11institutetext: Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69,
53121, Bonn, Germany 22institutetext: Institute of Astronomy, Faculty of
Physics, Astronomy and Informatics, Nicolaus Copernicus University, ul.
Grudziądzka 5, 87-100 Toruń, Poland 33institutetext: Max Planck Insitute for
Astronomy, Königstuhl 17, 69117 Heidelberg, Germany 44institutetext:
Department of Astronomy, University of Florida, P.O. Box 112055, Gainesville,
FL 32611-2055, USA 55institutetext: Tecnologico de Monterrey, Escuela de
Ingeniería y Ciencias, Avenida Eugenio Garza Sada 2501, Monterrey, 64849,
Mexico 66institutetext: Max-Planck-Institut für extraterrestrische Physik,
Giessenbachstrasse 1, D-85748 Garching, Germany 77institutetext: I.
Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln,
Germany 88institutetext: IRAM, 300 rue de la Piscine, Domaine Universitaire de
Grenoble, 38406 St.-Martin-d’Hères, France 99institutetext: Department of
Astrophysics, University of Vienna, Turkenschanzstrasse 17, 1180 Vienna,
Austria

# The Cygnus Allscale Survey of Chemistry and Dynamical Environments: CASCADE.
II. A detailed kinematic analysis of the DR21 Main outflow

I. M. Skretas 11 A. Karska 1122 F. Wyrowski 11 K. M. Menten 11 H. Beuther 33
A. Ginsburg 44 A. Hernández-Gómez 5511 C. Gieser 66 S. Li 33 W.-J. Kim 77 D.
A. Semenov 33 L. Bouscasse 88 I. B. Christensen 11 J. M. Winters 88 A. Hacar
99

(Received May 5, 2023; Accepted Sep 17, 2023)

###### Abstract

Context. Molecular outflows are believed to be a key ingredient in the process
of star formation. The molecular outflow associated with DR21 Main in Cygnus-X
is one of the most extreme, in mass and size, molecular outflows in the Milky
Way. The outflow is suggested to belong to a rare class of explosive outflows
which are formed by the disintegration of protostellar systems.

Aims. We aim to explore the morphology, kinematics, and energetics of the DR21
Main outflow, and compare those properties to confirmed explosive outflows to
unravel the underlying driving mechanism behind DR21.

Methods. Line and continuum emission are studied at a wavelength of 3.6 mm
with IRAM 30 m and NOEMA telescopes as part of the Cygnus Allscale Survey of
Chemistry and Dynamical Environments (CASCADE) program. The spectra include
($J=1-0$) transitions of HCO+, HCN, HNC, N2H+, H2CO, CCH (among others)
tracing different temperature and density regimes of the outflowing gas at
high-velocity resolution ($\sim$ 0.8 km s-1). The map encompasses the entire
DR21 Main outflow and covers all spatial scales down to a resolution of
3$\arcsec$ ($\sim$ 0.02 pc).

Results. Integrated intensity maps of the HCO+ emission reveal a strongly
collimated bipolar outflow with significant overlap of the blue- and red-
shifted emission. The opening angles of both outflow lobes decrease with
velocity, from $\sim 80$ to 20∘ for the velocity range from 5 to 45 km s-1
relative to the source velocity. No evidence is found for the presence of
elongated, $``$filament-like$"$ structures expected in explosive outflows.
N2H+ emission near the western outflow lobe reveals the presence of a dense
molecular structure which appears to be interacting with the DR21 Main
outflow.

Conclusions. The overall morphology as well as the detailed kinematics of the
DR21 Main outflow is more consistent with that of a typical bipolar outflow
instead of an explosive counterpart.

###### Key Words.:

¡Stars: formation - Stars: protostars - Stars: winds, outflows - ISM: jets and
outflows - ISM: kinematics and dynamics - ISM: molecules

##  1 Introduction

Molecular outflows are a ubiquitous part of star formation arising from both
high and low mass protostars (Arce et al. 2007; Frank et al. 2014; Bally
2016). A new type of outflows, formed by the disintegration of protostellar
systems due to a merging event, has been proposed and tied to regions of high-
mass star formation (Bally & Zinnecker 2005; Zapata et al. 2009). The massive
outflow of DR21 Main is one of the proposed candidates for such explosive
outflows (Zapata et al. 2013). The large angular extent of the DR21 Main
outflow allows for a detailed analysis of its structure and properties, and
their comparison to those of other explosive outflow candidates.

Molecular protostellar outflows range from highly collimated molecular jets
like HH211 (Gueth & Guilloteau 1999) all the way to wide-angled outflows from
high mass sources (Beuther & Shepherd 2005a). In general, outflows tend to
appear “narrower” at higher velocities (Bachiller & Tafalla 1999), and become
less collimated as they evolve (Beuther & Shepherd 2005b; Arce & Sargent 2006;
Offner et al. 2011; Hsieh et al. 2023). They can vary significantly in size
and energetics, with sizes from 0.1 pc up to pc scales and momentum rates
between 10-5 and 10-2 M⊙ km s-1 yr-1, for low mass sources and some O type
stars, respectively (e.g., Maud et al. 2015). Some outflow properties, such as
their mass, force and mechanical luminosity, correlate well with intrinsic
parameters of their driving sources, e.g., the bolometric luminosity (Bally &
Lada 1983; Cabrit & Bertout 1992; Wu et al. 2004) and the mass of the
molecular gas envelope of their driving source (Bontemps et al. 1996; Beuther
et al. 2002), suggesting a common driving mechanism in both the low and high
mass sources. Clearly, molecular outflows play a critical role in regulating
star formation by removing excess angular momentum and thus facilitating the
further mass growth of a protostellar system (Blandford & Payne 1982; Machida
2014), and partly in dispersing the surrounding envelope, reducing the
available mass reservoir (Arce & Sargent 2006).

Due to their large size and the energy they carry, molecular outflows can have
a significant impact to the surrounding interstellar medium (ISM) over
different spatial scales. Firstly, at envelope scales (103-104 AU), powerful
young outflows entrain and clear-out dense material giving rise to bipolar
cavities (e.g. Gueth et al. 1997; Velusamy & Langer 1998; Arce & Sargent 2004,
2005). At core scales (0.1–0.3 pc), outflows are considered a significant
contributor to the turbulence (Myers et al. 1988; Zhang et al. 2005). In
addition, outflows from high mass young stellar objects (YSOs) might impact
the morphology and even break apart the host molecular cloud (Fuente et al.
1998; Benedettini et al. 2004). Finally, the propagation of outflows through
the surrounding dense material leads to the formation of shocks, which locally
compress and heat the gas, and drive chemical processes enriching the ISM
(e.g. Kaufman & Neufeld 1996; Flower & Pineau Des Forêts 2010; Burkhardt et
al. 2019).

A newly proposed type of molecular outflows are the so–called “explosive
dispersal outflows”, whose origin appears to linked to the disintegration of
young stellar systems (Bally et al. 2017; Rivera-Ortiz et al. 2021) or to
protostellar mergers (Bally & Zinnecker 2005). The interpretation is limited
due to the small sample of explosive-outflow candidates: Orion-KL (Zapata et
al. 2009), DR21 Main (Zapata et al. 2013), G5.89 (Zapata et al. 2019), and
IRAS 16076-5134 (Guzmán Ccolque et al. 2022). Nevertheless, these explosive
outflows share the following characteristics (Zapata et al. 2009, 2017): (i)
they consist of multiple straight, narrow and relatively isotropically
distributed filament-like structures; (ii) these filament-like structures
should all point towards the origin point of the explosive outflow and show an
increase in velocity with the distance from the origin point akin to a Hubble
flow; (iii) have a significant overlap of their blue- and red-shifted emission
components. The filament-like structures of explosive outflows form because
all material is simultaneously accelerated in the explosion. As a result,
faster moving material has traveled further away from the source and is
trailed by the slower parts of the outflow. Overall, the properties of those
outflows have been mostly studied using low$-J$ CO transitions at high angular
resolution (e.g. Zapata et al. 2009, 2013, 2019). The inner parts of the
molecular outflows revealed multiple filament-like structures that make up the
explosive outflows. At the same time, the lack of similar observations in
other molecular tracers limits our understanding of their chemistry and
various physical gas components.

