11institutetext: Institut de Radioastronomie Millimétrique (IRAM), 300 rue de
la Piscine, 38406 Saint-Martin-d’Hères, France
e-mail<EMAIL_ADDRESS>22institutetext: Université Grenoble Alpes, 38000
Grenoble, France 33institutetext: Université Grenoble Alpes, CNRS, IPAG, 38000
Grenoble, France 44institutetext: INAF, Osservatorio Astrofisico di Arcetri,
Largo E. Fermi 5, I-50125, Firenze, Italy 55institutetext: Università degli
Studi di Firenze, Dipartimento di Fisica e Astronomia, Via G. Sansone 1, 50019
Sesto Fiorentino, Italy 66institutetext: Center for Astrochemical Studies, Max
Planck Institute for Extraterrestrial Physics, Garching, 85748, Germany
77institutetext: The Institute of Physical and Chemical Research (RIKEN),
Saitama 351-0198, Japan 88institutetext: Department of Physics, The University
of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan 99institutetext: IRAP, Université
de Toulouse, 9 avenue du colonel Roche, 31028 Toulouse Cedex 4, France

# OMC-2 FIR 4 under the microscope: Shocks, filaments, and a highly collimated
jet at 100 au scales

L. Chahine 1122 A. López-Sepulcre 1133 L. Podio 44 C. Codella 4433 R. Neri 11
S. Mercimek 4455 M. De Simone 3344 P. Caselli 66 C. Ceccarelli 33 M. Bouvier
33 N. Sakai 77 F. Fontani 33 S. Yamamoto 88 F. O. Alves 66 V. Lattanzi 66 L.
Evans 9944 C. Favre 33

(Received day month year; Accepted …)

###### Abstract

Context. Star-forming molecular clouds are characterised by the ubiquity of
intertwined filaments. The filaments have been observed in both high- and low-
mass star-forming regions, and they are thought to split into collections of
sonic fibres. The locations where filaments converge are termed hubs, and
these are associated with the young stellar clusters. However, the
observations of filamentary structures within hubs at distances of 75-300 pc
require a high angular resolution $<$2$\arcsec$ ($\sim$ 150-600 au) that
limits the number of such studies conducted so far.

Aims. The integral shaped filament (ISF) of the Orion A molecular cloud is
noted for harbouring several hubs within which no filamentary structures have
been observed so far. The goal of our study is to investigate the nature of
the filamentary structures within one of these hubs, which is the chemically
rich hub OMC-2 FIR 4, and to analyse their emission with high density and
shock tracers.

Methods. We observed the OMC-2 FIR 4 proto-cluster using Band 6 of the Atacama
Large (sub-)Millimetre Array (ALMA) in Cycle 4 with an angular resolution of
$\sim$0.26$\arcsec$ (100 au). We analysed the spatial distribution of dust,
the shock tracer SiO, and dense gas tracers (i.e., CH3OH, CS, and H13CN). We
also studied the gas kinematics using SiO and CH3OH maps.

Results. Our observations for the first time reveal interwoven filamentary
structures within OMC-2 FIR 4 that are probed by several tracers. Each
filamentary structure is characterised by a distinct velocity as seen from the
emission peak of CH3OH lines. They also show transonic and supersonic motions.
SiO is associated with filaments and also with multiple bow-shock features.
The bow-shock features have sizes between $\sim$500 and 2700 au and are likely
produced by the outflow from HOPS-370. Their dynamical ages are $<$800 yr. In
addition, for the first time, we reveal a highly collimated SiO jet ($\sim$1∘)
with a projected length of $\sim$5200 au from the embedded protostar VLA15.

Conclusions. Our study unveiled the previously unresolved filamentary
structures as well as the shocks within OMC-2 FIR 4. The kinematics of the
filamentary structures might be altered by external and/or internal mechanisms
such as the wind from H II regions, the precessing jet from the protostellar
source HOPS-370, or the jet from VLA 15. While the complexity of the region,
coupled with the limited number of molecular lines in our dataset, makes any
clear association with these mechanisms challenging, our study shows that
multi-scale observations of these regions are crucial for understanding the
accretion processes and flow of material that shape star formation.

###### Key Words.:

Stars: formation, Stars: jets, Methods: observational, ISM: molecules, ISM:
kinematics and dynamics, ISM: individual objects: OMC-2 FIR 4

