 12 | –1.9 $\pm$ 0.4
110 | 194 $\pm$ 14 | 138 $\pm$ 9 | –1.7 $\pm$ 0.5
115 | 691 $\pm$ 36 | 701 $\pm$ 36 | 0.1 $\pm$ 0.4
120 | 785 $\pm$ 41 | 770 $\pm$ 39 | –0.1 $\pm$ 0.4
151 | 692 $\pm$ 38 | 610 $\pm$ 33 | –0.6 $\pm$ 0.4
152 | 893 $\pm$ 46 | 564 $\pm$ 29 | –2.3 $\pm$ 0.4
159 | 369 $\pm$ 21 | 317 $\pm$ 17 | –0.8 $\pm$ 0.4
161 | 164 $\pm$ 12 | 160 $\pm$ 10 | –0.1 $\pm$ 0.5

aWe only report sources with spectral index errors smaller than or equal to
0.5.

###  4.2 Rapid time variability

We constructed maps for the data corresponding to different observing days
(May 3, May 4 and May 8) to search for rapid variability in the CRSs. We
applied BLOBCAT to each map using a signal to noise ratio of $3~{}\sigma$
because the individual maps are noisier than the map produced from the
complete $uv$-data set.

The remaining faintest CRSs that BLOBCAT did not detect on single-day maps
were measured using the task IMFIT. Then, we compared the fluxes of the CRSs
between all the possible combinations of pairs of days by using the relation
$V=(|S_{\rm day1}-S_{\rm day2}|\times 100)/S_{\rm max}$, where $S_{\rm day1}$
and $S_{\rm day2}$ are the integrated fluxes on different days, and $S_{\rm
max}$ is the larger of the two flux densities. We considered highly variable
sources those with $V\geq 50\%$ (e.g., Dzib et al., 2013a), considering that
the average variability of sources that are not highly variable is $30\%$.
Table 5 shows a total of $72$ CRSs with rapid high variability, around $40\%$
of our catalog. Note that this fraction is a lower limit because some sources
determined as non variable may be highly variable but be caught at an epoch in
which their emission is steady. Assuming that our CRSs are YSOs, this lower
limit is consistent with the result found in other regions (Dzib et al.,
2013a, 2015; Ortiz-León et al., 2015a). The error in V was determined
following standard error propagation theory. In Figure 2 we show an example of
two CRSs close to each other that present rapid variability. The southern
source decreases in luminosity in the lapse of a few days while the northern
source exhibits the opposite behavior.

Table 5: Rapid highly variable sources

ID | $May~{}3-4$ | $May~{}4-8$ | $May~{}3-8$ | ID | $May~{}3-4$ | $May~{}4-8$ | $May~{}3-8$
---|---|---|---|---|---|---|---
# | % | % | % | # | % | % | %
2 | 19 $\pm$ 4 | 55 $\pm$ 12 | 44 $\pm$ 9 | 83 | 13 $\pm$ 2 | 64 $\pm$ 9 | 58 $\pm$ 6
3 | 40 $\pm$ 10 | 75 $\pm$ 15 | 59 $\pm$ 14 | 84 | 41 $\pm$ 4 | 42 $\pm$ 5 | 66 $\pm$ 7
4 | 26 $\pm$ 6 | 59 $\pm$ 11 | 70 $\pm$ 17 | 93 | 0 $\pm$ 0 | 75 $\pm$ 26 | 75 $\pm$ 16
6 | 63 $\pm$ 18 | 54 $\pm$ 17 | 19 $\pm$ 5 | 96 |  $\geq$ 14 $\pm$ 6 |  $\geq$ 50 $\pm$ 18 | 57 $\pm$ 15
9 |  $\geq$ 75 $\pm$ 26 |  $\geq$ 88 $\pm$ 31 | 52 $\pm$ 8 | 97 |  $\geq$ 54 $\pm$ 24 | 32 $\pm$ 10 |  $\geq$ 68 $\pm$ 25
11 | 54 $\pm$ 9 | 11 $\pm$ 3 | 49 $\pm$ 11 | 101 | 35 $\pm$ 9 |  $\geq$ 64 $\pm$ 25 |  $\geq$ 44 $\pm$ 17
12 | 21 $\pm$ 4 | 41 $\pm$ 9 | 53 $\pm$ 10 | 104 |  $\geq$ 24 $\pm$ 10 |  $\geq$ 38 $\pm$ 14 | 53 $\pm$ 16
14 | 17 $\pm$ 2 | 60 $\pm$ 11 | 67 $\pm$ 11 | 106 | 64 $\pm$ 23 |  $\geq$ 61 $\pm$ 24 |  $\geq$ 10 $\pm$ 4
15 | 38 $\pm$ 10 |  $\geq$ 62 $\pm$ 25 |  $\geq$ 77 $\pm$ 27 | 111 |  $\geq$ 59 $\pm$ 26 | 34 $\pm$ 11 |  $\geq$ 73 $\pm$ 26
16 |  $\geq$ 90 $\pm$ 32 | 27 $\pm$ 4 |  $\geq$ 93 $\pm$ 32 | 114 |  $\geq$ 17 $\pm$ 7 |  $\geq$ 55 $\pm$ 19 | 62 $\pm$ 16
19 | 52 $\pm$ 14 | 45 $\pm$ 8 | 14 $\pm$ 3 | 117 |  $\geq$ 43 $\pm$ 20 |  $\geq$ 75 $\pm$ 26 |  $\geq$ 86 $\pm$ 29
21 | 8 $\pm$ 2 | 84 $\pm$ 15 | 82 $\pm$ 12 | 123 | 6 $\pm$ 1 | 69 $\pm$ 16 | 67 $\pm$ 10
23 | 30 $\pm$ 5 |  $\geq$ 77 $\pm$ 28 |  $\geq$ 84 $\pm$ 29 | 124 | 9 $\pm$ 3 | 50 $\pm$ 23 | 54 $\pm$ 22
24 | 44 $\pm$ 4 | 66 $\pm$ 6 | 39 $\pm$ 3 | 125 | 7 $\pm$ 2 | 66 $\pm$ 17 | 63 $\pm$ 11
25 | 26 $\pm$ 3 | 74 $\pm$ 15 | 65 $\pm$ 13 | 127 | 3 $\pm$ 1 | 62 $\pm$ 10 | 63 $\pm$ 8
26 | 23 $\pm$ 8 |  $\geq$ 41 $\pm$ 18 |  $\geq$ 55 $\pm$ 21 | 131 | 88 $\pm$ 15 | 65 $\pm$ 7 | 66 $\pm$ 12
27 |  $\geq$ 88 $\pm$ 33 | 39 $\pm$ 10 |  $\geq$ 81 $\pm$ 32 | 136 |  $\geq$ 92 $\pm$ 32 | 69 $\pm$ 11 |  $\geq$ 76 $\pm$ 27
32 | 7 $\pm$ 1 | 50 $\pm$ 4 | 54 $\pm$ 5 | 139 | 35 $\pm$ 17 | 57 $\pm$ 23 | 72 $\pm$ 25
35 | 8 $\pm$ 2 | 77 $\pm$ 21 | 75 $\pm$ 15 | 144 |  $\geq$ 43 $\pm$ 20 |  $\geq$ 16 $\pm$ 6 |  $\geq$ 52 $\pm$ 20
36 | 63 $\pm$ 18 |  $\geq$ 47 $\pm$ 20 |  $\geq$ 81 $\pm$ 28 | 146 | 49 $\pm$ 19 | 17 $\pm$ 5 | 58 $\pm$ 20
40 |  $\geq$ 7 $\pm$ 3 |  $\geq$ 90 $\pm$ 30 | 91 $\pm$ 22 | 152 | 3 $\pm$ 0.2 | 53 $\pm$ 4 | 52 $\pm$ 4
42 | 22 $\pm$ 6 |  $\geq$ 61 $\pm$ 24 |  $\geq$ 50 $\pm$ 18 | 158 |  $\geq$ 17 $\pm$ 7 |  $\geq$ 84 $\pm$ 28 | 86 $\pm$ 23
46 | 37 $\pm$ 13 |  $\geq$ 32 $\pm$ 15 |  $\geq$ 57 $\pm$ 20 | 162 | 17 $\pm$ 7 | 68 $\pm$ 25 | 61 $\pm$ 13
52 | 50 $\pm$ 15 | 24 $\pm$ 6 | 62 $\pm$ 18 | 163 | 66 $\pm$ 13 | 23 $\pm$ 4 | 55 $\pm$ 11
54 | 7 $\pm$ 2 |  $\geq$ 52 $\pm$ 22 |  $\geq$ 55 $\pm$ 20 | 164 | 33 $\pm$ 12 | 69 $\pm$ 20 | 79 $\pm$ 22
56 | 34 $\pm$ 16 | 25 $\pm$ 11 | 51 $\pm$ 18 | 167 | 30 $\pm$ 9 | 76 $\pm$ 19 | 66 $\pm$ 17
57 |  $\geq$ 10 $\pm$ 4 |  $\geq$ 58 $\pm$ 21 | 62 $\pm$ 16 | 170 | 36 $\pm$ 9 | 24 $\pm$ 7 | 52 $\pm$ 10
61 | 27 $\pm$ 13 | 48 $\pm$ 20 | 62 $\pm$ 18 | 171 |  $\geq$ 60 $\pm$ 40 |  $\geq$ 77 $\pm$ 26 | 91 $\pm$ 54
62 | 33 $\pm$ 11 | 37 $\pm$ 11 | 57 $\pm$ 15 | 173 | 0 $\pm$ 0 | 88 $\pm$ 11 | 88 $\pm$ 16
65 | 15 $\pm$ 5 |  $\geq$ 55 $\pm$ 23 |  $\geq$ 46 $\pm$ 17 | 174 | 17 $\pm$ 3 | 52 $\pm$ 10 | 42 $\pm$ 5
66 | 61 $\pm$ 18 | 45 $\pm$ 9 | 78 $\pm$ 20 | 175 |  $\geq$ 47 $\pm$ 16 |  $\geq$ 83 $\pm$ 29 | 68 $\pm$ 10
71 | 52 $\pm$ 8 | 85 $\pm$ 10 | 93 $\pm$ 12 | 177 |  $\geq$ 41 $\pm$ 14 |  $\geq$ 51 $\pm$ 18 | 16 $\pm$ 3
73 | 5 $\pm$ 2 | 65 $\pm$ 18 | 63 $\pm$ 11 | 178 |  $\geq$ 87 $\pm$ 31 | 70 $\pm$ 15 |  $\geq$ 56 $\pm$ 21
78 | 17 $\pm$ 4 |  $\geq$ 61 $\pm$ 24 |  $\geq$ 53 $\pm$ 19 | 179 | 12 $\pm$ 6 | 53 $\pm$ 28 | 59 $\pm$ 13
82 |  $\geq$ 51 $\pm$ 22 | 56 $\pm$ 17 |  $\geq$ 78 $\pm$ 28 | 182 | 53 $\pm$ 23 | 16 $\pm$ 7 | 44 $\pm$ 10