The DR21 Main outflow is a particularly interesting explosive outflow
candidate (Zapata et al. 2013), as it is one of the most massive
($M_{\mathrm{out}}>3000$ M⊙) and energetic ($E_{\text{kin}}>$ 2 $\times
10^{48}$ erg) outflows detected in our Galaxy (Garden et al. 1986, 1991b),
first in vibrationally excited $2.12\leavevmode\nobreak\ \mu$m line of shock-
excited molecular hydrogen (H2). DR21 Main itself is a compact HII region
prominent at radio wavelengths. It is located in the Cygnus-X high-mass star-
forming region/molecular cloud complex (Leung & Thaddeus 1992), at the
southern end of the DR21 molecular ridge (Dickel et al. 1978), and at a
distance of 1.5 kpc (Rygl et al. 2012). The outflow appears bipolar, with the
outflow lobes extending in a East–West direction (Garden et al. 1986, 1991a;
Garden & Carlstrom 1992; Schneider et al. 2010). High velocity low$-J$ CO
emission has also been reported in the North–South direction (Garden et al.
1991b). The blue- and red-shifted parts of the outflow overlap significantly,
suggesting that it extends very close to the plane of the sky (Cruz-González
et al. 2007). It was initially suggested that the DR21 Main outflow is driven
by a massive protostar, with $L_{\mathrm{bol}}$ of $\sim 10^{5}-10^{6}$
$L_{\odot}$ (Garden et al. 1991b; Garden & Carlstrom 1992), but such a source
has not been yet identified (Cruz-González et al. 2007). The absence of a
clearly detected driving source, along with the detection of some filament-
like structures in CO (1 – 0) emission, led Zapata et al. (2013) to suggest a
possible explosive nature for the DR21 Main outflow.

In this work, we aim to study the morphology, kinematics and energetics of the
DR21 Main outflow using observations in multiple molecular lines, sensitive to
a range of physical conditions. We also aim to determine whether those
characteristics of the DR21 Main outflow are consistent with those expected
for explosive outflows or, rather, for typical protostellar outflows.

This work is a part of the Max Planck IRAM Observatory Program (MIOP) ”Cygnus
Allscale Survey of Chemistry and Dynamical Environments (CASCADE)” (Beuther et
al. 2022). CASCADE aims to map significant parts of the Cygnus-X molecular
cloud complex at high angular resolution and with a broad bandpass using the
Northern Extended Array for Millimeter Astronomy (NOEMA) and the 30 m
telescope, both operated by the Institut de Radioastronomie Millimètrique
(IRAM). The combination of velocity-resolved single dish and interferometric
observations offers the high resolution necessary to resolve the outflow
structure without losing information on extended emission. CASCADE aims to
take advantage of these high quality observations to connect the transition of
gas all the way from the large scales of molecular clouds down to the small
scales of cores, to look for signs of collapse or feedback, to investigate the
impact of star-forming cores to their surrounding, and to search for possible
trends with evolutionary stage and more. The scope and goals of CASCADE are
discussed in detail by Beuther et al. (2022).

Table 1: Continuum and spectral line parameters for all lines covered by the
CASCADE observations

Species | Transition |  | Frequency
---
[GHz]
| $\sigma_{\text{rms}}$
---
$\left[\frac{\rm\text{mJy}}{\rm\text{beam}}\right]$
| Beam
---
[arcsec]
| Eup
---
[K]
| Log(Aij)
---
[s-1]
Continuum | - | 82.028 | 0.05 | 2.80 $\times$ 2.54 | - | -
DCO+ | (1-0) | 72.039 | 15 | 3.57 $\times$ 3.19 | 3.46 | -4.16
CCD | (1-0) | 72.108 | 15 | 3.57 $\times$ 3.19 | 3.46 | -6.06
DCN | (1-0) | 72.415 | 16 | 3.56 $\times$ 3.18 | 3.47 | -4.88
SO2 | (60,6-51,5) | 72.758 | 14 | 3.54 $\times$ 3.16 | 19.15 | -5.56
HCCCN | (8-7) | 72.784 | 14 | 3.54 $\times$ 3.16 | 15.72 | -4.53
H2CO | (10,1-00,0) | 72.838 | 16 | 3.90 $\times$ 3.28 | 3.50 | -5.09
CH3CN | (4k-3k) | 73.590 | 12 | 3.51 $\times$ 3.13 | 8.83 | -4.66
DNC | (1-0) | 76.306 | 10 | 3.24 $\times$ 2.90 | 3.66 | -4.79
CH3OH | (50,5-41,3)E | 76.510 | 10 | 3.24 $\times$ 2.89 | 47.93 | -6.05
NH2D | (11,1-10,1) | 85.926 | 10 | 2.77 $\times$ 2.51 | 20.68 | -5.71
H13CN | (1-0) | 86.340 | 9 | 2.75 $\times$ 2.50 | 4.14 | -4.65
H13CO+ | (1-0) | 86.754 | 7 | 2.74 $\times$ 2.49 | 4.16 | -4.41
SiO | (2-1) | 86.847 | 11 | 3.25 $\times$ 2.70 | 6.25 | -4.53
HN13C | (1-0) | 87.091 | 9 | 2.73 $\times$ 2.48 | 4.18 | -4.73
CCH | (1-0) | 87.329 | 10 | 2.72 $\times$ 2.47 | 4.19 | -5.90
HNCO | (40,4-30,3) | 87.925 | 8 | 2.71 $\times$ 2.45 | 10.55 | -5.06
HCN | (1-0) | 88.632 | 13 | 3.18$\times$ 2.64 | 4.25 | -4.62
HCO+ | (1-0) | 89.189 | 10 | 3.29 $\times$ 2.74 | 4.28 | -4.38
HNC | (1-0) | 90.664 | 11 | 3.12 $\times$ 2.57 | 4.35 | -4.57
HCCCN | (10-9) | 90.979 | 9 | 2.64 $\times$ 2.39 | 24.01 | -4.24
CH3CN | (5k-4k) | 91.987 | 9 | 2.61 $\times$ 2.36 | 13.24 | -4.85
H41$\alpha$ | - | 92.034 | 6 | 2.60 $\times$ 2.36 | - | -
13CS | (2-1) | 92.494 | 10 | 2.59 $\times$ 2.35 | 6.66 | -4.85
N2H+ | (1-0) | 93.174 | 11 | 2.58 $\times$ 2.33 | 4.47 | -4.44
111 The rms noise level, $\sigma_{\text{rms}}$, is calculated for channels of
0.8 km s-1 except for the case of H41$\alpha$ for which the channel width is
3.0 km s-1. Rest frequencies are from NIST recommended rest frequencies (Lovas
2004) and the Cologne Database for Molecular Spectroscopy (CDMS, Müller et al.
(2001)), while Eup and Aij are taken from CDMS. The conversion factor from
Jy/beam to K, in the Rayleigh-Jeans limit, is $\sim$ 23.75 for a frequency of
89.189 GHz.