##  1 Introduction

The interstellar medium (ISM) is highly filamentary on all scales. Networks of
intricate filaments are seen in low- and high-mass star-forming regions, and
even in clouds where no star formation is yet ongoing (e.g. Goldsmith et al.
2008; André et al. 2014; Molinari et al. 2014; Orkisz et al. 2019; Chung et
al. 2021). In recent years, high angular resolution studies have shown that
the filaments split into small-scale and velocity-coherent elongated
structures, called fibres, which in turn fragment into the dense cores we
observe today (e.g. Hacar et al. 2013, 2017b; Tafalla & Hacar 2015; Henshaw et
al. 2017; Sokolov et al. 2019). Among star-forming regions, the Orion A cloud
is noted for its prominent filamentary structures in both CO and dust maps
(e.g. Bally et al. 1987; Chini et al. 1997; Johnstone & Bally 1999; Shimajiri
et al. 2011; Polychroni et al. 2013; André et al. 2014; Suri et al. 2019).
Within this cloud lies the well-known integral shaped filament (ISF; Bally et
al. 1987), the most massive filament in the solar neighbourhood with a total
mass per unit length (M${}_{lin}\sim$500 M⊙ pc-1; Bally et al. 1987). The ISF
is located at a distance of $\sim$393$\pm$25 pc (Grossschedl et al. 2018).
Several studies have found that the ISF consists of substructures of small-
scale ($<$1 pc) sub-filaments or fibres (e.g. Martin-Pintado et al. 1990;
Rodriguez-Franco et al. 1992; Shimajiri et al. 2011; Li et al. 2013; Hacar et
al. 2017a, 2018; Zhang et al. 2020) and condensations or hub-filament
structures (hubs) (Mezger et al. 1990; Chini et al. 1997; Myers 2009;
Shimajiri et al. 2011; Hacar et al. 2018). In turn, the latter are thought to
contain complex nets of fibres (Hacar et al. 2018) or of filamentary
structures. However, the filamentary structures within the hubs along the ISF
were not resolved at 4$\aas@@fstack{\prime\prime}$5 resolution ($\sim$ 2000
au; Hacar et al. 2018). To our knowledge, filamentary structures within a hub
were only observed within IRAS4A at 300 pc. They have sizes $\leq$ 450 au (De
Simone et al. 2022). To observe similar structures within the hubs of the ISF,
an angular resolution of about 2$\arcsec$ or smaller ($<$ 600 au) is therefore
required. Because the filament and fibre nomenclature refers to structures at
larger scales, we decided to adopt the name ”filamentary structures” for those
at $<$1000 au scales.
In this work, we focus on the intermediate-mass hub OMC-2 FIR 4, an
archetypical Sun-like star-forming region along the ISF. This region was
identified for the first time as a bright continuum source at 1.3 mm by Mezger
et al. (1990). Later, it was observed, studied, and characterised at several
wavelengths. Based on its envelope mass and luminosity ($\sim$30 M⊙ and
$\sim$400 L⊙; Mezger et al. 1990), OMC-2 FIR 4 was considered as an
intermediate-mass protostar (Johnstone et al. 2003). However, Shimajiri et al.
(2008) discovered that OMC-2 FIR 4 is actually a protostellar cluster hosting
several dusty cores and not a single protostar. Since then, OMC-2 FIR 4 was
thoroughly studied with different telescopes and at different wavelengths
(e.g. Takahashi et al. 2008; Kama et al. 2013, 2015; Furlan et al. 2014;
López-Sepulcre et al. 2017; Evans et al. 2022). Its envelope size and
luminosity were estimated to be $\sim$104 au and $\leq$1000 L⊙ (Crimier et al.
2009; Furlan et al. 2014). Its systemic velocity is $\sim$11.4 km s-1
(Shimajiri et al. 2008; Favre et al. 2018). The clustered nature of OMC-2 FIR
4 was further confirmed by several studies (López-Sepulcre et al. 2013b;
Kainulainen et al. 2017; Tobin et al. 2019; Chahine et al. 2022). In addition,
more of its physical properties have been revealed. Its outer shell is
strongly irradiated by a far-ultraviolet (FUV) field ($\sim$1500 $G_{\rm 0}$,
where $G_{0}$ is the FUV radiation field in Habing units), arising from the
high-mass stars located in the nearby Trapezium OB association (López-Sepulcre
et al. 2013a; Favre et al. 2018), and its interior envelope is subject to high
irradiation and ionisation by local cosmic ray-like particles with an
ionisation rate of $\zeta\sim$ 4 $\times$ 10-14 s-1 (Ceccarelli et al. 2014;
Fontani et al. 2017; Favre et al. 2018). Due to these characteristics, OMC-2
FIR 4 is considered as the nearest analogue of our solar birth environment,
which is thought to be a clustered dense environment (Adams 2010; Pfalzner et
al. 2015) that was subject to internal irradiation from energetic particles
(¿10 MeV; Gounelle et al. 2013).
Despite all this progress, several questions still swirl around OMC-2 FIR 4.
The observations at parsec and sub-parsec scales using quiescent gas tracers
(e.g. N2H+; Hacar et al. 2018; Fontani et al. 2020, NH3; Wu et al. 2018, HC3N;
Tatematsu et al. 2008) have shown that OMC-2 FIR 4 is a hub-filament
structure. In this context, the open questions are whether filamentary
structures exist within this hub, and if they might be observed at
protocluster scale ($\leq$ 0.1 pc) using dynamic gas tracers (such as SiO and
CH3OH), as in the case of NGC 1333 IRAS 4A (De Simone et al. 2022). The
differences that might arise from this type of observations compared to the
studies performed at large scales are unclear. The same applies to the
differences between the dynamic gas (traced by, e.g., SiO and CH3OH) and the
quiescent gas (traced by, e.g., N2H+ and NH3). Another important question we
also address is the presence of jets and outflows. It is known that within the
protocluster, the hot corino HOPS-108 drives a jet towards the south-east
(Lattanzi et al. 2022). In addition, OMC-2 FIR 4 is located to the south of
the intermediate-mass protostar HOPS-370 that drives an outflow whose southern
lobe overlaps, at least in projection, with OMC-2 FIR 4 (Shimajiri et al.
2008). The outflow was detected in several tracers such as CO and [O I] (e.g.
Williams et al. 2003; Shimajiri et al. 2008; Takahashi et al. 2008; González-
García et al. 2016; Tobin et al. 2019), and several non-thermal jet knots were
also discovered (Osorio et al. 2017). However, whether it has triggered the
star formation in OMC-2 FIR 4 is still an open question.
Two key molecules that can help to answer these questions and to study the
diverse processes that occur in molecular clouds and stellar protoclusters are
SiO and CH3OH. First, SiO is a useful tracer of shocked gas and outflows.
During shock processes, gas-grain collisions (sputtering) and grain-grain
collisions (shattering) release Si from the grains into the gas phase (Draine
et al. 1983; Flower & Pineau des Forets 1994; Schilke et al. 1997; Caselli et
al. 1997). Si is oxidised rapidly in SiO, resulting in an increase of SiO
abundance in the shocks by several orders of magnitude (Gusdorf et al. 2008a,
b; Guillet et al. 2011). On the other hand, CH3OH is present in different
environments such as molecular clouds, photo-dissociation regions (PDR), and
hot corinos (e.g. Leurini et al. 2004, 2010; Cuadrado et al. 2017; Maret et
al. 2005; Bouvier et al. 2020). It is prevalently formed on dust grains (e.g.
Tielens & Hagen 1982; Watanabe & Kouchi 2002; Rimola et al. 2014) and is
released into the gas phase via thermal and/or non-thermal processes (e.g.
Duley & Williams 1993; Flower & Pineau des Forets 1994; Minissale et al. 2016;
Dartois et al. 2019). Hence, CH3OH can probe different mechanisms that can
inject it into the gas phase, such as thermal desorption, shocks, irradiation
by UV photons, and cosmic ray bombardment. In this vein, we present the first
study of the filamentary structures and shocks within the protostellar cluster
OMC-2 FIR 4 at an unprecedented angular resolution of $\sim$100 au, where we
targeted eight different molecular tracers, including SiO and CH3OH, with the
Atacama Large (sub-)Millimetre Array (ALMA) at 1.2 mm.
The paper is structured as follows: In Sect. 2 we describe the observations.
In Sect. 3 we present the molecular line maps, together with the main results
of the analysis. In Sect. 4 we discuss the results, and, finally, in Sect. 5
we summarise the conclusions.

##  2 Observations

OMC-2 FIR 4 was observed with ALMA during its Cycle 4 operations, between 25
October 2016 and 5 May 2017, as part of the project 2016.1.00376.S (PI: Ana
López-Sepulcre). The observations were performed using the ALMA main array in
its C-5 configuration, probing angular scales from
0$\aas@@fstack{\prime\prime}$19 to 11$\aas@@fstack{\prime\prime}$2\. Several
spectral windows (spw) were placed within the spectral range (218-234 GHz and
243-262 GHz). They were observed using spectral channels of 122.070 kHz
($\sim$ 0.14–0.17 km s-1). A subsequent spectral smoothing by a factor 3 was
applied for some spws. The phase-tracking centre was R.A.
(J2000)$=05^{h}35^{m}26^{s}.97$, Dec.
(J2000)$=-05^{\circ}09^{\prime}54^{\prime\prime}.50,$ and the systemic
velocity was set to $V_{LSR}$ = 11.4 km s-1. The quasars J0510+1800 and
J0522-3627 were used for bandpass and flux calibration, while J0607-0834 and
J0501-0159 were used for phase calibration. The absolute flux calibration
uncertainty is estimated to be $<$10%.
The data calibration was performed using the standard ALMA calibration
pipeline with the Common Astronomy Software Applications package
(CASA111https://casa.nrao.edu/; McMullin et al. 2007), while self-calibration,
imaging, cleaning, and data analysis were performed using the IRAM-GILDAS
software package222http://www.iram.fr/IRAMFR/GILDAS/. The continuum images
were produced by averaging line-free channels from the wide spectral windows
at 232 GHz and 246 GHz in the visibility plane. The resulting beam size and
rms are (0$\aas@@fstack{\prime\prime}$28 $\times$
0$\aas@@fstack{\prime\prime}$23 and 95.7 $\mu$Jy beam-1) and
(0$\aas@@fstack{\prime\prime}$50 $\times$ 0$\aas@@fstack{\prime\prime}$27 and
91.7 $\mu$Jy beam-1). Phase self-calibration was performed on the continuum
emission, and the gain solutions were applied to the line cubes. Continuum
subtraction was performed on the cubes in the visibility plane before line
imaging. The data were cleaned in GILDAS MAPPING using the Clark algorithm
(Clark 1980). The resulting synthesised clean beam, velocity resolution, and
channel rms for each spectral window are summarised in Table 1. The line
identification was performed using the Cologne Database for Molecular
Spectroscopy (CDMS333https://cdms.astro.uni-koeln.de; Müller et al. 2001,
2005). The maps shown in the paper are not corrected for the primary beam
attenuation because we are more interested in the structure morphology (rather
than the flux) that was not affected by the primary beam attenuation. However,
we took the correction into account in the spectra.