(a) May $3$ (b) May $4$ (c) May $8$

Figure 2: Detailed view of rapid variability on two sources close to each
other. The northern source is the number 82 and the southern source is the
number 84. The color scale is the same in the three panels. In (a) we show the
map on May 3. Contours are $-3,3,10,20$ and $40$ times $8~{}\mu$Jy beam-1. In
(b) we show the map on May 4. Contours are $-3,3,10,15$ and $18$ times
$12~{}\mu$Jy beam-1. In (c) we show the map on May 8. Contours are
$-3,3,10,15$ and $18$ times $12~{}\mu$Jy beam-1.

###  4.3 Polarization

We searched for circular polarization, which is indicative of gyrosynchrotron
emission. To this end we used the CASA task CLEAN to map the Stokes V
parameter. The rms noise was $4~{}\mu$Jy beam-1 in the central region. Stokes
V and I are measures of the circular polarization and total intensity (i.e.
final map of Sect 2), respectively, of the radiation, where the V/I flux
ration gives the degree of circular polarization. The polarization was
considered significant only when the sources are close to the phase center
($<1^{\prime}$; sources more distant from the phase center suffer from the
’beam squint’ effect that produces spurious circular polarization) and the
flux density ratio ($S_{\rm peakV}\times 100/S_{\rm peakI}$) is greater than
$3\%$, where $S_{\rm peakV}$ is the peak flux density from the Stokes V map
and $S_{\rm peakI}$ is the peak flux density obtained in the Stokes I map.
Table 6 shows $S_{\rm peakV}$ and $S_{\rm peakI}$ of the CRSs that fulfill
these criteria and their corresponding flux density ratio. The error in the
flux density ratio was determined following standard error propagation theory.

Table 6: Circularly polarized sources. ID | $S_{peakV}$ | $S_{peakI}$ | Flux density ratio
---|---|---|---
# | ($\mu$Jy bm-1) | ($\mu$Jy bm-1) | %
56 | 17 $\pm$ 3 | 42 $\pm$ 6 | 40 $\pm$ 9
62 | –16 $\pm$ 4 | 34 $\pm$ 6 | 47 $\pm$ 5
95 | 17 $\pm$ 4 | 79 $\pm$ 7 | 21 $\pm$ 5
113 | –14 $\pm$ 5 | 46 $\pm$ 6 | 30 $\pm$ 12
122 | 16 $\pm$ 5 | 61 $\pm$ 6 | 26 $\pm$ 9

##  5 Discussion

The extraordinary large number of radio sources found in M 17 is possibly
related with the hundreds of sources reported in previous surveys at other
wavelengths (e.g., 771 X-ray sources with stellar counterparts and 96
candidate YSOs: Broos et al., 2007; Povich et al., 2009), as suggested by the
counterparts reported in Sect. 3.4. As pointed out by RGM2012, their detected
radio sources probably correspond to the same class of objects detected at
other wavelengths and any lack of counterparts is caused by heavy extinction.
In order to confirm the same trend in our CRSs, in Sect. 4 we explored some
source properties, from which we attempt to unveil their emission mechanism.
These properties will be discussed in this section and compared, when
possible, to what is found for other regions.

In Fig. 3 we plot the spectral index vs. flux density for our brighter CRSs.
Other than the predominance of negative spectral indices highlighted in the
previous sections, there are no significant trends in the diagram.

Given the deeper detection level achieved by Forbrich et al. (2016) in Orion,
a significant trend was revealed, where the brighter sources in Orion (at
least a few mJy) have positive spectral indices, while the sources with
negative spectral indices are significantly weaker. This behavior is a
consequence of the smaller emitting region of non-thermal emission in YSOs,
usually limited to the stellar corona. At the same time, this implies that
non-thermal emission associated to stellar objects usually appears point-like.
While the presence of thermal sources of a few mJy in Orion is consistent with
the 1.7 mJy source in M 17 seen in the diagram (scaled by distance), our
detection limit prevents us from searching for the aforementioned trend for
fainter sources. However, while the highly variable CRSs corresponds to only
10% of the CRSs plotted in Figure 3, Table 6 shows that the fraction of highly
variable CRSs represents 40% of our catalog, indicating that most of the
highly variable CRSs are faint objects. Although variablity is likely
associated to non-thermal emission, additional factors are ususally required.
If we assume that the trend of Forbrich et al. (2016) also applies to the
sources of M 17, then variability could be tied to non-thermal emission, at
least for a subtantial part of our CRSs (most of which do not appear in the
graph).

Figure 3: Spectral index against integrated flux density at 10 GHz of the
brightest CRSs. Squares are the highly variable sources. Triangles are the
sources with polarization. The filled symbols are those with X-ray counterpart
and the unfilled are the sources without an X-ray counterpart. The UC1 source
is not included in the plot. The dashed line marks a spectral index of $-0.1$.

Gyrosynchrotron radiation from stellar coronae is a common type of point-like,
non-thermal emission found in regions of young stellar objects. In the case of
gyrosynchrotron emission associated with low-mass stars, electrons accelerated
during magnetic reconnection events are responsible both for the radio
emission when they propagate in the stellar magnetic field, and the X-ray
emission through their heating of the coronal gas (Benz & Güdel, 2010). This
type of emission can present variability and also shows a significant degree
of cicular polarization. Therefore, from the total catalog of CRSs, we
identify polarized and variable sources as likely gyrosynchrotron candidates
(e.g., Dulk, 1985; Güdel et al., 2002; Dzib et al., 2013a, 2015; Ortiz-León et
al., 2015a, hereafter referred to as Gy-CRS). We consider polarization and
variability as complementary since some CRSs might be variable but present
flux density stability during the short periods covered by our observations.

The rest of the CRSs cannot be uniquely categorized and we leave their nature
as undetermined (hereafter referred to as Und-CRS). A small fraction of Und-
CRS might be thermal emitters and they could be small trapped HCHII regions or
proplyds. Moreover, some Und-CRS have IR counterparts that could indicate the
presence of heated dust. The typical fluxes detected by us ($<1$ mJy) are
similar to those of proplyds detected in Orion (Zapata et al., 2004) scaled to
the M 17 distance and, thus, we favor the proplyd scenario. Other Und-CRS
could be non-thermal emitters that present low or no variability (e.g.,
collision wind region between massive stars where electrons are highly
accelerated). Observations at higher frequencies with higher angular
resolution are needed to explore the nature of Und-CRS.

###  5.1 The X-ray–Radio connection

As we mentioned earlier, it is thought that the X-ray and radio emissions in
stellar coronas are both caused by electrons accelerated during magnetic
reconnection events. The electrons accelerated during these events generate
the radio emission via the gyro-synchrotron mechanism, but they also heat the
corona to temperatures sufficient to produce thermal bremsstrahlung X-ray
emission (Dulk, 1985; Feigelson & Montmerle, 1999; Güdel et al., 2002). This
correlation between radio and X-ray emission was first investigated
empirically by Güdel & Benz (1993) and Benz & Güdel (1994) for a population of
coronally active stars and this relation was found:

$\frac{L_{X}}{L_{R}}=\kappa 10^{15.5\pm 0.5}$ (2)

with $\kappa\leq 1$ accounting for different types of stars. For instance,
several works show that $\kappa$ is significantly lower than 1 for YSOs (Gagné
et al., 2004; Dzib et al., 2013a, 2015).

From our catalog, 24 of 182 CRSs have X-ray counterparts with measured
luminosities reported in Broos et al. (2007), and some of them are absorption
corrected. They followed a similar methodology as that in Feigelson et al.
(2002): wide band fluxes were obtained and converted to total-band
luminosities (0.5-8 kev) by fitting a thermal plasma model to the data under
the maximum likelihood method. The fit also gives the absorbing column of
interstellar material and, thus, corrected luminosities by absorption can also
be obtained. When the fit yielded unrealistic physical parameters, Broos et
al. employed a power-law model to fit the data. Besides, some corrected
luminosities measured only the hard X ray band (2-8 kev) and hence are
meaningless because they correspond to highly obscured sources. Thus, the
absorption-corrected luminosities are omitted for these sources and only their
observed luminosity are reported. The X-ray luminosities and our derived radio
luminosities for CRSs in M 17 are shown in Figure 4. In this plot, we excluded
those Gaia counterparts not belonging to the region (i.e., foreground
sources). A linear least squares fit to the 21 remaining data points shown in
Figure 4 gives:

log $L_{\mathrm{X}}$ = ($26.6\pm 5.7$) + ($0.26\pm 0.32$) log $L_{\mathrm{R}}$

with a correlation coefficient (R) of 0.17. Since this coefficient has a low
value, we conclude that the radio and X-ray luminosities are not correlated
and, thus, do not follow a Güdel-Benz relation, where a slope consistent with
1 is expected. Note that, in general, our data are underluminous in X-ray
emission. Lower X-ray luminosities than those predicted by the Güdel-Benz
relation are also found in other star forming regions regions (Kounkel et al.,
2014; Dzib et al., 2013a; Ortiz-León et al., 2015a; Dzib et al., 2015;
Forbrich et al., 2016), meaning that we are likely picking up objects of the
same nature. Also, Gy-CRS and Und-CRS follow the same behavior and there is no
segregation between them.