Figure 1: United Kingdom Infra-Red Telescope (UKIRT) Wide Field Camera (WFCAM)
continuum image of the DR21 Main region at 2.2 $\mu$m (Warren et al. 2007) and
the line emission in key gas tracers observed as part of CASCADE. Shock
excited H2 emission makes a significant contribution to the 2.2 $mu$m image,
in particular to the lobes off the central region. White contours mark the
5$\sigma$ HCO+ (left), N2H+ (middle) and SiO (right) emission.Intensities are
integrated between -50 and 50, -20 and 10 ,and -20 and 20 km s-1 for the HCO+,
N2H+ and SiO emisison respectively. The adopted origin point of the outflow is
marked in all cases with a red cross. Red dashed lines (middle) mark the three
separate areas of the DR21 Main region used to extract the spectra in Fig. 2,
with “E” marking the eastern outflow lobe, “C” the central area and “W” the
western outflow lobe. The magenta dashed box (right) marks the location of the
interaction region shown in Fig. 11. Finally orange contours (right) show the
5$\sigma$ integrated intensity of H41$\alpha$ from -30 to 30 km s-1. Table 2:
Molecular line detections (at 5$\sigma$ level) in the area of the DR21 Main
outflow

Location | Species
---|---
Outflow (E – W) | HCO+, HCN
DR21 ridge - Dense gas (N – S) |  13CS, CCH, H2CO, H13CO+, HCCCN, HNC, N2H+, H13CN, HN13C
Sporadic or compact emission | CH3CNaa$a$Only a 3$\sigma$ detection., CH3OH, DCN, DCO+, DNC, H41$\alpha$, NH2D, SiO
Non-detections | CCD, HNCO, SO2

222

Figure 2: Averaged line profiles of the (1 – 0) transitions of HCO+, HCN, HNC,
N2H+, and the (2 – 1 ) transition of SiO toward the east lobe, center, and
west lobe of the DR21 Main outflow (see Fig. 1). For HCN and N2H+, the
velocities corresponding to hyperfine structure components are marked with
black ticks. Red dashed lines show the source velocity, $-3$ km s-1, green
dashed lines mark the location of the absorption feature at 9 km s-1 and the
grey horizontal lines show the baselines.

The paper is organized as follows. Section 2 describes the observations from
CASCADE. Section 3 presents line detections and maps of the DR21 Main outflow
in several molecular transitions, and provides the analysis of outflow
properties. In Section 4 the results are discussed and scenarios for the
origin of the DR21 Main outflow are explored along with its interactions with
the surrounding molecular cloud. Finally, Section 5 contains the summary and
conclusions.

##  2 Observations

A detailed overview of the CASCADE program is given in Beuther et al. (2022).
In brief, CASCADE covers all high column density areas in the Cygnus-X
molecular cloud complex using 40 mosaics, each covering 16 arcmin2. Each of
the mosaics corresponds to 78 NOEMA pointings and was observed in both the C
and D configurations. The observations have a total bandwidth of 16 GHz, 8 in
each sideband, at the 3.6 mm window. The full bandwidth is covered with a
spectral resolution of 2.0 MHz, but selected parts, surrounding the most
important lines, are also covered by additional high resolution correlator
units providing a spectral resolution of 62.5 kHz. The DR21 Main outflow is
covered by two of the NOEMA mosaics, which were observed between 2020 May 29
and November 6. During that time, the array consisted of 10 antennas, yielding
baselines between 15 m and 365 m. The strong quasars 3C345 and 3C273 were used
as bandpass calibrators, MWC349 and 2010+723 were used for flux calibration
and 2005+403, 2120+445, 2050+363 and 2013+370 were used for gain calibration.
Complementary single-dish observations were carried out with the IRAM 30m
telescope between 2020 February and July, in order to provide the missing
short spacing information. These observations will be presented in detail in
an upcoming paper by Christensen et al. (in prep.).

The calibration and imaging of the data was done using CLIC and MAPPING
software, which are part of the GILDAS
package333https://www.iram.fr/IRAMFR/GILDAS/). The NOEMA observations are
combined with the IRAM 30m data using the UV_SHORT task. The resulting single
channel $\sigma_{\text{rms}}$ noise, for a channel width of 0.8 km s-1, and
beam sizes for all observed lines are summarized in Table 1.

Figure 3: Channel maps of the DR21 Main outflow in HCO+. Contours of the blue-
shifted HCO+ emission (in blue) and red-shifted emission (in red) are
integrated over velocity steps of 5 km s-1 and are plotted over the
corresponding gray-scale. The full velocity range is from 5 to 45 km s-1
relative to source velocity ($-$3 km s-1) and the contour levels correspond to
5, 10 and 20 $\sigma_{\text{rms}}$. The black, dashed circle shows the area of
the DR21–1 core in Cao et al. (2019). The green dashed lines denote the half-
circles used to derive the outflow opening angles (see Fig. 7). Figure 4: HCO+
emission integrated from -50 to 50 km s-1. Red and blue contours mark the
redshifted (5 to 45 km s-1) and blueshifted ($-$5 to $-$45 km s-1) HCO+
emission respectively. Contours correspond to 5 $\sigma_{\text{rms}}$ emission
and the velocity ranges are given relative to the source velocity ($-$3 km
s-1). Black arrows mark the cuts for the PV diagrams (Figs. 5 and 6) and the
ticks mark distances of 50 arcseconds along the arrows. Figure 5: Position –
velocity diagram for HCO+ emission along the DR21 Main outflow. The offset is
measured from the edge of the eastern lobe towards the west. The arrow points
to the 9 km s-1 absorption feature (Dickel et al. 1978), the white arrow
highlights the location of the interaction region (this work), the magenta
dashed contour marks the H41$\alpha$ emission from the H ii region, and the
light green arrows mark examples of sawtooth pattern structures (Santiago-
García et al. 2009).

##  3 Results and analysis

We present the CASCADE observations for an area surrounding DR21 Main that
covers the entirety of its outflow. Our data allow us to analyze the
kinematics, morphology and energetics of the DR21 Main outflow and to
contribute to a discussion about its nature.

###  3.1 Molecular detections

Several molecular lines are detected in the CASCADE observations of the DR21
Main (see Table 2). The spatial distribution of the emission can be divided
into three cases: (i) tracing the outflow (extended emission in the west–east
(W–E) direction), (ii) tracing the DR21 ridge (extended emission in the
north–south (N–S) direction) , and (iii) sporadic (compact emission that
appears in multiple locations) or compact emission (see Fig. 1). The different
tracers can therefore be used to examine the morphology of the various gas
components in DR21 Main. Integrated intensity contour maps for all the
emission lines are shown in Appendix A.

The contour map of the HCO+ integrated intensity (Fig. 1, left panel) shows
that most of the emission arises from the area of the outflow lobes and also
appears to be in close agreement with the H2 emission at 2.2 $\mu$m (see also
Garden et al. 1986; Davis et al. 2007), associated with outflowing shocked
gas. Interestingly, the HCO+ (1 – 0) contours reveal also the presence of
hollowed out cavities in both outflow lobes lacking line emission, similar to
early findings by Garden & Carlstrom (1992). The cavities are more prominent
in the eastern outflow lobe, which appears entirely separated from the center
of DR21 Main area. Therefore, while HCO+ is well associated with the
outflowing material, it traces most accurately the outer parts of the outflow
cavities. In a similar fashion, line emission in HCN (1 – 0) is also
associated with the outflow, but shows a more compact pattern in the direction
of the peaks of HCO+ emission (see Fig. 18).

In contrast, the N2H+ emission appears to trace the DR21 ridge along the N–S
direction (the middle panel of Fig. 1, Wilson & Mauersberger 1990; Motte et
al. 2007). This is expected as N2H+ is known to trace dense, cold, CO depleted
gas (Caselli et al. 2002; Jørgensen et al. 2004). In addition to the ridge,
N2H+ reveals also the presence of a molecular structure near the western lobe.
This structure was previously detected in CS (2 – 1) by Plambeck & Menten
(1990) who also reported the detection of a collisionally excited class I
methanol maser in its interaction region with the outflow traced by H2
emission. Other molecules that are often associated with dense gas, like CCH
(1 – 0), HCCCN (both the (10 – 9) and (8 – 7) transitions), 13CS (1 – 0) and
HNC (1 – 0) show distribution similar to that of N2H+ (1 – 0) (see Figs. 18,
19 and 20).

Finally, SiO is detected close to the center of the outflow, but also shows
very localized emission in an area of the western outflow lobe (Fig. 1, right
panel). Even though SiO typically traces shocks in the ISM (e.g. Martin-
Pintado et al. 1992; Schilke et al. 1997; Gusdorf et al. 2008), its emission
peak in the center of DR21 might also originate from photo-evaporating ice
mantles in the dusty envelope of a driving source(s) (e.g. Walmsley et al.
1999; Schilke et al. 2001). On the other hand, the SiO emission detected near
the western lobe could result from the interaction of the outflow and the the
dense structure seen, for example, in N2H+, located there. This scenario is
discussed in more detail in Sec 4.3.