Table 1: List of transitions, their parameters, and spectral window
parameters.

$\mathrm{Molecule}$ | $\mathrm{Transition}$ | $\mathrm{\nu\,^{a}}$ | $\mathrm{E_{up}\,^{a}}$ | $\mathrm{S_{ij}\,\mu^{2}\,{}^{a}}$ | $n_{cr}\,^{c}$ | $\mathrm{dV}$ | $\mathrm{Beam\,(PA)}$ | $\mathrm{Chan.\,rms}$
---|---|---|---|---|---|---|---|---
|  | (MHz) | (K) |  (D2) |  (104 cm-3) |  (km s-1) |  ($\arcsec$ $\times$ $\arcsec$, ∘) |  (mJy beam-1)
HC3N | 24-23 | 218324.72 | 131 | 334.2 | 8.3 |  0.5 d |  0.52 $\times$ 0.29 ($-$69) | 5.4
CH3OH |  4-2,3 $-$ 3-1,2 E | 218440.06 | 45 | 13.9 | 2.2 | 0.17 |  0.55 $\times$ 0.29 ($-$73) | 5.5
|  42,3 $-$ 51,4 A | 234683.37 | 61 | 4.5 | 0.5 | 0.16 |  0.49 $\times$ 0.27 ($-$71) | 5.8
|  51,4 $-$ 41,3 A | 243915.78 | 50 | 15.5 | 1.9 | 0.15 |  0.27 $\times$ 0.22 ($-$95) | 4.7
C18O | 2-1 | 219560.35 | 16 | 0.02 | 0.5 | 0.17 |  0.53 $\times$ 0.29 ($-$71) | 5.0
CS | 5-4 | 244935.56 | 35 | 19.1 | 130 | 0.15 |  0.33 $\times$ 0.27 ($-$68) | 3.7
$\mathrm{CH_{3}CN}$ |  14$\rm{{}_{1}}-$ 131 | 257522.42 | 100 | 295.2 | 9.3 | 0.42 d | 0.31 $\times$ 0.26 (-71) | 2.8
|  14$\rm{{}_{0}}-$ 130 | 257527.38 | 93 | 296.7 | 10.2
H13CN b |  J=3$-$2, F=4$-$3 | 259011.86 | 25 | 34.4 | 410 |  0.45 d |  0.33 $\times$ 0.27 ($-$68) | 2.1
SiO | 6-5 | 260518.02 | 44 | 57.6 | 50 |  0.42 d |  0.30 $\times$ 0.26 ($-73$) | 2.5
CCH b |  N= 3$-$2, J=5$/$2$-$3$/$2, F=3$-$2 | 262064.99 | 25 | 4.3 | 6.5 |  0.42 d |  0.30 $\times$ 0.26 ($-$73) | 2.5

Notes:
a Frequencies and spectroscopic parameters have been extracted from the CDMS
catalogue (Müller et al. 2001, 2005).
b H13CN and CCH consist of two hyperfine components in 0.4 MHz interval. Those
with the higher S${}_{ij}\,\mu^{2}$ are reported here.
c The critical densities of CS and H13CN were obtained from Shirley (2015) at
50 K, and those of the other tracers were estimated at 50 K using the LAMDA
database (Schöier et al. 2005).
d The spectral channels of these spws were smoothed by a factor of 3 to gain
in signal-to-noise ratio.

##  3 Results

We have imaged the emission from eight different molecules: SiO, CH3OH, C18O,
CS, CH3CN, H13CN, HC3N, and CCH at a high angular and spectral resolution
($\sim$0$\aas@@fstack{\prime\prime}$33 and $\sim$0.16 km s-1). We mapped three
different transitions of CH3OH and one transition for the other molecules (see
Table 1).

###  3.1 Spatial distribution of molecular emission

####  3.1.1 CS and C18O

In Fig. 1 we show the velocity-integrated maps444The maps were integrated
between the following ranges: 7.4 and 15.4 km s-1 for C18O, -0.2 and 24.4 km
s-1 for CS, -6.2 and 24.4 km s-1 for SiO, 3.4 and 19.4 km s-1 for CH3OH, 7.6
and 23.6 km s-1 for CH3CN, 1.5 and 21.8 km s-1 for H13CN, 5 and 21.4 km s-1
for HC3N, and 2.1 and 20.6 km s-1 for CCH. of the observed molecular tracers
in colours, on which we overlaid CH3OH (5${}_{1,4}-$41,3 A) contours. CS, a
typical high-density tracer ($n_{cr}$=1.3$\times$106 cm-3 for the 5-4
transition, see Table 1), shows the brightest and most extended emission in
our maps. It probes the overall structures of the gas in the FIR 4
protocluster, a ridge in the east side extending from the north-east (NE) to
the south-west (SW) (hereafter called eastern ridge), and several filamentary
structures. The eastern ridge has a length of $\sim$20$\arcsec$ ($\sim$7900
au) and extends over $\sim$3300 au. The other filamentary structures are
relatively shorter, with a length ranging between $\sim$4$\arcsec$ and
10$\arcsec$ ($\sim$1500-4000 au). C18O traces the eastern ridge, but its
emission is less extended (see Sect. 3.2). All the other molecules are
emitting from the region covered by CS, except for SiO.

####  3.1.2 SiO

SiO traces the shocked regions within the FIR 4 protocluster. From the overlap
of its velocity-integrated map with the CH3OH contours, we can notice a
different morphology and a shift in the emission. More specifically, two
regions of the cloud can be distinguished: a region dominated by SiO located
to the west, and another dominated by CH3OH to the east. Furthermore, SiO
reveals multiple bow-shock features: an arc-like extended bow shock to the
west of the cluster (hereafter called Arc 1) and a smaller bow shock located
south-west of the cluster (hereafter called arc 2). Arc 1 extends over
$\sim$2700 au, while arc 2 extends over $\sim$500 au. SiO emission had already
been reported towards the west of FIR 4 region, but at lower angular
resolution ($\sim$3$\arcsec-$7$\arcsec$; Shimajiri et al. 2008). In addition,
we report a collimated monopolar molecular jet from the protostar VLA15. The
jet extends along the S-N direction, with a length of 13.3$\arcsec$
($\sim$5200 au). The source driving the jet was recently identified as a young
and deeply embedded protostar (Osorio et al. 2017; Tobin et al. 2019) that
appears to have an edge-on horizontal disk (Tobin et al. 2019). The class of
this protostar has not yet been determined.