Figure 4: X-ray luminosity vs. Radio luminosity at 10 GHz of the CRSs in M 17.
Black dots represents the Und-CRS, green diamonds are sources with circular
polarization and red squares are variable sources (these two types of sources
are Gy-CRS), with the horizontal red bars representing the range of
variability between our observing epochs. If the variable source is undetected
in one or more epochs, we report upper limits as arrows at the lower end of
the range. The black solid and blue dashed lines represent the Güdel-Benz
relation with $\kappa$ = 1 and 0.03, respectively, the latter employed by Dzib
et al. (2013a) to match radio and X-ray luminosites of YSOs.

The lack of correlation between X-ray and radio data can be due to several
reasons. First, our data sample is contaminated by a number of possible
thermal sources because points from the undetermined category (black dots) are
dominant in the graph, while the Güdel-Benz relation only applies for non-
thermal sources. Second, our sample is small and CRSs with luminosities below
log ($L_{\mathrm{R}}$) $\sim 17.0$ erg s-1Hz-1 are not detected, perhaps due
to a lack of completeness, adding uncertainty in the slope determination. This
is not surprising since M 17 is further away than other nearby regions
observed in similar studies (Dzib et al., 2013a; Ortiz-León et al., 2015a) and
suggests that the faintest objects of our region remain undetected by our
observations.

As discussed by Forbrich et al. (2016), a similar trend is observed for the
250 ONC sources with both X-ray and radio emission. If we fit their respective
luminosities we obtain:

log $L_{\mathrm{X}}$ = ($24.1\pm 1.6$) + ($0.37\pm 0.10$) log $L_{\mathrm{R}}$

again suggesting that the X-ray and radio luminosities are poorly correlated,
if at all (R$=0.23$). This is not unexpected since a large fraction of the
radio sources in the ONC are thermal and their intensity is unrelated to the
mildly relativistic electrons that produce the X-ray and gyrosynchrotron
emission in active stars (Güdel & Benz 1993). If, in contrast, for Orion we
consider only the 82 sources with X-ray emission that are also VLBI sources
(Forbrich et al., 2021; Dzib et al., 2021) and whose emission is thus clearly
non-thermal111We consider a VLBI source as a non-thermal emitter since the
VLBI technique filters out the thermal emission leaving only the most compact
emission with high brightness temperatures ($>10^{6}$ K), a strong indicator
of non-thermal emission associated to young stars (e.g., Feigelson &
Montmerle, 1999)., a clear correlation is obtained. These data are fitted by:

log $L_{\mathrm{X}}$ = ($17.1\pm 2.4$) + ($0.81\pm 0.14$) log
$L_{\mathrm{R}}$.

consistent with a linear fit (R$=0.53$). The data and fit are shown in Figure
5. The obtained fit for this case is also about an order of magnitude lower in
$L_{\mathrm{X}}$ than the original Güdel-Benz relation. This result confirms
the notion that the radio emission is probably produced by gyrosynchrotron
radiation from the same mildly relativistic electrons that are responsible for
the X-ray emission. Unfortunately, the same test for the M 17 data yields
unreliable results because we have only 8 sources with X-ray emission and non-
thermal characteristics in the radio. Nevertheless, we have plotted this
handful of sources in Figure 5 to show that they fall near the high-brightness
portion of the Orion data. We speculate that more sensitive observations of M
17 will reveal a similar population of related radio - X-ray sources. As
discussed below, for a given radio luminosity M 17 has about three times the
number of sources of Orion. In practice, however, given its larger distance it
will be very difficult to obtain an M 17 survey with similar luminosity limits
as those obtained for Orion.

Figure 5: X-ray luminosities as a function of radio luminosity. The points
mark the position of the 82 X-ray sources that are also detected with VLBI
techniques (blue dots). The red stars are data points of the M 17 region with
X-ray emission and non-thermal characteristics in the radio. The green line is
the fit discussed in the text. The black line is the original Güdel-Benz
relation.

###  5.2 The luminosity function

The high number of detected CRSs towards M 17 (182 CRSs) makes this region the
second richest in radio source detections after the ONC (556 CRSs), and
followed by NGC 6334 (83 CRSs). Thus, a comparison between these regions is
tempting. In Figure 6 we show histograms for the radio luminosity distribution
(number of CRSs per logarithmic luminosity interval) of the three regions of
star formation considered here. Unfortunately, because of their larger
distances, we are severely limited in the dynamic range for M 17 and NGC
6334D-F due to incompleteness, and a comparison can be made only for the two
bins of higher luminosity.

M 17 has about three times more sources at a given radio luminosity than the
other two regions. This result is probably related with the significantly
higher global luminosity (stars plus dust) that M 17 has over the two other
regions (Johnson 1973). We also conclude that the negative slope between these
two luminosity bins is, within error, the same for all three regions. We find
that, approximately, the number of sources drops inversely (in a linear
relation) with their radio luminosity.

Figure 6: Radio luminosity distribution of M 17 (red bars, this work), NGC
6334D-F (green bars, Medina et al. 2018) and Orion (blue bars, Forbrich et al.
2016). The error bars are the uncertainty assuming that the number of sources
in each luminosity bin follows a Poisson distribution. The vertical dashed
lines indicate the adopted detection limit for each region according to the
respective color. Note that the position of these lines makes the log(Radio
luminosity) = 17.0 erg/s/Hz bins incomplete for M 17 and NGC 6334D-F. The
width of each bin is 0.5 dex.

###  5.3 Age and spatial distribution of the CRSs

The VLA observations from the Gould’s Belt Very Large Array Survey studied the
radio emission from hundred of YSOs in nearby ($<500$ pc) star-forming regions
(Dzib et al., 2013b, 2015; Kounkel et al., 2014; Ortiz-León et al., 2015b;
Pech et al., 2016). The results have shown that the properties of the radio
emission (variability, spectral index, and circular polarization) change with
the infrared classification of YSOs. Class 0, and Class I young stars are
mostly associated with thermal free-free radio emission, Class III with
gyrosynchrotron radio emission and Class II is an intermediate case. However,
some Class I YSOs have been confirmed to be gyrosynchrotron radio emmiters
(e.g., Dzib et al., 2010). Thus, gyrosynchrotron sources usually correspond to
relatively evolved young stars with clean surroundings and their photosphere
visible at optical wavelengths (Class III YSOs). On the other hand, thermal
sources are likely younger objects still embedded in large envelopes of
circumstellar material that are strongly accreting and at the same time
producing jets whose free-free emission is dominant. The large number of gy-
CRSs detected by us (36% of the total CRSs, twice the VLBI sources found in
Orion: $\sim 20$% of the total radio sources) suggests the existence of an
important evolved population of young objects in the M 17 region. The Und-CRS,
on the other hand, probably include young thermal sources.

In the past, objects of different ages have been found in M 17. For instance,
Nielbock et al. (2001) analyzed 22 compact NIR sources and deduced that most
of them are massive stars of Class I type, indicating that these objects
formed very recently. In contrast, Broos et al. (2007) analyzed 138 stars and
found that only 12% are Class I and II, indicating a predominance of older
Class II and III sources toward M 17. Our results agree with the scenario of
an age dispersion for the objects in M 17 and suggest that the region has been
forming stars at least ten million years (i.e. the age of Class III objects).

In Figure 7 we show the distribution of CRSs in the M 17 region highlighting
the Gy-CRSs in pink. The distribution of the two populations looks centrally
peaked and smoothly fades in the outskirts of the map. Thus, to analyze the
spatial distribution of radio sources in M 17 we have fitted the data to a
Gaussian ellipsoid following Rodriguez-Gomez et al. (2019). In Table
LABEL:tab:average_sigma we present the parameters (centroid, major and minor
axis and position angle of the major axis) for Gaussian ellipsoids fitted to
the three populations of radio sources in M 17 and in Orion, aiming to compare
any segregation between populations in both regions. Note that the axes of the
distributions are significantly smaller than the primary beam size.

We conclude that for the case of Orion all three populations have parameters
that overlap to within one-sigma. Perhaps this result is related to the lack
of expansion or contraction motions found by Dzib et al. (2017) from
observations of 88 young stars with compact radio emission. It also suggests
that, despite their age, the stars will remain similarly distributed.

For M 17, the centroids are consistent within the noise and fall within a few
arcsec of the positions of the massive binary system formed by CEN 1a and CEN
1b. Also the position angles are consistent within one-sigma. However, the
major and minor axes of the non-thermal population seem to be about 20 percent
larger than those of the other two populations, and significantly smaller than
the primary beam size. The rough coincidence of the centroids of all three
distributions in M 17 with CEN 1 is consistent with a scenario where the
potential well of the cloud centers on this massive binary star, and most of
the objects (including the most massive) are formed around this position.
Besides, the larger dispersion of the Gy-CRS is marginal and a direct
measurement of their proper motions is required to provide a conclusive
statement.

We finally note that the radio population in M 17 is not only about three
times more abundant (a result inferred from the comparison of the most
luminous sources) than in Orion, but also that it is more extended spatially.
Correcting for the distance ($388\pm 5$ pc, Kounkel et al., 2017), the major
axis of the distribution of all sources in Orion corresponds to 0.17 pc, while
it is 0.84 pc for the case of M 17.