Figure 6: Position – velocity diagrams for HCO+ (top) and N2H+ (bottom)
emission along the DR21 ridge. The offset is measured from South to North (see
Fig. 4 for the exact location of the cut). the magenta dashed contour marks
the H41$\alpha$ emission from the H ii region while the orange dashed lines
mark the source velocity ($-$3 km s-1). For the N2H+ line, the velocities of
the three resolved hyperfine structure components are marked at 4, $-$3 and
$-$10 km s-1, respectively.

Figure 2 shows the profiles of the strongest lines, averaged over the eastern
outflow lobe, the central area of DR21 Main and the western outflow lobe (Fig.
1). Similar spectra for the rest of the lines are presented in Appendix B. The
peak of the line emission lies at a velocity of $-$3 km s-1, which corresponds
to the velocity of the DR21 ridge (Dickel et al. 1978). The 9 km s-1 feature,
caused by more diffuse foreground material in the so-called “extended W75
cloud” (Dickel et al. 1978; Nyman 1983) can also be seen in some of the lines,
but is most prominent in HCO+ and HCN. The profiles of both the HCO+ and HCN
lines display extended line wings and strong emission in both outflow lobes,
thus confirming their association with the outflowing material. In contrast,
emission from molecules associated with the denser gas and the bulk of the
DR21 ridge, like N2H+ or HNC have narrower emission lines and relatively
weaker emission from the outflow lobes.

In summary, the CASCADE observations offer a clear view of the different gas
components in the DR21 Main area. Most importantly, the HCO+ is found to trace
well the molecular outflow, N2H+ highlight the dense filament while SiO
emission suggests the possibility of interaction between the outflow and the
surrounding ISM, a scenario further explored in Section 4.3. The release of a
full line list, including unidentified lines for all targets of the CASCADE
survey, will be presented in a future paper of the collaboration.

###  3.2 The molecular outflow of DR21 Main

The HCO+ emission is one of the best tracers of the molecular outflow in DR21
Main and its distribution closely follows that of the H2 emission (Section
3.1, Fig. 1). Therefore, we use it to explore the kinematics as well as the
outflow morphology at different velocities. In particular, we investigate the
change of the opening angle with gas velocity.

Figure 3 shows the spatial distribution of the HCO+ (1 – 0) emission
integrated over velocity intervals of 5 km s-1, in the range from 5 and 45 km
s-1 relative to source velocity ($v_{\text{source}}=-3\text{ km s}^{-1}$) for
the red-shifted emission, and from $-5$ to $-45$ km s-1, for the blue-shifted
emission. Most of the HCO+ emission is elongated in the West–East direction,
tracing the outflow lobes of a bipolar outflow (Fig. 1). Some emission extends
also in the North–South direction in a narrow range of velocities, suggesting
that it is associated with the DR21 ridge. However, some of this emission
might also originate from the outflowing gas, as suggested by CO (1 – 0) maps
(Garden et al. 1991b).

Overall the velocity-channel maps (Fig. 3) show a rather symmetric morphology,
but some small asymmetries can be noted. Namely, the blue-shifted part of the
outflow appears stronger and extends to higher velocities than its red-shifted
counterpart. Similar behaviour is also seen between the two lobes of the
outflow, with the western lobe appearing both brighter and having higher
velocities that the eastern one. These small asymmetries are likely to arise
due to the relative position of the outflow driving source compared to the
bulk of material in the surrounding ISM. In addition, the higher velocity HCO+
emission seems to be detached from the origin point of the outflow (see also,
Garden & Carlstrom 1992). Due to the higher angular resolution of the current
observations, we find cleared-out cavities in the outflow lobes. Finally, the
known overlap of the blue- and red-shifted parts of the DR21 Main outflow,
indicating that the outflow extends close to the plane of the sky, is clearly
seen (see Fig. 4, and Schneider et al. 2010). We note, however, that we cannot
estimate the inclination of the outflow more precisely because to the
complexity of the ISM surrounding DR21 Main, e.g., the interaction region.

Figure 5 shows the position–velocity diagram of HCO+ along the DR21 Main
outflow illustrating several key outflow structures. Firstly, the bright
negative peak near the offset of 200$\arcsec$ indicates the location of the
interaction region, where material is deflected into the line of sight.
Secondly, an extended absorption feature is detected near the middle of the
outflow, which corresponds to the H ii region. Thirdly, the known absorption
feature at 9 km s-1, caused by more diffuse foreground material associated
with W75, is also detected along the entire length of the outflow (Dickel et
al. 1978). Finally, several structures are detected in the less disrupted
Eastern lobe, which resemble a sawtooth pattern associated with the extremely
high velocity component (EHV) of the low mass protostar IRAS 04166+2706
(Santiago-García et al. 2009). A first estimate using approximate values,
derived from the PV diagram, for the maximum velocity of the structures ($\sim
20$ km s-1) and the separation between them ($\sim 10\arcsec$) yields upper
limits for the timescales between these knots of the order of $\sim 10^{3}$
yrs. This result corresponds to the upper limits of the timescales found
between knots in the outflows of W43–MM1 (Nony et al. 2020).

Figure 6 compares the velocities of HCO+ with those of N2H+ in the direction
of the DR21 ridge. N2H+ is exclusively associated with the ridge (Fig. 1) and
shows three peaks corresponding to its hyperfine-splitted lines. All those
components show a velocity gradient along the DR21 ridge, with velocities
becoming increasingly blue-shifted North of DR21 Main. A similar gradient
cannot be probed in the corresponding HCO+ position–velocity diagram (Fig. 6)
due to the complex line profiles. Similar to Figure 5, a strong central
absorption feature exists, associated with the H ii region. In addition, the
extended emission in the North – South is detected as a blue-shifted structure
between offsets 50$\arcsec$ and 100$\arcsec$. The lack of corresponding red-
shifted emission favors the scenario that this emission is associated with the
ridge and not an additional outflow, extending in the N–S direction.

An important characteristic often used to describe outflows is their opening
angle, a measure of how wide or collimated an outflow actually is. Here we
calculated the opening angles for the DR21 Main outflow, separately for each
lobe and for red- and blue-shifted emission, by examining the spectra of HCO+
emission along a half-circle with radius approximately equal to half the
extent of the corresponding outflow lobe (The exact location of these half-
circles is shown in Fig. 3). The opening angle then corresponds to the angle
between the location where the emission first becomes significant and the
location where it drops further to noise level. The resulting opening angles
for all different cases and for different velocities are plotted in Fig. 7
over the corresponding velocity. Interestingly, the opening angles for all
cases appear to be decreasing for higher velocities, a behavior that is
expected in the case of a typical bipolar outflow, powered by a narrow and
well collimated jet (e.g. Zhang et al. 2019; Rabenanahary et al. 2022).

Figure 7: Opening angle of the DR21 Main outflow as a function of velocity
using HCO+ 1-0 line profiles. The angles are calculated every 5 km s-1 from
the source velocity up to $v_{\mathrm{max}}=22,32,-38$ and $-48$ km s-1 for
the east-red, west-red, east-blue and west-blue outflow lobe respectively.
Top: Opening angles for blue-shifted outflow velocities. Bottom: Opening
angles for the red-shifted outflow velocities.

###  3.3 Energetics of the outflow

The spatially- and velocity-resolved observations of the DR21 Main outflow
allow us to calculate key outflow properties such as, the outflow force, $F$,
the rate at which the outflow injects momentum into its surrounding
interstellar medium, the outflow mass, $M$, and its kinetic energy,
$E_{\text{kin}}$. For all calculations, we use the HCO+ (1 – 0) emission found
to trace well the molecular outflow (Section 3.1).

Table 3: Inclination correction factors used in the different methods of
outflow force calculation

$i$aa$a$$i$ is measured from the line of sight. ($\degr$) | 10 | 30 | 50 | 70 | Ref
---|---|---|---|---|---
$c_{1}$ | 0.28 | 0.45 | 0.45 | 1.1 | 1,2
$c_{2}$ | 1.6 | 3.6 | 6.3 | 14 | 1
$c_{3}$bb$b$Values are interpolated from Table 6 of Downes & Cabrit (2007) | 0.6 | 1.3 | 2.4 | 3.8 | 3

(1) Cabrit & Bertout (1990); (2) Cabrit & Bertout (1992); (3) Downes & Cabrit
(2007) 444 , where $\alpha=90-i$.