####  3.1.3 CH3OH

Similarly to CS, the CH3OH velocity-integrated maps shown in the middle panel
of Fig. 1 reveal several cavities from NE to SW and widespread interwoven
filamentary structures. The origin of these structures is discussed in Sect.
4.1.

For the CH3OH (4${}_{-2,3}-$3-1,2 E) transition, known to be a class I-type
CH3OH maser (Hunter et al. 2014; Chen et al. 2019), we noted that in three
positions (P1, P2, and P3) along the eastern ridge (see panel (d) in Fig. 1),
the line intensities were ¿300 K km s-1. This suggests the presence of maser
spots at these positions. Maser spots were previously identified at the same
position, with different CH3OH maser transitions (e.g. Kogan & Slysh 1998;
Kang et al. 2013; Fontani et al. in prep). The maser at P2 was observed at 7
mm, 3 mm, and 2 mm (Kang et al. 2013), while those at P1 and P3 were observed
at 3 mm (Fontani et al. in prep). Hence, we report the detection of these
masers at 1 mm. The locations of the maser spots we detect along the eastern
ridge are reported in Table. 2. The narrow spectra extracted at the peak of
the above-mentioned spots are shown in Fig. 9 of Appendix A. It is worth
noting that on one hand, the class I-type CH3OH masers result from collisional
pumping (e.g. Sobolev et al. 2007; Ladeyschikov et al. 2020; Nesterenok 2022),
which can be associated with young stellar object outflows (Voronkov et al.
2006; Chen et al. 2011; Ladeyschikov et al. 2020), expanding H II regions
(Voronkov et al. 2014), expanding supernova remnants (Pihlström et al. 2014),
and rapidly moving cloudlets (Voronkov et al. 2010). Hence, they indicate the
presence of compressed gas. On the other hand, Kang et al. (2013) suggested
that the exciting source of the maser at P2 could be one of the protostars in
the OMC-2 FIR 3/4 clusters, and Osorio et al. (2017) have reported radio
emission associated with the jet from the protostar HOPS-370 close to the
maser at P3.

Table 2: Position of the detected CH3OH (4${}_{-2,3}-$3-1,2 E) maser spots along the eastern ridge. $\mathrm{Spot}$ | $\mathrm{R.A.(J2000)}$ | $\mathrm{Dec.(J2000)}$
---|---|---
|  (${}^{h\;m\;s}$) |  (∘ ${\>{}^{\prime}\>{}^{\prime\prime}}$)
P1 | 05:35:27.792 |  $-$05:09:42.82
P2 | 05:35:27.717 |  $-$05:09:45.62
P3 | 05:35:27.401 |  $-$05:09:52.38

We detected three CH3OH lines with close upper state energies. Therefore, we
were not able to perform a non-local thermodynamic equilibrium analysis (non-
LTE).

####  3.1.4 H13CN, CH3CN, HC3N, CCH, and dust

The emission of H13CN, CH3CN, HC3N, and CCH is fainter, but agrees reasonably
well with the CS and CH3OH morphology. The eastern ridge is detected in all
the tracers. The hot corino HOPS-108 (Tobin et al. 2019; Chahine et al. 2022)
is also observed in all the molecules except for CCH, which is a traditional
probe of UV illuminated regions (e.g. Gratier et al. 2017; Bouvier et al.
2020). Moreover, the bright central filament ($\sim$4$\arcsec$ in length)
probed by CS is very well detected in H13CN, the other tracer of high-density
gas ($n_{cr}$=4.1$\times$106 cm-3 for the 3-2 transition, see Table 1),
suggesting that this filament might be one of the densest within the
protocluster.

Finally, in addition to probing the protostars within OMC-2 FIR 4, the
continuum emission also traces some filamentary structures. The overall
distribution of CH3OH emission follows the filament morphology observed in the
continuum (see Fig. 2). The eastern ridge is well probed by dust emission,
suggesting that it is in high-density conditions.

Figure 1: Velocity-integrated maps of the different tracers used to probe the
emission towards OMC-2 FIR 4.
Upper panels: Velocity-integrated maps of C18O, CS and SiO in colours, with
superimposed contours of the velocity-integrated emission of CH3OH
(5${}_{1,4}-$41,3 A) line (white). The colour images are shown at the top of
each panel. The different cores are depicted with magenta stars and are
labelled in white. The synthesised beam of CH3OH and the other molecules are
depicted in the lower left corner in white and yellow, respectively. In the
SiO panel, the positions of the SiO peaks from this study are depicted with
filled grey triangles. The primary beam at the full width at half maximum is
depicted in white.
Middle panels: Velocity-integrated maps of CH3OH lines at Eu=45 K and Eu=50 K
and of CH3CN. On the latter, we superimposed the contours of the velocity-
integrated emission of CH3OH (5${}_{1,4}-$41,3 A) line (white). The colour
images are shown at the top of each panel. The positions of the three masers
detected in CH3OH (4${}_{-2,3}-$3-1,2 E) are depicted with filled blue
squares.
Lower panels: Velocity-integrated maps of H13CN, HC3N and CCH in colours with
superimposed contours of the velocity-integrated emission of CH3OH
(5${}_{1,4}-$41,3 A) line (white). The colour images are shown at the top of
each panel The methanol contours start at 10$\sigma$ with $\sigma$ = 12.3 mJy
beam-1 km s-1. Figure 2: Continuum emission towards OMC-2 FIR 4 at 232 GHz in
colours, with superimposed contours of the velocity-integrated emission of
CH3OH (4${}_{-2,3}-$3-1,2 E) at 15$\sigma$ with $\sigma$ = 20.6 mJy beam-1 km
s-1. The different cores are depicted with magenta stars and are labelled in
white. The synthesised beam of CH3OH and continuum are depicted in the lower
left corner in white and green, respectively.

###  3.2 Kinematics of molecular emission

We provide velocity maps (moment 1) and velocity-dispersion maps (moment 2)
for CH3OH and SiO (Figs. 3 and 4). We also show the velocity channel maps for
CH3OH, C18O, HC3N, CS, and SiO (see Appendix C, Figs. 14-18). C18O and CS are
plausibly affected by missing flux due to structures larger than the maximum
recoverable scale (MRS$\sim$11$\aas@@fstack{\prime\prime}$2). Therefore, we
did not analyse the kinematics using these tracers. The remaining molecular
tracers are either faint or have a hyperfine structure, hence the study of
their kinematics would not be reliable.