Figure 7: RGB image toward M 17 of $\sim 7\arcmin\times 7\arcmin$ of field.
Blue corresponds to the SOFIA-FORCAST $20~{}\mu$m image, green is the SOFIA
$37~{}\mu$m image and red is the Herschel $70~{}\mu$m image. Pink stars
represents the position of the Gy-CRS and white stars are the Und-CRS. The
black star shows the UC1 position. The SOFIA images are taken from Lim et al.
(2020).  Table 7: Statistical parameters of the spatial distribution of CRSs

CRS Type |  |  | Size
---|---|---|---
| Mean Position | Major Axis | Minor Axis | Position Angle
|  $\alpha$ (J2000) |  $\delta$ (J2000) |  (′′) |  (′′) | (deg.)
M17
All CRS | $18^{\mathrm{h}}20^{\mathrm{m}}29\hbox to0.0pt{.\hss}^{\mathrm{s}}84\pm 0\hbox to0.0pt{.\hss}^{\mathrm{s}}33$ | $-16^{\circ}10^{\prime}47\hbox to0.0pt{.\hss}^{\prime\prime}9\pm 4\hbox to0.0pt{.\hss}^{\prime\prime}5$ |  66.9 $\pm$ 3.8 |  55.2 $\pm$ 3.0 |  52.0 $\pm$ 12.6
Gy-CRS | $18^{\mathrm{h}}20^{\mathrm{m}}29\hbox to0.0pt{.\hss}^{\mathrm{s}}54\pm 0\hbox to0.0pt{.\hss}^{\mathrm{s}}61$ | $-16^{\circ}10^{\prime}49\hbox to0.0pt{.\hss}^{\prime\prime}7\pm 8\hbox to0.0pt{.\hss}^{\prime\prime}2$ |  79.2 $\pm$ 6.7 |  61.9 $\pm$ 5.2 |  53.6 $\pm$ 19.1
Und-CRS | $18^{\mathrm{h}}20^{\mathrm{m}}30\hbox to0.0pt{.\hss}^{\mathrm{s}}04\pm 0\hbox to0.0pt{.\hss}^{\mathrm{s}}37$ | $-16^{\circ}10^{\prime}46\hbox to0.0pt{.\hss}^{\prime\prime}6\pm 5\hbox to0.0pt{.\hss}^{\prime\prime}3$ |  56.6 $\pm$ 3.9 |  50.0 $\pm$ 3.5 |  47.8 $\pm$ 16.3
Orion Nebula Cluster
All sources | $5^{\mathrm{h}}35^{\mathrm{m}}16\hbox to0.0pt{.\hss}^{\mathrm{s}}36\pm 0\hbox to0.0pt{.\hss}^{\mathrm{s}}21$ | $-5^{\circ}22^{\prime}55\hbox to0.0pt{.\hss}^{\prime\prime}3\pm 3\hbox to0.0pt{.\hss}^{\prime\prime}4$ |  87.1 $\pm$ 2.6 |  67.7 $\pm$ 2.0 |  -17.4 $\pm$ 7.1
VLBI radio sourcesa | $5^{\mathrm{h}}35^{\mathrm{m}}16\hbox to0.0pt{.\hss}^{\mathrm{s}}45\pm 0\hbox to0.0pt{.\hss}^{\mathrm{s}}45$ | $-5^{\circ}23^{\prime}2\hbox to0.0pt{.\hss}^{\prime\prime}2\pm 7\hbox to0.0pt{.\hss}^{\prime\prime}8$ |  90.0 $\pm$ 5.8 |  69.0 $\pm$ 4.4 |  -28.7 $\pm$ 15.1
non-VLBI radio sourcesb | $5^{\mathrm{h}}35^{\mathrm{m}}16\hbox to0.0pt{.\hss}^{\mathrm{s}}34\pm 0\hbox to0.0pt{.\hss}^{\mathrm{s}}22$ | $-5^{\circ}22^{\prime}53\hbox to0.0pt{.\hss}^{\prime\prime}4\pm 4\hbox to0.0pt{.\hss}^{\prime\prime}1$ |  86.4 $\pm$ 2.9 |  66.9 $\pm$ 2.3 |  -14.0 $\pm$ 8.0

aForbrich et al. (2021) and Dzib et al. (2021)
bForbrich et al. (2016)

###  5.4 The Hyper-Compact HII region UC1 and the Arc structure

In Figure 8 we focus on the X band emission at the western part of the cloud.
The left panel of the figure shows the arc-shaped structure found previously
by Felli et al. (1980) and discussed by Felli et al. (1984) and Johnson et al.
(1998). This structure is identified as the ionizing front of the central HII
region of M 17. For the whole arc-like structure, we measure a flux density of
0.1 Jy and a bounding box of $\sim$ 5 $\times$ 30 arcsec. These values are
significantly lower than those measured by Felli et al. (1984) because our
data suffer from severe spatial filtering.

Near the focus of the arc morphology, Felli et al. (1980) found a compact
thermal radio source they named UC1 and which they categorized as an UCHII
region. Felli et al. (1984) determined a size ($\sim 0.004$ pc) and spectral
index ($\sim 1$) that RGM2012 found more typical of an HCHII region. Our
measured flux density value at 10 GHz, $98\pm 5$ mJy, is roughly consistent
with the results of the spectral analysis of RGM2012. However, RGM2012 showed
that the spectral index derived for UC1 actually results from averaging a
gradient along the cometary morphology highlighted in the right panel of
Figure 8 (countours). The gradient on the spectral index is likely a
consequence of a gas density gradient in the source, being maximum at the tip
of the cometary morphology at the south-east part of the source (i.e.,
implying emission measures of at least $\sim 10^{9}$ cm-6 pc).

We obtained archival ALMA band 6 data centered on UC1 in order to assess the
scenario of RGM2012 described above (project 2015.1.01163). In agreement with
this scheme, the mm emission integrated over the entire band (1.875 GHz) shows
a bow-like structure oriented toward the SE, where high densities, either of
dust or ionized material are expected to be found. In addition, a bright
central source possibly associated with the stellar object of UC1 is also
revealed in the mm map. The total integrated flux density of the mm emission
centered at 228 GHz is about 125 mJy, 7 mJy of which come from this central
source. The deconvolved size of the latter is $(30\pm 10$ mas$)\times(22\pm
17$ mas$)$ (P.A. of $70\arcdeg\pm 50\arcdeg$) that at the distance of M 17
corresponds to $\sim 60\times 40$ AU. If this mm emission comes from a
protostellar disk, its derived size is significantly lower than the sizes
inferred for disks of other massive stars ($\sim 300$ AU and a few hundreds of
mJy at similar wavelegths, Lugo et al., 2004; Girart et al., 2018). A
possibility is that the disk is being ablated by the energetic radiation of
the protostellar object exciting UC1, although we cannot discard alternative
scenarios involving other elements than the presence of a disk.

Exploring UC1 at IR wavelengths, Nielbock et al. (2007) found hints of a disk-
like structure in absorption across UC1, which was interpreted as a cool disk
of 1000 AU in size. This size is more than an order of magnitude larger than
the size of the central peak of the ALMA image. The fact that more extended
material from this cool disk is not detected at the frequency of the ALMA
observations suggests that it corresponds to a different structure, perhaps
associated to the molecular part of the M 17 region. The potential role of
disks around massive stars, in particular in their early stages (i.e., HCHII
region) is presently unclear and their firm detection would have important
implications on the paradigm of their formation and deserves further
investigation.

Figure 8: Left Panel: X band emission (color and contour scale) highlighting
the arc shape structure (West part of the image). Contour levels are C times
-20, 21/2, 21, 23/2, 22, 25/2, 23, 27/2 and 24, where C is given by 2.5 times
the rms noise measured over the region of the map shown in the panel (70
$\mu$Jy beam-1). The beam size is the same as shown in Fig. 1. The dotted
lines show the angular extend of the Arc shape. The dashed box represents the
area shown enlarged in the right panel enclosing the UC1 source. Right Panel:
Detail of the X band emission (contours) of UC1 superimposed on the ALMA 240
GHz continuum image (color scale). Note the compact ALMA source that we
identify as a disk associated with the ionizing star of the region. Contours
are 3, 6, 12, 20, 35, 53 and 68 times the rms noise of the region of the map
around UC1 (25 $\mu$Jy beam-1). The corresponding beams are shown in the
bottom left corner (VLA) and in the top right corner (ALMA).

##  6 Conclusions

We have carried out deep X band observations with the VLA operating in A
configuration. After self-calibrating the obtained data of M 17 and extracting
radio sources by applying the BLOBCAT software on the resulting map, we
cataloged them and inspected their basic emission properties. We reached the
following conclusions:

  1. 1.

We found a total of 194 radio sources in the M 17 region, 12 of them extended
and 182 compact. This catalog constitutes the most complete catalog of radio
sources known of M 17. After Orion, M 17 becomes the second richest region in
radio source detection to our knowledge.

  2. 2.

The inspection of the properties of our radio sources reveals a population
(about 40% of our catalog) of objects emitting non-thermal radiation. These
are probably low-mass stars with coronal gyrosynchrotron emission, given their
observed variability and polarization.

  3. 3.

In X-ray emission, these non-thermal radio sources are underluminous with
respect to the Güdel Benz relation, but a reasonable correlation of their
radio and X-ray emission is still found, provided that only sources with
evident non thermal nature are selected from Orion and M 17. This suggests
that radio and X-ray emission are both caused the same underlying process,
presumably magnetic reconnection in the stellar coronae.

  4. 4.

The luminosity function of the M 17 radio sources shows a decreasing trend
with luminosity (i.e. weak sources are more abundant than bright ones). The
same trend is observed for the radio sources of the Orion Nebula Cluster and
NGC 6334D-F reported in other works, though there is an important lack of
completeness for M 17 and NGC 6334D-F that prevents us from exploring this
similarity for the weakest sources. In any case, M 17 has about three more
times sources at any given radio luminosity than the other two regions perhaps
due to its higher intrinsic luminosity.