Table 4: Outflow parameters of the DR21 Main outflow |  | $v_{\text{max}}$
---
[km s-1]
| $R_{\text{lobe}}$
---
[pc]
| $t_{\text{dyn}}$
---
[yr]
| $M$
---
[M⊙]
| $P$
---
[M⊙ km s-1]
| $E_{\text{kin}}$
---
[erg]
| $\dot{M}$
---
[M⊙ yr-1]
| $F$
---
[M⊙ km yr-1 s-1]
| $L_{\text{kin}}$
---
[erg yr-1]
East lobe: red | 38.6 | 0.74 | 4900 | 7 | 87 | 1.2 $\times$ 1046 | 0.002 | 0.02 | 4.87 $\times$ 1042
blue | -62.2 | 0.75 | 3100 | 47 | 669 | 2.4 $\times$ 1047 | 0.015 | 0.22 | 1.57 $\times$ 1044
West lobe: red | 52.2 | 0.88 | 4300 | 13 | 135 | 1.8 $\times$ 1046 | 0.003 | 0.03 | 8.14 $\times$ 1042
blue | -70.2 | 0.98 | 3600 | 57 | 1037 | 2.3 $\times$ 1047 | 0.016 | 0.29 | 1.26 $\times$ 1044
East+West: red | – | – | – | 20 | 222 | 3.0 $\times$ 1046 | 0.005 | 0.05 | 1.30 $\times$ 1043
blue | – | – | – | 104 | 1706 | 4.7 $\times$ 1047 | 0.031 | 0.51 | 2.83 $\times$ 1044
Entire outflow | – | – | – | 124 | 1928 | 5.0 $\times$ 1047 | 0.036 | 0.56 | 2.96 $\times$ 1044

To measure the force of the DR21 Main outflow, we use the so-called separation
method introduced in van der Marel et al. (2013), where the outflow force is
calculated as:

$F_{\text{HCO}^{+}}=c_{3}\times\frac{\displaystyle
K\left(\sum_{\text{j}}\left[\int_{v_{\text{in}}}^{v_{\text{out,j}}}T(v^{\prime})v^{\prime}\text{d}v^{\prime}\right]_{\text{j}}\right)v_{\text{max}}}{\displaystyle
R_{\text{lobe}}}.$ (1)

Here, $c_{3}$ is a correction factor for a given inclination angle of the
outflow (Table 3), $K$ is a conversion factor between the line integrated
intensity and the molecular gas mass, the integral
$\int_{v_{\text{in}}}^{v_{\text{out,j}}}T(v^{\prime})v^{\prime}\text{d}v^{\prime}$
corresponds to the velocity weighted integrated intensity, $v_{\text{max}}$ is
the maximum line-of-sight velocity in the outflow lobe, and $R_{\text{lobe}}$
is the length of the outflow lobe, while the sum runs over all pixels (j) that
are part of the outflow. The conversion factor $K$ (see Appendix C of van der
Marel et al. 2013) is given by:

$K=\mu m_{\text{H}}A\frac{8\pi
k_{\text{B}}\nu^{2}}{hc^{3}A_{\text{ul}}}\left[\frac{\text{H}_{2}}{\text{HCO${}^{+}$}}\right]\frac{Q{(T_{\text{exc}})}}{g_{\text{u}}}\text{e}^{E_{\text{u}}/T_{\text{exc}}}$
(2)

where $\mu$ is the mean molecular weight, $m_{\text{H}}$ is the hydrogen mass,
$A$ is the observed area of the outflow, $[\text{H}_{2}/\text{HCO}^{+}]$ is
the abundance ratio between H2 and HCO+, $Q{(T_{\text{exc}})}$ is the
partition function at a specific excitation temperature $T_{\text{exc}}$,
$g_{\text{u}}$ is the degeneracy of the upper level of the observed
transition, $E_{\text{u}}$ is the upper level energy in Kelvins, $\nu$ is the
frequency of the observed transition in Hz, $c$ is the speed of light,
$k_{\text{B}}$ is Boltzmann’s constant, $h$ is Planck’s constant and
$A_{\text{ul}}$ is Einstein A coefficient for the transition in s-1. We assume
a single excitation temperature of 40 K (Garden et al. 1991b). The abundance
ratio of H2 over HCO+ in high-mass star-forming regions has been found to
range between 2 $\times$ 109 down to 3 $\times$ 107 (Godard et al. 2010;
Gerner et al. 2014). In this work, we adopt an abundance ratio of H2 over HCO+
of $1.6\times 10^{8}$ (Garden & Carlstrom 1992), which is well within the
above range and was estimated for DR21 Main. The corresponding value of the
partition function and the remaining molecular data are taken from
Splatalogue555https://splatalogue.online/advanced1.php using the CDMS
catalogue (Müller et al. 2001).

The inner velocities for the integration ($v_{\text{in}}$) are $-$5 and 5 km
s-1 relative to the source velocity for the blue- and red-shifted parts of the
emission, excluding the innermost 10 km s-1 in order to avoid contamination
from the cloud material.

Figure 8: 3mm continuum emission in the area of DR21 Main. The peak of the
continuum emission is marked with a red star, while the green circle marks the
location, and the green dashed line the FWHM, of the dense core DR21-1 (Cao et
al. 2019). Blue contours mark the integrated HCO+ intensity (-50 to 50 km
s-1), while the beam of the continuum observations is noted with the red
ellipse.

The calculation of the length of the outflow lobe, $R_{\mathrm{lobe}}$,
requires information about the origin point of the outflow. Here, we adopt it
as the position of the dense core DR21-1 from Cao et al. (2019) (see also core
N46 in Motte et al. 2007), which is located close to the center of the two
outflow lobes and at the DR21 ridge. The location of the core agrees also well
with the peak of the 3mm continuum emission, as shown in Fig. 8. Higher
resolution observations of this area would be required to determine the exact
location and nature of the driving source, which is outside of the scope of
this paper.

Figure 9: Distribution of gas velocities associated with the outflows in DR21
(left) and Orion KL (right, Bally et al. 2017). Each point shows a local
intensity peak of the HCO+ emission (above 2 $\sigma$) integrated in velocity
steps of 5 km s-1. The color of the points signifies the corresponding
velocity steps and are shared for the two plots. Background grey-scale shows
the integrated HCO+ (left panel) and CO emission (right panel) over the entire
velocity range of the outflows (from $-$70 to 70 km s-1 for DR21 Main and
$-$100 to 100 km s-1 for Orion KL).

The gas mass carried by the outflow is obtained from:

$M=K\left(\sum_{\text{j}}\left[\int_{v_{\text{in}}}^{v_{\text{out,j}}}T(v^{\prime})\text{d}v^{\prime}\right]_{\text{j}}\right).$
(3)

Subsequently, the time-averaged kinetic energy of the outflow is calculated
as:

$E_{\text{kin}}=\frac{1}{2}M\langle v\rangle^{2},$ (4)

and its momentum as:

$P=\displaystyle
K\left(\sum_{\text{j}}\left[\int_{v_{\text{in}}}^{v_{\text{out,j}}}T(v^{\prime})v^{\prime}\text{d}v^{\prime}\right]_{\text{j}}\right).$
(5)

The dynamical time, which is an estimate of the lifetime of the outflow, is
then measured from:

$t_{\text{dyn}}=\frac{R_{\text{lobe}}}{v_{\text{max}}}$ (6)

This, in turn, allows the calculation of the mass loss rate:

$\dot{M}=\frac{M}{t_{\text{dyn}}},$ (7)

and the power of the outflow:

$L_{\text{kin}}=\frac{E_{\text{kin}}}{t_{\text{dyn}}}.$ (8)

The calculations are performed for the east and west outflow lobes, and for
the red- and blue-shifted emission, separately. The resulting outflow
properties are presented in Table 4.