The average systemic velocity of the FIR 4 protocluster is Vsys=11.4 km s-1
(Shimajiri et al. 2008; Favre et al. 2018). Remarkably, from the velocity maps
and channel maps of CH3OH, we can clearly see that each filament has a
different velocity and that the filaments at the centre of the cloud have
higher velocities ($>$2 km s-1) than the external filaments in both red- and
blueshifted emission. The velocity gradient in CH3OH between the east and the
west of the FIR 4 protocluster is about $\pm$4 km s-1 with respect to the
systemic velocity. From the velocity-dispersion maps of CH3OH in Fig 4, it
seems that we have three distinct spatial distributions of the velocity
dispersion:

  1. 1.

The external filamentary structures that have the lowest velocity dispersion,
$<$2 km s-1.

  2. 2.

The intermediate filaments around the central region that have a velocity
dispersion of 2-4 km s-1.

  3. 3.

The central region showing (in the moment 2 map) a velocity dispersion of 4-7
km s-1.

However, the high dispersion in the central region is actually due to the
presence of multiple structures along the line of sight (see Appendix A, Fig.
10). In Fig. 10 we show the spectra extracted from three different positions
that are marked by magenta triangles in the moment-2 map.

A completely different velocity distribution is observed in SiO. Its emission
extends up to much higher blueshifted velocities than the other tracers,
covering a wide velocity range of $-$17.6 $<$ V-Vsys $<$ +8 km s-1. From the
SiO velocity and channel maps, three velocity regimes with different spatial
distribution can be identified:

  1. 1.

Blueshifted high-velocity (HV) regime in the range $-$17.6 $<$ V-Vsys $<-$8.1
km s-1, where SiO reveals the decelerating jet from VLA15, the south-west bow-
shock (arc 2) and the elongated eastern ridge.

  2. 2.

Blueshifted low-velocity (LV) regime in the range $-$8 $<$ V-Vsys $<$ 0 km
s-1, where SiO probes the extended arc-like bow (arc 1) west of the cluster.

  3. 3.

Redshifted low-velocity (LV) regime in the range +0.1 $<$ V-Vsys $<$ +8 km
s-1, where SiO probes more compact filaments than the blueshifted parts.

It is worth noting that the redshifted counterpart of the jet from VLA15 is
not detected in our data as it is $\sim$3$\arcsec$ out of our primary beam.
However, the possibility of having a monopolar jet cannot be excluded (e.g.
Codella et al. 2014). Interestingly, the jet can also be observed in CS at
high blueshifted velocities in the range $-$0.2$<$ V$<+$3.3 km s-1 (see the
bottom left panel in Fig. 17 of Appendix C).

We overlaid the integrated contours of each velocity regime on the velocity-
integrated maps of CH3OH, CS, and H13CN (see Fig. 5). We note that the bow
shocks in SiO are spatially unrelated to the bulk emission that is traced by
the other molecules, but SiO traces some filamentary structures within OMC-2
FIR 4 at certain velocities. At higher blueshifted velocities (HV) SiO traces
the eastern ridge, which is probed by all the other molecular tracers and by
the dust emission. It also traces the filamentary structure located west of
the protostar HOPS-64 and the cavity located west of the source HOPS-108.
While in the red LV regime, SiO probes the central collimated filament, which
is also observed in CS and H13CN. The detection of SiO in some filamentary
structures indicates that these filaments contain shocked gas, where the gas
is compressed and warmed up. This is also confirmed by the detection of the
class-I CH3OH maser spots along the eastern ridge.

Figure 3: Velocity maps of CH3OH and SiO towards OMC-2 FIR 4.
Left panel: Velocity maps (integrated between 9.9 and 12.9 km s-1; colour
scale in km s-1) of CH3OH (4${}_{-2,3}-$3-1,2 E) (left) and CH3OH
(5${}_{1,4}-$41,3 A) (right). Right panel: Velocity map (integrated between
-6.2 and 24.4 km s-1; colour scale in km s-1) of SiO(6-5). The different cores
are depicted with magenta stars and are labelled in black. The positions of
the SiO peaks from this study are depicted with filled magenta triangles. We
draw four arrows from HOPS-370, one perpendicular to the disk, two towards the
SiO peaks, and one similar to the direction of the jet in [O I] by González-
García et al. (2016). Figure 4: Velocity-dispersion maps (integrated between
7.4 and 15.4 km s-1; colour scale in km s-1) of CH3OH (4${}_{-2,3}-$3-1,2 E)
(left) and CH3OH (5${}_{1,4}-$41,3 A) (right). The positions (A, B, and C)
from which we extracted the spectra in Fig. 10 are labelled with filled
magenta triangles. Figure 5: Velocity-integrated maps of CH3OH and CS reported
in Fig. 1 with superimposed contours of the velocity-integrated emission of
SiO at different velocity regimes. The colour images are as in Fig. 1. The SiO
contours are at 5$\sigma$ with $\sigma$= 5.8 mJy beam-1 km s-1. They
correspond to the blueshifted high-velocity regime (blue), the blueshifted
low-velocity regime (white), and the redshifted low-velocity regime (red) (see
Sect. 3.2). The different cores are depicted with magenta stars and are
labelled in black. The positions of the SiO peaks from this study are depicted
with filled magenta triangles.

Figure 6: N2H+(1–0) integrated intensity emission from Hacar et al. (2018) in
colour scale and black contours, with superimposed 5$\sigma$ contours of the
velocity-integrated emission of CH3OH (5${}_{1,4}-$41,3 A) (left) and CS
(right) from this work. N2H+ contours are equally spaced every 1 Jy beam-1 km
s-1. The CH3OH and CS contours are shown at 5$\sigma$ with $\sigma$= 12.3 mJy
beam-1 km s-1 and 23.2 mJy beam-1 km s-1, respectively. The different cores
are depicted with magenta stars and are labelled in white. The direction of
Fibre 43 is shown with a green line. The synthesised beams of N2H+, CH3OH
(left), and CS (right) are depicted in the lower left corner in green and
white, respectively.

##  4 Discussion

###  4.1 Origin of the filaments

At $\sim$1 pc scale and $\sim$1800 au angular resolution, Hacar et al. (2018)
identified 55 fibres along the OMC-1 and OMC-2 clouds of the ISF using the
quiescent gas tracer N2H+. They have suggested that the fibres towards the
OMC-1 south region, the OMC-1 ridge, and towards the centres of OMC-2 FIR 4
and OMC-2 FIR 6 are distributed in fan-like arrangements, while they are
sparse outside these regions (see their Sect. 3.5). Furthermore, they noted
that the fibre hubs coincide with the positions of different (stars plus gas)
mass concentrations along the ISF. The algorithm HiFive used in their study
was not able to decompose the most complex structures, such as OMC-2 FIR 4,
which consists of multiple sub-branches. However, one main fibre within this
region was clearly identified in their study, and it is fibre number 43 (see
their Fig. 4).