  5. 5.

The spatial distribution of sources in M 17 is centrally peaked towards the
position of the most massive stars (e.g., CEN 1) and fades radially and
smoothly toward the outskirts of the region, with a dispersion that is
marginally higher for non thermal sources. The same analysis carried out for
the radio sources in the ONC shows no segregation between thermal and non
thermal sources. The radio cluster in M 17 is about 5 times larger than that
in the ONC.

  6. 6.

A previously known arc structure that has the HCHII region UC1 at its focus is
also detected in our image. The cometary morphology of UC1 is highlighted in
our data with a probable increase of density towards the ionization front
traced by the arc. Exploring ALMA archival data at 1 mm, we found emission
associated to the possible exciting source of UC1. If this emission comes from
a protoplanetary disk, its size is considerably smaller than typical sizes
inferred from disks around massive protostars.

## Acknowledgements

V.Y. acknowledges the financial support of DAIP, UG, and MPIfR during the
internship at the Max Planck Institute for Radioastronomy. V.Y. acknowledges
the financial support of CONACyT, México. L.L. acknowledges the support of
CONACyT-AEM grant 275201, CONACyT-CF grant 263356, and UNAM DGAPA/PAPIIT
grants IN112417 and IN112820. This research has made use of the NASA/IPAC
Infrared Science Archive, which is funded by the National Aeronautics and
Space Administration and operated by the California Institute of Technology.
Counterparts to radio sources were found by using SIMBAD database.

Facilities: This work makes use of the Spitzer, 2MASS, Gaia, Vary Large Array
(VLA), and Atacama Large Millimiter Array (ALMA) telescopes. The VLA
observations presented here were part of NRAO program 18A-002. The National
Radio Astronomy Observatory is a facility of the National Science Foundation
operated under cooperative agreement by Associated Universities, Inc. This
paper makes use of the following ALMA data: ADS/JAO.ALMA#2015.1.01163. ALMA is
a partnership of ESO (representing its member states), NSF (USA) and NINS
(Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI
(Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA
Observatory is operated by ESO, AUI/NRAO and NAOJ.

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##  Appendix A Catalog of the compact radio sources of M17