The outflow mass and kinetic energy can be compared with the results from
Garden & Carlstrom (1992), where observations of the transition of HCO+ (1 –
0) were analyzed. Here, we use the high velocity component from Garden et al.
(1991b) and scale it to the same distance of DR21, as adopted in this work.
The outflow mass for the high-velocity component, of $\sim$120 M⊙ (see Table
4) is a factor of 2-5 higher than the corresponding outflow mass in Garden &
Carlstrom (1992). The outflow extent and the area covered by the observations
are similar in both studies; Garden et al. (1991b) obtain the
$R_{\mathrm{lobe}}$ of $\sim$1.7 pc. Thus, the difference is likely due to the
velocity limits adopted in Garden & Carlstrom (1992), which exclude a
significant part of gas mass at velocities close to the source velocity
($v_{\mathrm{in}}$ from $-$12.5 to $-$42.5 km s-1). In fact, the outflow
kinetic energy, which accounts for the relevant range of velocities, is fully
consistent: $E_{\text{kin}}$ of 5.0$\times 10^{47}$ erg (Table 4) is within
the range of $\sim$2.5–5.0 $\times 10^{47}$ erg reported in Garden & Carlstrom
(1992) using HCO+ and about a factor of 4 lower than the total energy measured
using CO (Garden et al. 1991b). For a more thorough discussion of the DR21
Main outflow properties, with respect to both low- and high-mass protostars,
see Section 4.2.

The calculation of the outflow parameters includes a few assumptions that need
to be addressed. Firstly, the conversion factor $K$ is accurate only when the
observed emission is optically thin. In the case of DR21 Main, the HCO+
emission is optically thin in the outflow lobes, but not in the central area
(Garden & Carlstrom 1992). Using our H13CO+ observations, which appear to
trace the dense ridge (Section 3), we estimate a $\tau$ of $12.5$ for the DR21
Main center. For that, the central region is excluded from the calculation of
the outflow parameters. Similarly, gas with velocities within $\sim$5 km s-1,
from the velocity of the N–S filament is also excluded. Noteworthy, even
though a significant part of the outflow material is often found at low
velocities, its impact on $F$ or $E_{\text{kin}}$ is not as significant due to
the dependence of those parameters on $v^{2}$. Secondly, the correction factor
of inclination angle, $c_{3}$, is available only for the inclination angles of
10$\degr$, 30$\degr$, 50$\degr$, and 70$\degr$ (Table 3). In this work,
$c_{3}$ of 3.8 is assumed (corresponding to 70$\degr$), but since the outflow
of DR21 Main appear to be close to 90$\degr$ (see Section 3.2), the correction
is most likely underestimated by a factor of $\lesssim 2$. Finally, we assumed
a uniform excitation temperature of the gas along the outflow, which likely
differs by a factor of a few depending on the position of line emission. The
increase of excitation temperature from 40 to 80 K would lead to the increase
of the $K$ parameter by a factor of $\sim 1.9$. Thus, the variations of
$T_{\text{ex}}$ along the outflow are not expected to significantly impact the
results.

To summarize, the parameters of the DR21 Main outflow, calculated using the
HCO+, provide a useful diagnostic of outflow energetics. The calculations are
consistent with previous work by Garden & Carlstrom (1992) using the same
tracer and transition. Due to the optical thickness of the emission and the
adopted velocity limits, the mass of the outflow as well as the related
parameters are lower limits to the actual parameters.

##  4 Discussion

###  4.1 The nature of the DR21 Main outflow

The nature of the DR21 Main outflow is still a topic of discussion since it
has been proposed to belong to the class of explosive outflows (Zapata et al.
2013). The detailed analysis of the morphology and the kinematics of the
outflow using HCO+ observations reveals a rather well defined bipolar outflow
structure reminiscent of that of a typical protostellar outflow (Section 3.2).
The strong overlap of red- and blue-shifted HCO+ emission is indeed a property
attributed to explosive outflows (Zapata et al. 2017), however it can also
appear in the case of a bipolar outflow that extends along the plane of the
sky with the red- and blue-shifted emission arising from the sideways
expansion of the outflow lobes. Additionally, the apparent decrease of the
DR21 Main outflow’s opening angle with increasing velocities (see Fig. 7) is
characteristic of bipolar outflows that are powered by a narrow collimated jet
but seems unlikely to happen in the case of explosive outflows.

Figure 9 shows a side-by-side comparison of local intensity peaks at multiple
velocity steps between the DR21 Main outflow and the Orion KL outflow. For the
DR21 Main outflow the HCO+ data presented in this work have been used, while
for Orion archival ALMA CO (2 – 1) data were used (Project ID: 2013.1.00546.S,
PI: John Bally, Bally et al. (2017)). The emission in Orion (right) can be
seen to consist of multiple, well defined filament-like structures which also
display clear velocity gradients along their length with higher absolute
velocities being further away from the outflows point of origin. This
behaviour is probably the most distinctive characteristic of explosive
outflows and is absent in the case of the DR21 Main outflow (left). More
precisely, in DR21 Main there are very few distinct filament-like structures
which also do not show any significant velocity gradients along their length.
Moreover, the structures that do exist appear to be tracing the cavity walls
of the outflow lobes contrary to the more random distribution that the
filament-like structures have in Orion. We note though that DR21 is located
significantly further, at 1.5 kpc (Rygl et al. 2012), than Orion KL at
$\sim$400pc (Menten et al. 2007; Kounkel et al. 2017), which means that the
physical scales of the two outflows in Fig. 9 are significantly different.

Taking all the above into consideration, it appears that the outflow of DR21
Main resembles more a typical bipolar outflow that is driven by a protostar
rather than an explosive outflow. However, the observations presented here do
not reveal a single, compact emission that could be associated with the
driving source (Appendix A). Further observations are required in order to
discern the protostar or protostellar system behind such an exceptionally
powerful bipolar outflow.

###  4.2 Outflow energetics

Figure 10:  Outflow force over the envelope mass of the driving source for
various protostellar sources. Right facing triangles represent low-mass
sources from Mottram et al. (2017), left facing triangles mark sources taken
from Yıldız et al. (2015) and upwards are from van der Marel et al. (2013)
while in blue are the Class I sources and in red the Class 0. Black diamonds
mark intermediate mass sources (van Kempen et al. 2009), grey squares mark
high mass sources from Maud et al. (2015), green “$\times$” mark high mass
sources from Beuther et al. (2002) and black stars mark high mass sources in
Cygnus (Skretas & Kristensen 2022). The red crosses mark a sample of high-mass
70$\mu$m dark sources (Li et al. 2020). The Cyan star marks G5.89-0.39, the
magenta one marks Orion KL and the green one represents the DR21 Main outflow.
The dashed, black line shows the best fit to the outflow force - envelope mass
correlation for all sources, while red and grey show the best fits for the
low- and high-mass sources respectively.

Multiple correlations have been reported connecting the energetics of
protostellar outflows with the properties of their driving sources. For
example, the correlation of the outflow force with the envelope mass and
bolometric luminosity are attributed to a connection between mass accretion
rates and outflow activity (Beuther et al. 2002; Duarte-Cabral et al. 2013;
Mottram et al. 2017). The existence of such correlations allows for a direct
comparison of the properties of DR21 Main outflow and those of other
protostellar and explosive outflows.

Figure 10 shows the comparison of the outflow force of the DR21 Main outflow
and those of low-, intermediate- and high-mass protostars (Beuther et al.
2002; van Kempen et al. 2009; van der Marel et al. 2013; Yıldız et al. 2015;
Maud et al. 2015; Mottram et al. 2017; Li et al. 2020; Skretas & Kristensen
2022). We calculate the Pearson coefficients and the corresponding
significance ($\sigma$) for both correlations (see e.g., Marseille et al.
2010). The outflow forces of high-mass protostars are found to correlate
strongly both with their envelope masses (6.1$\sigma$) and bolometric
luminosities (6.5$\sigma$); the correlation extends also to lower masses. The
source sample from Li et al. 2020 represents sources at the very early stages
of their evolution (70 $\mu$m dark clumps), and are therefore expected to have
a low $L_{\text{bol}}/M_{\text{env}}$ ratio. That is why they display
relatively large envelope masses with respect to other sources of similar
energetic parameters.