Our observations of the OMC-2 FIR 4 protocluster at a spatial resolution of
$\sim$100 au reveal a collection of intertwined filamentary structures within
a field of view of $\sim$10000 au. To assess whether these structures are
associated with the fibres observed with N2H+ at larger scales, we
superimposed the CH3OH (5${}_{1,4}-$41,3 A) and CS contours emission from our
work on the N2H+ (1–0) emission map from Hacar et al. (2018) (see Fig. 6).
This figure shows that N2H+ emission surrounds that of CH3OH and CS. Our
eastern ridge is aligned with fibre 43 identified by Hacar et al. (2018). The
other shorter filaments could be connected to the multiple sub-branches of the
fibres’ hub that were not recovered by HiFive. Taking into account the
differences in scales, angular resolution, and tracers, we can say that the
two works are complementary. Whereas at 4.5$\arcsec$, Hacar et al. (2018)
observed the dense fibres, the higher angular resolution have allowed us to
resolve the filamentary structures within the hub. In addition, CH3OH has
offered us a probe of the dynamics. While with N2H+ the centroid velocities
VLSR(N2H+) of the fitted components in OMC-2 FIR 4 region range from $\sim$ 10
to 12.5 km s-1 from east to west with a velocity dispersion between 0.2 and
2.2 km s-1 (see their Fig. B.3), our CH3OH filamentary structures show a
higher velocity dispersion, ranging between 0.9 and 3.2 km s-1 (excluding the
line width of maser spots). Each of the CH3OH filaments has a different
velocity, with those at the centre being higher (V$\sim$ 7.4 km s-1 for the
most blueshifted filament and V$\sim$ 17.4 km s-1 for the most redshifted
one). The velocity range and dispersion are indicative of supersonic motions
within the protocluster. To verify this point, we compared the magnitude of
the line-of-sight non-thermal velocity dispersion $\sigma_{\rm{NT}}$ with the
local thermal sound speed of H2, $c_{s}(T_{K})$ (e.g. Hacar & Tafalla 2011;
Hacar et al. 2018). We used the ratio $\sigma_{\rm{NT}}/c_{s}(T_{K})$ to
identify whether the observed gas motions are subsonic
($\sigma_{\rm{NT}}/c_{s}(T_{K})\leq$1), transonic
(1$<\sigma_{\rm{NT}}/c_{s}(T_{K})\leq$2), or supersonic
($\sigma_{\rm{NT}}/c_{s}(T_{K})>$2) (Hacar et al. 2018, 2022). The line width
of a spectrum combines contributions from both thermal and non-thermal gas
motions. Consequently, to estimate the non-thermal component
$\sigma_{\rm{NT}}$, we subtracted the thermal component from the measured line
width of the spectra, assuming that the two contributions are independent of
one another (Myers 1983),

$\sigma_{\rm{NT}}=\sqrt{\frac{\Delta
V^{2}}{8\,\rm{ln}\,2}-\frac{k_{B}T_{K}}{\mu_{\rm{(CH_{3}OH)}}\,m_{p}}}.$ (1)

Thus, for line widths of [0.9–3.2 km s-1] as obtained from our spectra,
$\sigma_{\rm{NT}}$ will range between $\sim$0.4 and 1.4 km s-1. On the other
hand, for an isothermal medium, $c_{s}(T_{K})$ is given as

$c_{s}(T_{K})=\sqrt{\frac{k_{B}T_{K}}{\mu_{\rm{(H_{2})}}\,m_{p}}}.$ (2)

The kinetic temperature of the gas within OMC-2 FIR 4 was estimated to be
$\sim$ 40 K (Favre et al. 2018), so $c_{s}(T_{K})$ is $\sim$ 0.4 km s-1.
Hence, the ratio $\sigma_{\rm{NT}}/c_{s}(T_{K})$ of the filamentary structures
ranges between $\sim$ 1 and 3.3, implying the presence of transonic and
supersonic motions. As proposed by Henshaw et al. (2014) for the infrared dark
cloud (IRDC) G035.39–00.33, we suggest that the kinematics of the filamentary
structures could be affected by the outflows from the forming stars within the
protocluster. Gaseous CH3OH and CS would testify to a recent injection or
formation of these species into the gas-phase, which have not yet had the time
to accrete onto the grain mantles. For a density of about 105 cm-3 , the
timescale for methanol accretion is about 104 yr, and this timescale is
slightly longer for CS. This supports the idea that what we see within OMC-2
FIR 4 could be the result of the protostellar feedback (i.e. outflows, winds,
and radiation) that shaped the material in filament-like structures. Hence,
the results highlight the importance of stellar feedback on the evolution of
molecular hubs, where the material can be shaped and compressed within narrow
filaments.

###  4.2 SiO fingerprint in filaments

Another key molecule we detect along some of the filamentary structures is SiO
(see Fig. 5). Figure 5 shows the detection of SiO along three filamentary
structures: the eastern ridge, the filament located west of the protostar
HOPS-64, and the central collimated filament. Its detection in the filamentary
structures indicates that the gas is compressed and possibly shocked. The
shocks could be induced by internal or external feedback processes. For
example, the detection of narrow widespread SiO emission at parsec scale in
high-mass star-forming filaments was thought to be caused by either gentle
filament merging, that is, merging occurring at relatively low velocity
(Jiménez-Serra et al. 2010; Henshaw et al. 2013), or by a decelerated shocked
gas associated with large-scale outflows driven by the protostellar sources
(Jiménez-Serra et al. 2010; López-Sepulcre et al. 2016). Higher-resolution
studies have added a new possibility, which is the collision between the star-
forming cloud and the molecular gas pushed by H II regions or supernova
remnants (e.g. Cosentino et al. 2019, 2020). Recently, both SiO and CH3OH
emission were observed at sub-parsec scales and $\sim$2$\arcsec$ angular
resolution in the filamentary structures of IRAS 4, a low-mass protostellar
system located in the NGC 1333 star-forming region (De Simone et al. 2022).
The authors suggested that the emission arises from shock trains due to an
expanding gas bubble, coming from behind NGC 1333 and clashing against the
filament, where IRAS 4A lies. The narrow ($<$2 km s-1) SiO HV imprint along
the eastern ridge (see Appendix A, Fig. 11) suggests that a similar scenario
cannot be excluded in our case. Several molecular CO shells from B stars were
observed in the surroundings of OMC-2 FIR 4 (Feddersen et al. 2018), in
addition to an increase of CO emission at the eastern edge of the ISF
(Shimajiri et al. 2011). Hence, the shocks in the filamentary structures of
OMC-2 FIR 4 may be induced by the compression of the cloud due to some
external mechanisms such as an expanding bubble that impact the protocluster
or external winds from the OB stars located nearby. Alternatively, the SiO
signature along the filamentary structures can be produced by the feedback of
the protostars within the cluster, as discussed in the previous section. The
coincidence of SiO with dust emission (see Appendix C, Fig. 13) could be
indicative of dense or compressed gas that is not necessarily shocked. This is
also consistent with the detection of the class I type CH3OH masers along the
eastern ridge. To comprehend which feedback process might be compressing the
cluster and producing the SiO along the filamentary structures, we need to
observe more molecular line transitions to derive the SiO abundance and the
gas physical properties. An alternative interpretation of the SiO blueshifted
emission along the eastern ridge is discussed in section 4.4.