Table 8: Parameters of the CRSs with their counterparts at other wavelengths. ID | $\alpha$ | $\delta$ | $S_{peak}$ | $S_{int}$ |  X-raya |  IRb |  Opticalc |  Radiod
---|---|---|---|---|---|---|---|---
# | (J2000) | (J2000) | $(\mu Jy~{}beam^{-1})$ | $(\mu Jy)$ |  |  |  |
1 |  18 20 16.402 $\pm$ 0.002 |  -16 11 18.81 $\pm$ 0.03 |  149 $\pm$ 30 |  179 $\pm$ 30 |  |  |  |
2 |  18 20 16.405 $\pm$ 0.002 |  -16 11 10.73 $\pm$ 0.03 |  143 $\pm$ 29 |  169 $\pm$ 30 |  |  |  |
3 |  18 20 17.569 $\pm$ 0.001 |  -16 12 21.49 $\pm$ 0.02 |  188 $\pm$ 25 |  543 $\pm$ 118 |  |  |  |
4 |  18 20 18.968 $\pm$ 0.002 |  -16 11 27.98 $\pm$ 0.03 |  88 $\pm$ 17 |  185 $\pm$ 19 |  |  |  |
5 |  18 20 19.417 $\pm$ 0.001 |  -16 12 15.74 $\pm$ 0.01 |  549 $\pm$ 36 |  549 $\pm$ 36 | ✓ |  |  |
6 |  18 20 20.722 $\pm$ 0.001 |  -16 08 27.79 $\pm$ 0.02 |  177 $\pm$ 29 |  241 $\pm$ 30 | ✓ | ✓ | ✓ |
7 |  18 20 21.433 $\pm$ 0.001 |  -16 12 06.75 $\pm$ 0.02 |  180 $\pm$ 17 |  192 $\pm$ 17 | ✓ | ✓ |  | 4
8 |  18 20 21.636 $\pm$ 0.001 |  -16 11 17.91 $\pm$ 0.01 |  286 $\pm$ 22 |  400 $\pm$ 25 |  | ✓ | ✓ | 5
9 |  18 20 21.736 $\pm$ 0.001 |  -16 11 12.59 $\pm$ 0.01 |  222 $\pm$ 17 |  227 $\pm$ 16 |  |  |  |
10 |  18 20 21.849 $\pm$ 0.002 |  -16 09 05.43 $\pm$ 0.03 |  72 $\pm$ 15 |  94 $\pm$ 15 |  |  |  |
11 |  18 20 21.921 $\pm$ 0.002 |  -16 13 33.15 $\pm$ 0.03 |  201 $\pm$ 41 |  211 $\pm$ 41 |  |  |  |
12 |  18 20 22.339 $\pm$ 0.001 |  -16 11 28.55 $\pm$ 0.02 |  97 $\pm$ 15 |  189 $\pm$ 17 |  |  |  |
13 |  18 20 22.358 $\pm$ 0.001 |  -16 07 49.51 $\pm$ 0.01 |  187 $\pm$ 38 |  474 $\pm$ 44 |  |  |  |
14 |  18 20 22.363 $\pm$ 0.001 |  -16 12 04.87 $\pm$ 0.001 |  242 $\pm$ 19 |  242 $\pm$ 19 | ✓ |  |  | 6
15 |  18 20 22.468 $\pm$ 0.002 |  -16 09 51.39 $\pm$ 0.03 |  55 $\pm$ 10 |  58 $\pm$ 10 | ✓ |  |  |
16 |  18 20 22.852 $\pm$ 0.001 |  -16 11 48.51 $\pm$ 0.02 |  171 $\pm$ 15 |  239 $\pm$ 17 | ✓ |  |  |
17 |  18 20 23.082 $\pm$ 0.001 |  -16 13 05.23 $\pm$ 0.02 |  244 $\pm$ 29 |  270 $\pm$ 29 |  |  |  |
18 |  18 20 23.107 $\pm$ 0.002 |  -16 13 01.91 $\pm$ 0.03 |  123 $\pm$ 24 |  125 $\pm$ 24 | ✓ |  |  |
19 |  18 20 23.157 $\pm$ 0.002 |  -16 13 05.67 $\pm$ 0.03 |  134 $\pm$ 27 |  134 $\pm$ 27 | ✓ |  |  |
20 |  18 20 23.181 $\pm$ 0.001 |  -16 10 43.47 $\pm$ 0.02 |  119 $\pm$ 14 |  137 $\pm$15 |  |  |  |
21 |  18 20 23.763 $\pm$ 0.001 |  -16 11 57.99 $\pm$ 0.02 |  112 $\pm$ 16 |  458 $\pm$ 27 |  |  |  |
22 |  18 20 24.821 $\pm$ 0.001 |  -16 11 35.08 $\pm$ 0.01 |  18582 $\pm$ 1003 |  97836 $\pm$ 5092 | ✓ | ✓ |  | UC1
23 |  18 20 24.897 $\pm$ 0.001 |  -16 10 42.56 $\pm$ 0.02 |  122 $\pm$ 15 |  122 $\pm$ 15 | ✓ |  |  |
24 |  18 20 25.318 $\pm$ 0.001 |  -16 11 48.64 $\pm$ 0.02 |  230 $\pm$ 29 |  892 $\pm$ 65 |  |  |  | 7
25 |  18 20 25.496 $\pm$ 0.002 |  -16 10 53.80 $\pm$ 0.03 |  90 $\pm$ 17 |  207 $\pm$ 20 | ✓ | ✓ |  | 8
26 |  18 20 25.622 $\pm$ 0.002 |  -16 09 37.04 $\pm$ 0.03 |  45 $\pm$ 9 |  62 $\pm$ 9 |  |  |  |
27 |  18 20 25.847 $\pm$ 0.001 |  -16 08 32.16 $\pm$ 0.03 |  87 $\pm$ 13 |  93 $\pm$ 13 | ✓ | ✓ | ✓ |
28 |  18 20 26.116 $\pm$ 0.002 |  -16 09 13.92 $\pm$ 0.03 |  46 $\pm$ 9 |  100 $\pm$ 10 |  |  |  |
29 |  18 20 26.216 $\pm$ 0.001 |  -16 09 21.52 $\pm$ 0.02 |  70 $\pm$ 10 |  85 $\pm$ 10 |  | ✓ | ✓ |
30 |  18 20 26.240 $\pm$ 0.001 |  -16 11 15.84 $\pm$ 0.01 |  270 $\pm$ 21 |  345 $\pm$ 41 |  |  |  | 10
31 |  18 20 27.379 $\pm$ 0.001 |  -16 11 38.56 $\pm$ 0.01 |  262 $\pm$ 18 |  565 $\pm$ 30 |  |  |  | 12
32 |  18 20 27.588 $\pm$ 0.001 |  -16 11 33.40 $\pm$ 0.01 |  886 $\pm$ 49 |  886 $\pm$ 49 | ✓ |  | ✓ |
33 |  18 20 27.615 $\pm$ 0.001 |  -16 10 43.32 $\pm$ 0.03 |  45 $\pm$ 7 |  48 $\pm$ 7 | ✓ | ✓ | ✓ |
34 |  18 20 27.715 $\pm$ 0.001 |  -16 09 51.12 $\pm$ 0.02 |  79 $\pm$ 8 |  99 $\pm$ 8 | ✓ | ✓ |  |
35 |  18 20 27.742 $\pm$ 0.002 |  -16 14 16.76 $\pm$ 0.03 |  252 $\pm$ 49 |  340 $\pm$ 51 |  |  |  |
36 |  18 20 27.798 $\pm$ 0.001 |  -16 08 57.76 $\pm$ 0.02 |  87 $\pm$ 11 |  87 $\pm$ 11 | ✓ | ✓ | ✓ |
37 |  18 20 27.857 $\pm$ 0.001 |  -16 09 55.64 $\pm$ 0.02 |  79 $\pm$ 8 |  89 $\pm$ 8 | ✓ | ✓ | ✓ | 13
38 |  18 20 28.023 $\pm$ 0.001 |  -16 11 26.96 $\pm$ 0.01 |  208 $\pm$ 14 |  274 $\pm$ 16 |  |  |  | 14
39 |  18 20 28.034 $\pm$ 0.001 |  -16 10 42.36 $\pm$ 0.02 |  79 $\pm$ 7 |  80 $\pm$ 7 |  |  |  |
40 |  18 20 28.151 $\pm$ 0.001 |  -16 10 49.32 $\pm$ 0.03 |  41 $\pm$ 7 |  72 $\pm$ 7 | ✓ | ✓ | ✓ |
41 |  18 20 28.154 $\pm$ 0.002 |  -16 10 01.00 $\pm$ 0.03 |  31 $\pm$ 6 |  31 $\pm$ 6 |  |  |  |
42 |  18 20 28.206 $\pm$ 0.002 |  -16 11 05.64 $\pm$ 0.03 |  37 $\pm$ 7 |  37 $\pm$ 7 |  |  |  |
43 |  18 20 28.231 $\pm$ 0.001 |  -16 13 17.80 $\pm$ 0.01 |  1627 $\pm$ 89 |  1674 $\pm$ 85 | ✓ | ✓ |  | 15
44 |  18 20 28.273 $\pm$ 0.001 |  -16 10 42.36 $\pm$ 0.01 |  117 $\pm$ 8 |  151 $\pm$ 9 |  |  |  |
45 |  18 20 28.287 $\pm$ 0.001 |  -16 11 30.56 $\pm$ 0.02 |  128 $\pm$ 13 |  172 $\pm$ 14 | ✓ | ✓ |  | 16
46 |  18 20 28.367 $\pm$ 0.001 |  -16 10 30.68 $\pm$ 0.02 |  43 $\pm$ 6 |  43 $\pm$ 6 | ✓ | ✓ | ✓ |
47 |  18 20 28.384 $\pm$ 0.001 |  -16 10 14.16 $\pm$ 0.01 |  164 $\pm$ 11 |  222 $\pm$ 13 |  |  |  | 17
48 |  18 20 28.487 $\pm$ 0.001 |  -16 10 39.56 $\pm$ 0.01 |  99 $\pm$ 8 |  108 $\pm$ 8 |  |  |  |
49 |  18 20 28.503 $\pm$ 0.001 |  -16 10 55.36 $\pm$ 0.02 |  58 $\pm$ 7 |  226 $\pm$ 13 |  |  |  |
50 |  18 20 28.548 $\pm$ 0.001 |  -16 10 26.84 $\pm$ 0.01 |  110 $\pm$ 8 |  149 $\pm$ 10 |  |  |  |
51 |  18 20 28.720 $\pm$ 0.001 |  -16 10 22.16 $\pm$ 0.03 |  47 $\pm$ 6 |  47 $\pm$ 6 |  |  |  |
52 |  18 20 28.787 $\pm$ 0.002 |  -16 11 51.92 $\pm$ 0.03 |  40 $\pm$ 9 |  48 $\pm$ 8 |  |  |  |
53 |  18 20 28.787 $\pm$ 0.001 |  -16 10 49.32 $\pm$ 0.02 |  63 $\pm$ 6 |  71 $\pm$ 7 |  |  |  |
54 |  18 20 28.798 $\pm$ 0.001 |  -16 11 03.68 $\pm$ 0.03 |  41 $\pm$ 7 |  41 $\pm$ 7 |  |  |  |
55 |  18 20 28.839 $\pm$ 0.001 |  -16 10 56.08 $\pm$ 0.02 |  72 $\pm$ 7 |  72 $\pm$ 7 | ✓ | ✓ |  |
56 |  18 20 28.867 $\pm$ 0.001 |  -16 11 10.08 $\pm$ 0.03 |  42 $\pm$ 6 |  42 $\pm$ 6 | ✓ | ✓ |  |
57 |  18 20 28.998 $\pm$ 0.001 |  -16 10 33.44 $\pm$ 0.03 |  37 $\pm$ 6 |  54 $\pm$ 6 |  |  |  |
58 |  18 20 29.139 $\pm$ 0.001 |  -16 12 09.84 $\pm$ 0.03 |  50 $\pm$ 9 |  101 $\pm$ 10 |  |  |  |
59 |  18 20 29.167 $\pm$ 0.001 |  -16 09 43.04 $\pm$ 0.01 |  124 $\pm$ 9 |  156 $\pm$ 10 | ✓ | ✓ | ✓ |