The outflow force of DR21 is higher than those of other high-mass protostars,
including the other two explosive-outflow candidates: G5.89-0.39
($L_{\mathrm{bol}}$ of 4.1$\times$ $10^{4}$ L⊙, $M_{\mathrm{env}}$ of 140 M⊙,
van der Tak et al. 2013; Karska et al. 2014) and Orion BN/KL
($L_{\mathrm{bol}}$ of 5$\times$ $10^{4}$ L⊙, $M_{\mathrm{env}}$ of 150 M⊙,
Downes et al. 1981; Genzel & Stutzki 1989). For DR21, we adopt
$L_{\mathrm{bol}}$ of 1.0$\times$ $10^{4}$ L⊙ and $M_{\mathrm{env}}$ of 1355
M⊙, which account for the new distance to the source and the peak of the
Spectral Energy Distribution (Cao et al. 2019).

DR21 shows also enhanced values of the mass outflow rate and outflow kinetic
luminosity, but falls within the range of other high-mass protostars when the
outflow mass, power, and kinetic energy are concerned (Appendix C). Those
differences are therefore the largest for parameters involving the outflow
dynamical time, and as such are related with $v_{\mathrm{max}}$ which in turn
depends on the inclination angle. However, the inclination of DR21 on the sky
could only introduce a factor of $\sim$4 difference in the derived parameters
– much less than the enhancement in outflow properties with respect to typical
high-mass protostars.

Orion BN/KL and G5.89-0.39 also show relatively high mass outflow rates, and
kinetic energy and luminosity, which might suggest this could be a common
characteristic of explosive-outflow candidates. Assuming that the underlying
physical mechanism for explosive outflows is different than for typical
outflows, there is presently no theoretical expectation that explosive
outflows should follow the $F$-$M_{\mathrm{env}}$ and $F$-$L_{\mathrm{bol}}$
correlations (Figure 10). However, the sample of these objects is too small to
be conclusive.

The DR21 Main outflow is found to be a bipolar outflow (Section 4.1).
Therefore, the enhanced outflow force of DR21 might indicate the presence of
scatter for the high-mass sources, similar to the one measured in the outflow
properties of low-mass YSOs (Figure 10). In any case, the high outflow force
of DR21 Main outflow is consistent with it being one of the most powerful
outflows in the Galaxy.

###  4.3 Interaction at the western outflow lobe

Figure 11: Outflow-cloud interaction region in the western lobe of the DR21
Main outflow. The grey-scale shows the integrated HCO+ emission between $-$50
and 50 km s-1 relative to the source velocity. Red contours show line emission
of HCN (top), H2CO (middle), and HNC (bottom). Blue contours show the emission
of N2H+ and green contours show the emission of SiO, in all panels. Contour
levels are at 5, 10 and 20 $\sigma_{\text{rms}}$. The dashed black lines in
the middle plot mark the lines used to calculate the average intensities and
first moments across the interaction front, presented in Figs. 12-17. The
orange rectangle marks the area actively affected by the interaction as
derived from these intensities. Figure 12: Average integrated intensities of
HCO+ (in blue), H2CO (in red), and N2H+ (in green) across the interaction
region in the western lobe of DR21. Intensities are integrated from $-$70 to
70, $-$20 to 20 and $-$20 to 10 km s-1 for HCO+, H2CO and N2H+, respectively.
The x-axis shows the distance in arcseconds, covering the extent of the
relevant region where the outflow interacts with a dense structure (marked
also in Fig. 11). The orange rectangle shows the area actively affected by the
interaction. The intensities for H2CO and N2H+ are scaled up by a factor of 8
in order for their distributions to be more easily comparable. Figure 13:
First moment of HCO+ emission across the interaction front (see Fig. 12). The
black dashed line marks the DR21 cloud velocity of v${}_{\text{cloud}}$ = $-3$
km s-1. Figure 14: Average integrated intensities of SiO (in blue) and H2CO
(in red) across the interaction region in the western lobe of DR21.
Intensities are integrated from $-$25 to 25 and $-$20 to 20 km s-1 for SiO and
H2CO respectively. The SiO emission is scaled up by a factor of 2 for clarity.

Figure 15: HCN and HNC emission in the interaction region. Top: Average
integrated intensities of HCN (in blue) and HNC (in red) across the
interaction region in the western lobe of DR21. Intensities are integrated
from $-$35 to 35 and $-$15 to 10 km s-1 for HCN and HNC respectively. The HNC
emission is scaled up by a factor of 5 for clarity. Bottom: Ratio of HCN over
HNC across the interaction region.

Outflow activity from protostars can have an impact on the structure and
chemistry of their parental, or nearby, dense cores (e.g., van Kempen et al.
2009; Lis et al. 2016; Kahle et al. 2022). The energetic outflow from DR21
Main heavily interacts with its surrounding, and creates the “interaction
region” in the western outflow lobe, which has been associated with a
collisionally excited Class I methanol maser (Plambeck & Menten 1990).
Molecular line emission from CASCADE allows us to pin-point the detailed
characteristics of the outflow-cloud interaction.

The interaction region shows various patterns of molecular line emission with
HCN and H2CO peaking in its east part, and HNC and N2H+ emission extending to
the west and outer part of the outflow lobe (Fig. 11). The SiO emission shows
a compact pattern associated most closely with H2CO, suggesting the presence
of shocks in the interface between e.g., HCN and N2H+ gas.

We investigate the line emission across the interaction regions in Figures
12-17. HCO+ emission peaks in the interaction region, and is followed by H2CO
tracing warm gas and N2H+, which is sensitive to the sharp increase in cold
gas density (Fig. 12). In the case of HCO+, it is likely that some outflowing
material is deflected into the line of sight, giving rise to the strong, high
velocity, blue-shifted emission detected in this area (Fig. 3). This velocity
shift is also clearly seen in the first moment of HCO+, which shows that its
emission in the area of the dense structure is mainly red-shifted (Fig. 13). A
likely explanation could be that the dense structure is located closer to the
observer along the line of sight and is therefore interacting mostly with the
blue-shifted part of the outflow. The blue-shifted emission indeed shows a
high velocity components elongated almost vertical to the outflow axis, while
the red-shifted emission appears almost unperturbed (Fig. 3).

The H2CO emission displays a sharp increase in the interaction area, followed
by a significant decrease deeper into the dense structure (also shown in Fig.
12). Such an increase is likely related to the enhanced gas temperatures in
the interaction region, which lead to the sputtering of H2CO from the dust
grains (Benedettini et al. 2013). Alternatively, the H2CO emission could be
explained by shock chemistry (e.g. Viti et al. 2011), but the commonly
predicted double-peaked structure is not resolved in our observations. It is
possible, however, that averaging along the interaction front leads to the
blending of line emission making this feature less apparent. A close
association of H2CO with the SiO emission tracing shocks (Fig. 14), favors the
scenario that H2CO traces not only warm gas, but also the location of active
shocks as was suggested by Li et al. (2022). The multiple peaks of SiO and
H2CO likely arise due to averaging across the entire interaction front and the
clumpy nature of SiO emission (see Fig. 11).

HCN and HNC show some differences along the interaction region (Fig. 15),
which might reflect the changes in gas temperature (Hacar et al. 2020). The
pattern of emission in HNC is similar to that of N2H+, whereas HCN follows
closely H2CO. We refrain from using the Hacar et al. (2020) relation to
calculate gas temperatures, because the HCN over HNC line ratios partly exceed
the range where the experimental relation hold. Nevertheless, the ratio of the
two species suggests that the temperature increases rapidly at the front of
the interaction area and then drops steadily to a relatively low value. The
ratio shows two peaks that follow the peaks of the HCN intensity (Fig. 15),
and are found close to but not exactly at the same location as the peaks of
SiO. This suggests that HCN might be also enhanced in the warm gas behind the
shock front (see e.g., Mirocha et al. 2021). In addition, velocities of the
gas traced by HCN and HNC also differ (Fig. 16). HNC has velocities close to
the cloud velocity, especially from the middle of the interaction area and
into the dense structure, where its emission actually becomes significant. HCN
on the other hand follows HCO+ and shows significant blue-shifted emission,
which becomes red-shifted in the area dominated by the dense structure. The
blue-shifted peak appears deeper in the interaction area compared that of HCO+
showing that they trace different material.