###  4.3 SiO jet from VLA15

Figure 7 presents the SiO(6–5) clumpy elongated jet from VLA15. The jet is
very narrow and shows a wiggle, similarly to what has been reported in HH212,
another protostar located in Orion (Lee et al. 2015, 2017). To investigate the
jet collimation, we measured the width along several cuts perpendicular to the
jet axis where the SiO emission is $>$10$\sigma$. The widths were measured by
fitting the spatial profile of each cut with a Gaussian profile. Then the
deconvolved widths were derived by correcting the full width at half maximum
(FWHM) of the observed emission for the size of the beam perpendicular to the
jet axis employing the same equation as was used by Podio et al. (2021). We
plot the deconvolved sizes as a function of the distance from VLA15 in Fig. 8.
The width increases slightly with the distance. At 600-800 au from the source,
the widths were estimated to be $\sim$35-60 au, slightly lower than the width
of HH212 jet estimated at the same distances ($\sim$80 au; Cabrit et al.
2007). We derived the average collimation angle of the VLA15 jet by fitting
the measured cut widths with a straight line, taking the measurement errors
into account. We performed the fit using the Python function curve_fit,
according to the following equation:

$2R_{jet}=2\,tan(\alpha/2)z+2R_{0},$ (3)

where 2Rjet is the deconvolved width, $z$ is the distance from the source
VLA15, $\alpha$ is the apparent full opening angle of the flow555Due to
projection effects, the true opening angle is smaller:
tan$(\alpha_{\rm{true}})$= tan$(\alpha)\times$ sin$(i)$, where $i$ is the
inclination with respect to the line of sight. , and R0 is a constant offset.
For reference, we drew a dashed green line that represents a collimation with
a full opening angle of $\alpha$=3∘ and initial width of 30 au, as was done by
Podio et al. (2021). Our estimated widths fall belowthis line. We derived an
apparent full opening angle $\alpha$ = 0.9${}^{\circ}\pm$0.5∘ from the fit,
indicating that the jet of VLA15 is highly collimated ($<$2∘). The jet
collimation agrees with the values derived for atomic jets from class II
sources ($\sim$3∘), while it is smaller than those inferred for the class 0
SiO jets of the CALYPSO survey (4∘ \- 12∘ at $>$300 au distances; Podio et al.
2021). Our value is also lower than the value inferred for HH212 IRAM-PdBI
observations (Codella et al. 2007; Cabrit et al. 2007), possibly due to the
lower spatial resolution ($\sim$150 au). The very small opening angle that we
derive ($\sim$1∘) indicates that at the distance of the closest detected knot
($\sim$600 au) the jet is already collimated. Hence, our results suggest the
VLA15 SiO jet is one of the most highly collimated jets observed so far. This
is consistent with theoretical predictions that jets of young stellar objects
are driven by magnetohydrodynamic (MHD) mechanisms (Ray & Ferreira 2021). We
verified that the narrow opening angle of the jet is not the result of
interferometric filtering by modelling the interferometer response to
simulated structures with widths ranging from 1/6 to 10 times the size of the
synthesised beam. The widths of the structures were not narrowed, but rather
conserved, indicating that they were not resolved out by the interferometer
(see Appendix B). Hence, the jet width we observe is real, as is the narrow
opening angle we derived.

We estimated the dynamical age of three knots (A, B, and C; see Fig. 7) along
the jet assuming again a jet velocity Vjet = 100 km s-1 (Podio et al. 2021).
The disk associated with VLA15 appears to be nearly edge-on (Osorio et al.
2017; Tobin et al. 2019), hence its jet likely lies close to the plane of sky
($i$=0∘). In this case, the distances between VLA15 and knots A, B, and C
would be $d_{(\rm A)}\sim$420 au, $d_{(\rm B)}\sim$2260 au, and $d_{(\rm
C)}\sim$4795 au, respectively. Therefore, the dynamical ages would be
$\tau_{(\rm A)}\sim$20 yr, $\tau_{(\rm B)}\sim$107 yr, and $\tau_{(\rm
C)}\sim$227 yr. If the jet is inclined with respect to the plane of sky by 30∘
at most, the dynamical ages of the knots may be older than what was estimated
assuming $i$=0∘ by 16% at most.

We note that the radial velocity of the jet goes from V $\sim$ -6.2 km s-1 to
V$\sim$ +5.4 km s-1 (see Fig. 3). This might be due either to deceleration
from an interaction with the material within the FIR 4 cloud and/or to a
change in jet direction with respect to the line of sight. The high-velocity
dispersion, observed to the south of arc 1 along the direction of the jet from
VLA15, suggests that it could be interacting with this bow-shock.

Figure 7: Velocity integrated emission of the SiO jet driven by VLA15. The
colour images are for intensities higher than 3$\sigma$ with $\sigma$= 4.8 mJy
beam-1 km s-1. The cut positions at which the widths were measured are shown
with white horizontal lines. The knot positions at which the dynamical ages
are estimated are shown with arrows and are labelled in white. VLA15 is
depicted with a magenta star and labelled in white. The synthesised beam is
depicted in the lower left corner. Figure 8: Deconvolved widths (2Rjet) of the
SiO(6–5) emission along the jet axis as a function of the distance from VLA15.
The solid red line corresponds to the best fit of the data points. The dashed
green line corresponds to an opening angle of $\alpha$=3∘ and an initial width
of 30 au.

###  4.4 Possible role of the outflow from HOPS-370

To the north of the OMC-2 FIR 4 region lies the intermediate-mass protostar
HOPS-370. This source has a rotating disk with an east-west velocity gradient.
It also drives a high-velocity outflow going from -56 km s-1 to 66 km s-1 with
respect to the HOPS-370 systemic velocity (Tobin et al. 2019). The outflow
extends from the north-east (NE) to the south-west (SW) of HOPS-370, and it
was observed down to $\sim$0.09 pc south of HOPS-370 (Shimajiri et al. 2008).
It was identified through many outflow tracers such as CO (Aso et al. 2000;
Williams et al. 2003; Shimajiri et al. 2008, 2015; Wu et al. 2005; Takahashi
et al. 2008; González-García et al. 2016; Tobin et al. 2019), HCO+ (Aso et al.
2000; Shimajiri et al. 2015), CS, HCN (Shimajiri et al. 2015), and [O I]
(González-García et al. 2016). It was also observed with VLA at 9 mm and 5 cm
by Tobin et al. (2019) and Osorio et al. (2017), respectively. The blue- and
redshifted components of the outflow are observed in both directions (Aso et
al. 2000; Takahashi et al. 2008; Shimajiri et al. 2008; Tobin et al. 2019).
Some authors argued that the outflow directly interacts with clump FIR 4 and
triggers the star formation within it (e.g. Shimajiri et al. 2008; Manoj et
al. 2013; Shimajiri et al. 2015; González-García et al. 2016; Osorio et al.
2017), while others excluded interaction and suggested that spatial
coincidence between the outflow from HOPS-370 and clump FIR 4 is a projection
effect (Favre et al. 2018).