60 |  18 20 29.192 $\pm$ 0.001 |  -16 10 54.68 $\pm$ 0.01 |  28 $\pm$ 6 |  28 $\pm$ 6 |  |  |  |
61 |  18 20 29.217 $\pm$ 0.001 |  -16 10 38.80 $\pm$ 0.02 |  49 $\pm$ 6 |  61 $\pm$ 6 |  |  |  |
62 |  18 20 29.231 $\pm$ 0.002 |  -16 11 03.00 $\pm$ 0.03 |  34 $\pm$ 6 |  51 $\pm$ 6 |  |  |  |
63 |  18 20 29.267 $\pm$ 0.001 |  -16 10 41.32 $\pm$ 0.01 |  83 $\pm$ 7 |  83 $\pm$ 7 | ✓ | ✓ | ✓ |
64 |  18 20 29.317 $\pm$ 0.001 |  -16 10 47.96 $\pm$ 0.03 |  50 $\pm$ 6 |  51 $\pm$ 6 | ✓ |  |  |
65 |  18 20 29.317 $\pm$ 0.001 |  -16 09 50.60 $\pm$ 0.03 |  36 $\pm$ 6 |  41 $\pm$ 6 |  |  |  |
66 |  18 20 29.323 $\pm$ 0.002 |  -16 10 18.08 $\pm$ 0.03 |  31 $\pm$ 6 |  77 $\pm$ 7 |  |  |  |
67 |  18 20 29.367 $\pm$ 0.001 |  -16 11 08.88 $\pm$ 0.01 |  90 $\pm$ 8 |  90 $\pm$ 8 |  |  |  |
68 |  18 20 29.386 $\pm$ 0.002 |  -16 09 43.00 $\pm$ 0.03 |  34 $\pm$ 6 |  126 $\pm$ 9 | ✓ | ✓ |  |
69 |  18 20 29.436 $\pm$ 0.001 |  -16 10 49.84 $\pm$ 0.01 |  283 $\pm$ 16 |  656 $\pm$ 33 | ✓ |  |  | 19
70 |  18 20 29.445 $\pm$ 0.002 |  -16 09 16.64 $\pm$ 0.03 |  39 $\pm$ 8 |  51 $\pm$ 8 |  |  |  |
71 |  18 20 29.495 $\pm$ 0.002 |  -16 08 04.00 $\pm$ 0.03 |  77 $\pm$ 16 |  101 $\pm$ 16 |  |  |  |
72 |  18 20 29.625 $\pm$ 0.002 |  -16 11 41.56 $\pm$ 0.03 |  35 $\pm$ 6 |  37 $\pm$ 6 |  |  |  |
73 |  18 20 29.728 $\pm$ 0.001 |  -16 10 59.92 $\pm$ 0.03 |  48 $\pm$ 7 |  57 $\pm$ 7 |  |  |  |
74 |  18 20 29.731 $\pm$ 0.001 |  -16 11 04.48 $\pm$ 0.02 |  46 $\pm$ 6 |  60 $\pm$ 7 |  |  |  |
75 |  18 20 29.736 $\pm$ 0.001 |  -16 10 53.40 $\pm$ 0.01 |  121 $\pm$ 9 |  131 $\pm$ 9 |  |  |  |
76 |  18 20 29.745 $\pm$ 0.001 |  -16 09 55.64 $\pm$ 0.02 |  52 $\pm$ 6 |  60 $\pm$ 6 |  |  |  |
77 |  18 20 29.750 $\pm$ 0.002 |  -16 11 25.76 $\pm$ 0.03 |  29 $\pm$ 6 |  34 $\pm$ 6 |  |  |  |
78 |  18 20 29.767 $\pm$ 0.001 |  -16 10 44.72 $\pm$ 0.02 |  45 $\pm$ 6 |  45 $\pm$ 6 |  |  |  |
79 |  18 20 29.811 $\pm$ 0.001 |  -16 10 45.56 $\pm$ 0.01 |  692 $\pm$ 38 |  891 $\pm$ 45 | ✓ |  |  | 20 (CEN 1b)
80 |  18 20 29.822 $\pm$ 0.001 |  -16 11 11.80 $\pm$ 0.02 |  60 $\pm$ 6 |  60 $\pm$ 6 |  |  |  |
81 |  18 20 29.828 $\pm$ 0.001 |  -16 10 48.32 $\pm$ 0.01 |  109 $\pm$ 8 |  109 $\pm$ 8 |  |  |  |
82 |  18 20 29.842 $\pm$ 0.001 |  -16 10 40.72 $\pm$ 0.02 |  46 $\pm$ 6 |  59 $\pm$ 7 | ✓ |  |  |
83 |  18 20 29.842 $\pm$ 0.001 |  -16 10 31.04 $\pm$ 0.01 |  137 $\pm$ 9 |  198 $\pm$ 12 |  |  |  |
84 |  18 20 29.850 $\pm$ 0.001 |  -16 10 41.20 $\pm$ 0.01 |  230 $\pm$ 14 |  230 $\pm$ 14 | ✓ |  |  |
85 |  18 20 29.861 $\pm$ 0.001 |  -16 10 44.72 $\pm$ 0.01 |  173 $\pm$ 11 |  177 $\pm$ 11 |  |  |  |
86 |  18 20 29.897 $\pm$ 0.001 |  -16 10 44.44 $\pm$ 0.01 |  566 $\pm$ 31 |  566 $\pm$ 31 | ✓ |  |  | 21 (CEN 1a)
87 |  18 20 29.911 $\pm$ 0.002 |  -16 10 35.08$\pm$ 0.03 |  30 $\pm$ 6 |  30 $\pm$ 6 | ✓ |  |  |
88 |  18 20 29.917 $\pm$ 0.001 |  -16 10 33.60 $\pm$ 0.02 |  74 $\pm$ 7 |  81 $\pm$ 7 |  |  |  |
89 |  18 20 29.925 $\pm$ 0.001 |  -16 09 14.64 $\pm$ 0.01 |  39 $\pm$ 7 |  49 $\pm$ 8 |  |  |  |
90 |  18 20 29.953 $\pm$ 0.001 |  -16 10 50.80 $\pm$ 0.02 |  85 $\pm$ 7 |  94 $\pm$ 7 |  |  |  |
91 |  18 20 29.953 $\pm$ 0.002 |  -16 10 33.76 $\pm$ 0.03 |  32 $\pm$ 6 |  32 $\pm$ 6 |  |  |  |
92 |  18 20 29.968 $\pm$ 0.001 |  -16 11 15.36 $\pm$ 0.01 |  77 $\pm$ 7 |  78 $\pm$ 7 |  |  |  |
93 |  18 20 29.989 $\pm$ 0.001 |  -16 10 44.68 $\pm$ 0.02 |  44 $\pm$ 6 |  79 $\pm$ 7 |  |  |  |
94 |  18 20 30.003 $\pm$ 0.001 |  -16 10 35.28 $\pm$ 0.01 |  514 $\pm$ 28 |  627 $\pm$ 32 | ✓ |  |  | 22
95 |  18 20 30.031 $\pm$ 0.001 |  -16 10 34.68 $\pm$ 0.02 |  79 $\pm$ 7 |  79 $\pm$ 7 |  |  |  |
96 |  18 20 30.042 $\pm$ 0.002 |  -16 11 57.44 $\pm$ 0.03 |  35 $\pm$ 7 |  35 $\pm$ 7 | ✓ | ✓ | ✓ |
97 |  18 20 30.044 $\pm$ 0.002 |  -16 10 33.40 $\pm$ 0.03 |  32 $\pm$ 6 |  34 $\pm$ 6 |  |  |  |
98 |  18 20 30.094 $\pm$ 0.001 |  -16 10 41.44 $\pm$ 0.02 |  44 $\pm$ 6 |  54 $\pm$ 6 | ✓ |  |  |
99 |  18 20 30.133 $\pm$ 0.001 |  -16 10 39.48 $\pm$ 0.01 |  307 $\pm$ 17 |  404 $\pm$ 21 |  |  |  | 23
100 |  18 20 30.178 $\pm$ 0.001 |  -16 11 20.08 $\pm$ 0.02 |  58 $\pm$ 6 |  58 $\pm$ 6 |  |  |  |
101 |  18 20 30.194 $\pm$ 0.002 |  -16 10 50.48 $\pm$ 0.03 |  34 $\pm$ 6 |  34 $\pm$ 6 | ✓ |  |  |
102 |  18 20 30.208 $\pm$ 0.001 |  -16 10 12.04 $\pm$ 0.01 |  85 $\pm$ 7 |  89 $\pm$ 7 |  |  |  |
103 |  18 20 30.278 $\pm$ 0.001 |  -16 11 31.48 $\pm$ 0.03 |  37 $\pm$ 6 |  49 $\pm$ 6 | ✓ | ✓ | ✓ |
104 |  18 20 30.286 $\pm$ 0.001 |  -16 10 39.52 $\pm$ 0.03 |  34 $\pm$ 6 |  48 $\pm$ 6 |  |  |  |
105 |  18 20 30.325 $\pm$ 0.001 |  -16 10 50.68 $\pm$ 0.01 |  186 $\pm$ 11 |  198 $\pm$ 11 |  |  |  | 24
106 |  18 20 30.380 $\pm$ 0.002 |  -16 11 44.20 $\pm$ 0.03 |  28 $\pm$ 6 |  32 $\pm$ 6 |  |  |  |
107 |  18 20 30.394 $\pm$ 0.001 |  -16 10 52.64 $\pm$ 0.03 |  39 $\pm$ 6 |  48 $\pm$ 6 |  |  |  |
108 |  18 20 30.414 $\pm$ 0.001 |  -16 11 03.64 $\pm$ 0.01 |  179 $\pm$ 11 |  218 $\pm$ 13 |  |  |  | 26
109 |  18 20 30.436 $\pm$ 0.001 |  -16 11 01.56 $\pm$ 0.02 |  91 $\pm$ 8 |  105 $\pm$ 9 |  |  |  |
110 |  18 20 30.444 $\pm$ 0.001 |  -16 10 53.12 $\pm$ 0.01 |  152 $\pm$ 10 |  153 $\pm$ 10 | ✓ | ✓ | ✓ | 27
111 |  18 20 30.450 $\pm$ 0.001 |  -16 10 10.08 $\pm$ 0.03 |  35 $\pm$ 5 |  78 $\pm$ 6 | ✓ | ✓ | ✓ |
112 |  18 20 30.530 $\pm$ 0.002 |  -16 11 51.40 $\pm$ 0.03 |  32 $\pm$ 6 |  42 $\pm$ 6 |  |  |  |
113 |  18 20 30.541 $\pm$ 0.001 |  -16 10 39.56 $\pm$ 0.02 |  46 $\pm$ 6 |  53 $\pm$ 6 |  |  |  |
114 |  18 20 30.575 $\pm$ 0.001 |  -16 10 30.96 $\pm$ 0.03 |  36 $\pm$ 6 |  47 $\pm$ 6 |  |  |  |
115 |  18 20 30.578 $\pm$ 0.001 |  -16 11 04.16 $\pm$ 0.01 |  606 $\pm$ 33 |  670 $\pm$ 35 |  | ✓ |  | 28
116 |  18 20 30.633 $\pm$ 0.002 |  -16 10 28.52 $\pm$ 0.03 |  27 $\pm$ 5 |  27 $\pm$ 5 | ✓ | ✓ |  | 29
117 |  18 20 30.664 $\pm$ 0.001 |  -16 10 44.24 $\pm$ 0.03 |  35 $\pm$ 6 |  46$\pm$ 6 |  |  |  |
118 |  18 20 30.678 $\pm$ 0.001 |  -16 10 50.12 $\pm$ 0.02 |  61 $\pm$ 6 |  73 $\pm$ 7 |  |  |  |
119 |  18 20 30.711 $\pm$ 0.001 |  -16 10 31.96 $\pm$ 0.02 |  45 $\pm$ 6 |  45 $\pm$ 6 |  |  |  |
120 |  18 20 30.777 $\pm$ 0.001 |  -16 10 59.36$\pm$ 0.01 |  585 $\pm$ 32 |  826 $\pm$ 42 | ✓ |  |  | 30
121 |  18 20 30.780 $\pm$ 0.001 |  -16 11 00.24 $\pm$ 0.01 |  29 $\pm$ 6 |  75 $\pm$ 7 |  |  |  |
122 |  18 20 30.811 $\pm$ 0.001 |  -16 10 41.60 $\pm$ 0.02 |  61 $\pm$ 6 |  61 $\pm$ 6 | ✓ |  |  |
123 |  18 20 30.850 $\pm$ 0.001 |  -16 10 48.76 $\pm$ 0.02 |  74 $\pm$ 7 |  166 $\pm$ 10 |  |  |  |