HCCCN emission is detected in the area of the dense structure as expected
based on previous studies (e.g. Morris et al. 1976; Churchwell et al. 1978).
The slight enhancement in the interaction region suggest the origin in shocks
(Fig. 17), in agreement with Benedettini et al. (2013).

The intensity of CCH shows a significant increase close to the center of the
interaction region, likely due to UV radiation originating from the shocked
material in the interaction region (e.g. Gratier et al. 2017; Bouvier et al.
2020; Chahine et al. 2022).

In contrast, HNCO is not detected in the interaction region, even though it is
often associated with shocks. According to Yu et al. (2018), HNCO is
preferentially enhanced in slow moving shocks, but destroyed in high velocity
shocks, which might indeed be the case for the energetic outflow from DR21.

Finally, we note here that due to the irregular shape of the interaction
front, the simplistic average along it adopted for the above analysis suffers
from significant uncertainties. Still, it can offer an interesting qualitative
look into the behaviour of species across such an interaction.

Figure 16: Top: First moment of HCN emission across the interaction front of
the DR21 Main outflow and the dense structure located near the western outflow
lobe plotted over the corresponding distance. The distance is measured from
the outflow dominated part and extends into the dense structure. The black
dashed line marks the DR21 cloud velocity of v${}_{\text{cloud}}$ = $-3$ km
s-1. Bottom: Same as above for but for HNC. Figure 17: Average integrated
intensities of HCCCN $J=8-7$ (in blue), HCCCN $J=10-9$ (in red), and CCH (in
green) across the interaction region in the western lobe of DR21. Intensities
are integrated from -10 to 5km s-1 for all three lines.

##  5 Summary and conclusions

This work presents the results of the CASCADE observations in the area of the
DR21 Main outflow covering several molecular tracers, including HCO+, HCN,
HNC, N2H+, H2CO, and CCH at high spatial ($\sim 3\arcsec$) and spectral
resolution ($\sim$0.8 km s-1). These molecular tracers are split into three
separate categories according to their morphology, tracing the outflow (HCO+
and HCN), the DR21 ridge (13CS, CCH, H2CO, H13CO+, HCCCN (10 – 9), HCCCN (8 –
7), HNC, N2H+) and localized emission, e.g. CH3CN (4k – 3k), CH3CN (5k – 4k),
CH3OH, DCN, DCO+, DNC, H$\alpha$41, NH2D, SiO.

Based on the HCO+ emission, the DR21 Main outflow was found to mostly resemble
a typical bipolar outflow rather than an explosive one, as its emission shows
two well structured lobes that get progressively more collimated at higher
velocities and lack the filament-like structures that are prevalent in
established explosive outflows.

Adapting the separation method, and applying it to HCO+ emission allowed for
the estimation of the energetic parameters (outflow force $F$ = 0.56 M⊙ km
yr-1 s-1, mass $M$ = 124 M⊙ and kinetic energy $E_{\text{kin}}$ = 5 $\times
10^{47}$ erg) of the outflow. Comparison with other protostellar sources
showed that the outflow force of the DR21 Main outflow is about an order of
magnitude higher than sources of similar envelope mass. It remains uncertain
though whether the outflow of DR21 Main represents an upper limit of typical
protostellar outflows or is powered by a different mechanism.

Finally, a dense molecular structure was detected near the western lobe of the
outflow. The detection of SiO and H2CO emission in this area showed that there
is ongoing interaction between the outflow and this dense structure. This, in
turn, allowed for an analysis of the behaviour of different molecular species
across such an interaction and found them to be in good agreement with the
results of recent modelling predictions and observations of shocked regions.

Overall, the results presented in this paper firmly establish the outflow of
DR21 Main as one of the most interesting cases of bipolar, protostellar
outflows due to its exceptional size and power. Additionally, the CASCADE
observations offer a good glimpse into the chemistry of the interaction region
but further and more detailed modeling is required in order to properly
constrain the chemistry that takes place in this location.

###### Acknowledgements.

The authors are grateful to the staff at the NOEMA and Pico Veleta
observatories for their support of these observations. We thank in particular
P. Chaudet, operator at the NOEMA observatory, for his motivation and
dedication in developing and testing the advanced mosaic observing procedures
employed in this project. This work is based on observations carried out under
project number L19MA with the IRAM NOEMA Interferometer and [145-19] with the
30 m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN
(Spain). AK acknowledges support from the Polish National Agency for Academic
Exchange grant No. BPN/BEK/2021/1/00319/DEC/1. H.B. acknowledges support from
the European Research Council under the Horizon 2020 Framework Program via the
ERC Consolidator Grant CSF-648505 and from the Deutsche Forschungsgemeinschaft
in the Collaborative Research Center (SFB 881) “The Milky Way System”
(subproject B1). A.G. acknowledges support from NSF AAG 2008101 and NSF CAREER
2142300. D. S. acknowledges support from the European Research Council under
the Horizon 2020 Framework Program via the ERC Advanced Grant No.
832428-Origins.

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##  Appendix A Integrated intensity contour maps

Figures 18–23 show contour maps of the integrated intensity of all observed
molecules in the area of DR21 Main outflow. The detected molecules can broadly
be separated in three categories (see Section 3.1, Table 2) as i) tracing the
outflow, ii) tracing the dense ridge, and iii) displaying localized and
fragmented emission. The emission of the rare isotopologues (e.g, H13CO+,
H13CN, HN13C and 13CS) appears to trace the high density material along the
DR21 ridge. In contrast to the patters of emission in H13CO+ and H13CN, the
emission in their 12C counter-parts, is associated with outflowing material,
likely due to low abundances of rare isotopologues in the high-velocity gas.

Figure 18: UKIRT/WFCAM continuum image of the DR21 Main region at 2.2 $\mu$m
and the line emission in key gas tracers observed as part of CASCADE. White
contours mark the 5$\sigma$ HCN (left), H2CO (middle) and HNC (right)
emission. Figure 19: Similar to Fig. 18, but for 13CS (left), H13CO+ (middle)
and CCH (right). Figure 20: Similar to Fig. 18, but for HCCCN (10-9) (left),
NH2D (middle) and HCCCN (8-7) (right). Figure 21: Similar to Fig. 18, for
CH3CN (5-4) (left), CH3OH (middle) and CH3CN (4-3) (right). Figure 22: Similar
to Fig. 18, but for DNC (left), DCO+ (middle) and DCN (8-7) (right). Figure
23: Similar to Fig. 18, but for H13CN (left), H$\alpha$41 (middle) and HN13C
(right).

##  Appendix B Line profiles in detected molecules

Figures 24–27 show spectra of the different molecular lines observed in
CASCADE. The spectra are averaged over the east lobe, the center and the
western lobe (see Fig. 1).

Figure 24: Similar to 2, but for CCH, HCCCN (10 - 9), HCCCN (8 - 7), H13CO and
13CS  Figure 25: Similar to 2, but for H2CO, DNC, DCO+, DCN and NH2D Figure
26: Same as Fig 2, but for SO2, CCD, CH3OH, CH3CN (4k \- 3k), CH3CN (5k \- 4k)
and HNCO  Figure 27: Same as Fig 2, but for H41$\alpha$ emission.

##  Appendix C Correlation plots

Figures 28–32 show correlation plots between the different outflow parameters
and the bolometric luminosity and envelope mass of their driving sources.

Figure 28: Outflow mass versus envelope mass and bolometric luminosity. The
labels are the same as in Fig. 10.

Figure 29: Outflow mass rate versus envelope mass and bolometric luminosity.
The labels are the same as in Fig. 10.

Figure 30: Outflow momentum versus envelope mass and bolometric luminosity.
The labels are the same as in Fig. 10.

Figure 31: Outflow kinetic energy versus envelope mass and bolometric
luminosity. The labels are the same as in Fig. 10.

Figure 32: Outflow kinetic luminosity versus envelope mass and bolometric
luminosity. The labels are the same as in Fig. 10.