124 |  18 20 30.877 $\pm$ 0.002 |  -16 09 05.04 $\pm$ 0.03 |  40 $\pm$ 8 |  44 $\pm$ 8 | ✓ | ✓ | ✓ |
125 |  18 20 30.894 $\pm$ 0.001 |  -16 09 54.80 $\pm$ 0.02 |  60 $\pm$ 6 |  84 $\pm$ 7 | ✓ | ✓ | ✓ |
126 |  18 20 30.908 $\pm$ 0.001 |  -16 11 35.96 $\pm$ 0.03 |  34 $\pm$ 6 |  38 $\pm$ 6 | ✓ | ✓ | ✓ |
127 |  18 20 30.911 $\pm$ 0.001 |  -16 10 39.48 $\pm$ 0.01 |  98 $\pm$ 7 |  135 $\pm$ 9 | ✓ | ✓ |  |
128 |  18 20 31.111 $\pm$ 0.001 |  -16 09 29.84 $\pm$ 0.01 |  147 $\pm$ 10 |  147 $\pm$ 10 | ✓ | ✓ | ✓ | 31
129 |  18 20 31.305 $\pm$ 0.001 |  -16 10 50.96 $\pm$ 0.02 |  39 $\pm$ 5 |  39 $\pm$ 5 | ✓ |  |  |
130 |  18 20 31.336 $\pm$ 0.001 |  -16 10 52.80 $\pm$ 0.02 |  38 $\pm$ 5 |  41 $\pm$ 5 |  |  |  |
131 |  18 20 31.355 $\pm$ 0.001 |  -16 10 57.84 $\pm$ 0.01 |  125 $\pm$ 8 |  130 $\pm$ 8 |  |  |  |
132 |  18 20 31.424 $\pm$ 0.002 |  -16 11 03.48 $\pm$ 0.03 |  28 $\pm$ 5 |  28 $\pm$ 6 |  |  |  |
133 |  18 20 31.433 $\pm$ 0.001 |  -16 10 31.00 $\pm$ 0.01 |  75 $\pm$ 6 |  93 $\pm$ 7 |  |  |  |
134 |  18 20 31.469 $\pm$ 0.001 |  -16 09 13.08 $\pm$ 0.03 |  47 $\pm$ 8 |  74 $\pm$ 8 |  |  |  |
135 |  18 20 31.477 $\pm$ 0.002 |  -16 09 48.76 $\pm$ 0.03 |  28 $\pm$ 6 |  32 $\pm$ 6 |  |  |  |
136 |  18 20 31.516 $\pm$ 0.001 |  -16 09 10.40 $\pm$ 0.02 |  65 $\pm$ 8 |  78 $\pm$ 8 | ✓ |  | ✓ |
137 |  18 20 31.527 $\pm$ 0.002 |  -16 10 32.60 $\pm$ 0.03 |  28 $\pm$ 5 |  39 $\pm$ 6 | ✓ | ✓ |  | 32
138 |  18 20 31.602 $\pm$ 0.001 |  -16 11 05.88 $\pm$ 0.02 |  58 $\pm$ 6 |  58 $\pm$ 6 |  |  |  |
139 |  18 20 31.641 $\pm$ 0.002 |  -16 10 36.72 $\pm$ 0.03 |  28 $\pm$ 6 |  44 $\pm$ 6 |  |  |  |
140 |  18 20 31.716 $\pm$ 0.001 |  -16 11 06.80 $\pm$ 0.03 |  39 $\pm$ 6 |  39 $\pm$ 6 |  |  |  |
141 |  18 20 31.883 $\pm$ 0.001 |  -16 11 24.04 $\pm$ 0.03 |  34 $\pm$ 6 |  42 $\pm$ 6 | ✓ | ✓ |  |
142 |  18 20 31.977 $\pm$ 0.001 |  -16 09 25.16 $\pm$ 0.02 |  55 $\pm$ 7 |  55 $\pm$ 7 | ✓ | ✓ |  |
143 |  18 20 31.999 $\pm$ 0.002 |  -16 11 27.00 $\pm$ 0.03 |  30 $\pm$ 6 |  32 $\pm$ 6 | ✓ |  |  |
144 |  18 20 32.002 $\pm$ 0.002 |  -16 09 40.44 $\pm$ 0.03 |  31 $\pm$ 6 |  41 $\pm$ 6 |  |  |  |
145 |  18 20 32.324 $\pm$ 0.001 |  -16 10 48.92 $\pm$ 0.01 |  92 $\pm$ 7 |  102 $\pm$ 7 |  |  |  |
146 |  18 20 32.521 $\pm$ 0.002 |  -16 11 11.04 $\pm$ 0.03 |  32 $\pm$ 6 |  32 $\pm$ 6 |  |  |  |
147 |  18 20 32.549 $\pm$ 0.002 |  -16 10 06.20 $\pm$ 0.03 |  27 $\pm$ 5 |  38 $\pm$ 5 |  |  |  |
148 |  18 20 32.688 $\pm$ 0.002 |  -16 10 20.16 $\pm$ 0.03 |  26 $\pm$ 5 |  32 $\pm$ 5 |  |  |  |
149 |  18 20 32.932 $\pm$ 0.001 |  -16 10 35.68 $\pm$ 0.02 |  46 $\pm$ 6 |  46 $\pm$ 6 |  |  |  |
150 |  18 20 32.979 $\pm$ 0.001 |  -16 11 15.56 $\pm$ 0.01 |  107 $\pm$ 8 |  117 $\pm$ 8 |  |  |  | 33
151 |  18 20 33.060 $\pm$ 0.001 |  -16 11 21.60 $\pm$ 0.01 |  642 $\pm$ 35 |  642 $\pm$ 35 | ✓ | ✓ |  | 34
152 |  18 20 33.093 $\pm$ 0.001 |  -16 09 40.80 $\pm$ 0.01 |  153 $\pm$ 10 |  739 $\pm$ 37 |  |  |  | 35
153 |  18 20 33.199 $\pm$ 0.002 |  -16 11 20.84 $\pm$ 0.03 |  29 $\pm$ 6 |  29 $\pm$ 6 |  |  |  |
154 |  18 20 33.247 $\pm$ 0.001 |  -16 13 22.92 $\pm$ 0.03 |  109 $\pm$ 18 |  109 $\pm$ 18 |  |  |  |
155 |  18 20 33.324 $\pm$ 0.002 |  -16 11 11.00 $\pm$ 0.03 |  25 $\pm$ 5 |  25 $\pm$ 5 |  |  |  |
156 |  18 20 33.398 $\pm$ 0.002 |  -16 10 18.44 $\pm$ 0.03 |  29 $\pm$ 5 |  34 $\pm$ 6 |  |  |  |
157 |  18 20 33.404 $\pm$ 0.001 |  -16 09 44.40 $\pm$ 0.02 |  65 $\pm$ 7 |  77 $\pm$ 7 |  |  |  |
158 |  18 20 33.438 $\pm$ 0.002 |  -16 10 55.32 $\pm$ 0.03 |  29 $\pm$ 6 |  72 $\pm$ 7 |  |  |  |
159 |  18 20 33.504 $\pm$ 0.001 |  -16 10 44.60 $\pm$ 0.01 |  235 $\pm$ 14 |  352 $\pm$ 18 |  |  |  | 36
160 |  18 20 33.610 $\pm$ 0.001 |  -16 10 21.68 $\pm$ 0.01 |  115 $\pm$ 8 |  136 $\pm$ 9 |  |  |  | 37
161 |  18 20 33.768 $\pm$ 0.001 |  -16 10 46.44 $\pm$ 0.01 |  127 $\pm$ 9 |  176 $\pm$ 10 |  |  |  | 38
162 |  18 20 34.059 $\pm$ 0.001 |  -16 10 28.88 $\pm$ 0.02 |  50 $\pm$ 6 |  65 $\pm$ 7 |  |  |  |
163 |  18 20 34.079 $\pm$ 0.001 |  -16 10 44.52 $\pm$ 0.01 |  83 $\pm$ 7 |  91 $\pm$ 7 | ✓ | ✓ | ✓ |
164 |  18 20 34.209 $\pm$ 0.002 |  -16 10 21.36 $\pm$ 0.03 |  29 $\pm$ 6 |  53 $\pm$ 6 |  |  |  |
165 |  18 20 34.576 $\pm$ 0.001 |  -16 10 23.44 $\pm$ 0.01 |  85 $\pm$ 7 |  93 $\pm$ 7 |  |  |  |
166 |  18 20 34.673 $\pm$ 0.001 |  -16 11 23.40 $\pm$ 0.02 |  67 $\pm$ 7 |  79 $\pm$ 7 |  |  |  |
167 |  18 20 34.888 $\pm$ 0.002 |  -16 13 47.60 $\pm$ 0.03 |  160 $\pm$ 32 |  213 $\pm$ 33 |  |  |  |
168 |  18 20 35.048 $\pm$ 0.001 |  -16 09 48.28 $\pm$ 0.02 |  79 $\pm$ 8 |  135 $\pm$ 10 |  |  |  |
169 |  18 20 35.197 $\pm$ 0.001 |  -16 09 49.64 $\pm$ 0.02 |  72 $\pm$ 8 |  121 $\pm$ 9 |  |  |  |
170 |  18 20 35.479 $\pm$ 0.001 |  -16 11 11.84 $\pm$ 0.02 |  75 $\pm$ 7 |  77 $\pm$ 7 | ✓ | ✓ | ✓ |
171 |  18 20 35.858 $\pm$ 0.002 |  -16 10 18.95 $\pm$ 0.03 |  37 $\pm$ 7 |  69 $\pm$ 8 |  |  |  |
172 |  18 20 36.238 $\pm$ 0.002 |  -16 08 57.43 $\pm$ 0.03 |  54 $\pm$ 11 |  54 $\pm$ 11 |  |  |  |
173 |  18 20 36.449 $\pm$ 0.002 |  -16 08 17.03 $\pm$ 0.03 |  87 $\pm$ 17 |  167 $\pm$ 19 |  |  |  |
174 |  18 20 36.879 $\pm$ 0.001 |  -16 12 14.91 $\pm$ 0.02 |  99 $\pm$ 11 |  232 $\pm$ 15 |  |  |  |
175 |  18 20 37.762 $\pm$ 0.002 |  -16 08 43.03 $\pm$ 0.03 |  75 $\pm$ 15 |  83 $\pm$ 15 |  |  |  |
176 |  18 20 37.803 $\pm$ 0.002 |  -16 11 53.83 $\pm$ 0.03 |  53 $\pm$ 10 |  58 $\pm$ 10 |  |  |  |
177 |  18 20 37.895 $\pm$ 0.002 |  -16 12 19.99 $\pm$ 0.03 |  57 $\pm$ 12 |  58 $\pm$ 12 |  |  |  |
178 |  18 20 39.819 $\pm$ 0.002 |  -16 11 37.47 $\pm$ 0.03 |  57 $\pm$ 12 |  57 $\pm$ 12 |  |  |  |
179 |  18 20 40.895 $\pm$ 0.002 |  -16 10 08.46 $\pm$ 0.03 |  67 $\pm$ 14 |  67 $\pm$ 14 |  |  |  |
180 |  18 20 40.984 $\pm$ 0.002 |  -16 10 08.30 $\pm$ 0.03 |  71 $\pm$ 14 |  72 $\pm$ 14 |  |  |  |
181 |  18 20 43.625 $\pm$ 0.002 |  -16 10 38.57 $\pm$ 0.03 |  133 $\pm$ 26 |  178 $\pm$ 27 |  |  |  |
182 |  18 20 44.343 $\pm$ 0.002 |  -16 10 09.77 $\pm$ 0.03 |  207 $\pm$ 38 |  273 $\pm$ 39 |  |  |  |

  * •

a Chandra X-ray Observatory, (Broos et al., 2007)

  * •

b Two Micron All-Sky Survey, Spitzer Space Telescope Galactic Legacy infrared
Mid-Plane survey. IR counterparts are reported when the sources are detected
at least at one of the available bands.

  * •

c Gaia Data Release 2.

  * •

d Catalog number in RGM2012.