The jet driven by HOPS-370 is likely precessing because the direction of the
jet detected in [O I] by González-García et al. (2016) is not coincident with
that of the 9 mm jet, nor with the axis of outflow cavities detected in CO,
which are perpendicular to the disk of HOPS-370 (Tobin et al. 2019). This
precession pattern is also supported in our work by the detection of two
bright arcs seen in SiO emission, called arc 1 and arc 2. In Figs. 3-4 we draw
arrows along the directions of the [O I] jet, the axis of the outflow cavities
seen in CO, and the SiO peaks on the bow-shock features revealed by our
observations. The SiO moment 0 and 1 maps show multiple bow-shock features at
different velocities, whose emission is not associated with that of the
continuum, nor with that of the other tracers (see Figs. 1 and 13). The
geometry of the bow shocks suggests that this is due to a jet activity from
HOPS-370. More precisely, the bow shocks might be the signature of a
precessing jet that extends from east to west. Hence, the western part of the
protocluster is affected by the mass loss arriving from the north.
Furthermore, SiO leaves a narrow-line imprint along the eastern ridge. This
may be induced by one of the processes discussed in the previous section (i.e.
gentle filament merging, winds from nearby H II regions, expanding shells, or
stellar feedback). Alternatively, the jet from HOPS-370 may have also
contributed to compress and shock the filamentary structures in the
protocluster. Nonetheless, due to the limited number of molecular lines in our
dataset, we were not able to prefer one of the two different scenarios.

In this paragraph, we estimate the dynamical ages of the arc 1 and arc 2 bow-
shocks. The jet is thought to lie close to the plane of the sky (González-
García et al. 2016; Tobin et al. 2019). When we consider the CO jet radial
velocity ($\sim$50–60 km s-1) derived by Tobin et al. (2019) and assume a
typical jet velocity Vjet = 100 km s-1 (Podio et al. 2021), the inclination
angle would be $i<30^{\circ}$ with respect to the plane of the sky. If the jet
lies on the plane of the sky ($i=0^{\circ}$), the distance between HOPS-370
and the SiO peak in arc 1 and arc 2 would be $d_{\rm(Arc\,1)}\sim$10330 au and
$d_{\rm(Arc\,2)}\sim$14150 au. Hence, the dynamical ages would be
$\tau_{\rm(Arc\,1)}=d_{\rm(Arc\,1)}$/ V${}_{jet}\sim$490 yr and
$\tau_{\rm(Arc\,2)}\sim$670 yr, respectively. The jet may be inclined with
respect to the plane of the sky by $30^{\circ}$ at most, which implies that
the shock dynamical ages may be older than what was estimated assuming $i$=0∘
by 15% at most. The fact that arc 2 is older than arc 1 suggests that the jet
of HOPS-370 is precessing from east to west and then proceeds away.

Interestingly, the outflow of HOPS-370 (producing arc 2) does not appear to
affect the highly collimated jet from VLA15. This indicates that the outflow
and the jet are spatially separated, hence the proximity of arc 2 to the VLA15
jet is merely a projection effect. This result, as well as the spatial
dichotomy between the material traced in SiO and that traced by the other
molecular tracers, confirms that the outflow from HOPS-370 has a tangential
impact on OMC-2 FIR 4 rather than a direct one, as previously suggested by
Tobin et al. (2019).

##  5 Conclusions

We have investigated the internal structures of the protostellar cluster OMC-2
FIR 4 with ALMA observations at 0.04 pc scale and $\sim$100 au angular
resolution. We mapped the emission of eight different molecular tracers: SiO,
CH3OH, C18O, CS, CH3CN, H13CN, HC3N, and CCH, probing high-density gas and
shocks. The analysis was mainly focused on SiO and CH3OH, and our main
conclusions are summarised below.

  1. 1.

A net of intricate filamentary structures was revealed in all the tracers
probing extended structures. One of the filamentary structures is aligned with
one of the fibres observed in N2H+ by Hacar et al. (2018), and the other short
filamentary structures could be connected to the intra-hub fibres that are not
resolved at parsec scales.

  2. 2.

From the analysis of CH3OH kinematics, we showed that each filament moves at a
different velocity. The filamentary structures show supersonic motions likely
due to protostellar feedback.

  3. 3.

We observed SiO imprints on some filamentary structures suggesting that they
trace compressed and/or shocked gas.

  4. 4.

SiO emission unveiled multiple bow-shock features with sizes between $\sim$500
and 2700 au. Their geometry and age suggest that they are likely caused by a
precessing jet from HOPS-370 that extends from east to west. Their dynamical
ages are $<$800 yr.

  5. 5.

We observed a spatial dichotomy between SiO and CH3OH emission within the
protocluster, where SiO dominates the western region and CH3OH dominates the
eastern region, suggesting that the jet from HOPS-370 does not directly impact
OMC-2 FIR 4.

  6. 6.

We detected a highly collimated ($\sim$1∘) SiO jet extending along the S-N
direction with a projected length of $\sim$5200 au from the embedded protostar
VLA15.

In a nutshell, the observations at high-angular resolutions and sub-parsec
scales have allowed us to reveal the previously unresolved hub filamentary
structures within OMC-2 FIR 4. Our study complements the studies performed at
$\sim$1 pc scales using N2H+, and indicates that the formation of such complex
structures could show how cluster-forming regions evolve, with the material
being compressed within narrow filaments due to protostellar feedback. Imaging
multiple molecular lines is hence important to characterise the gas properties
and the processes at play within OMC-2 FIR 4 and other protoclusters.

###### Acknowledgements.

This project has received funding from the European Union’s Horizon 2020
research and innovation program under the Marie Skłodowska-Curie grant
agreement No 811312 for the Project ”Astro-Chemical Origins” (ACO). This paper
makes use of the following ALMA data: ADS/JAO.ALMA 2016.1.00376.S. 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. CCo and LP acknowledges the
project PRIN-INAF 2016 The Cradle of Life - GENESIS-SKA (General Conditions in
Early Planetary Systems for the rise of life with SKA), and the PRIN-MUR 2020
MUR BEYOND-2p (Astrochemistry beyond the second period elements, Prot.
2020AFB3FX). ALS, CC, CCo, MDS and MB acknowledge the funding from the
European Research Council (ERC) under the European Union’s Horizon 2020
research and innovation program, for the Project ”The Dawn of Organic
Chemistry” (DOC), grant agreement No 741002.

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##  Appendix A Spectra

We report in Fig. 9 the spectra of CH3OH (4${}_{-2,3}-$3-1,2 E) masers,
observed at three spots along the eastern ridge of OMC-2 FIR 4. In Fig. 10 we
show the CH3OH (5${}_{1,4}-$41,3 A) spectra extracted from the different
positions shown Fig. 4. In Fig. 11 we present an SiO spectrum extracted at a
position along the eastern ridge.

Figure 9: Spectra of the masers detected in CH3OH(4${}_{-2,3}-$3-1,2 E) at the
P1-P3 positions. The vertical dashed line stands for the systemic velocity of
OMC-2 FIR 4 Figure 10:  CH3OH (5${}_{1,4}-$41,3 A) spectra extracted from
three different positions (A, B, and C) shown in the moment-2 map (See Fig.
4). The first shows a region with low-velocity dispersion, the second shows a
region with intermediate-velocity dispersion, and the third shows the central
region with several components along the line of sight. Figure 11: SiO
emission representative of the eastern ridge extracted at the position
(6.5$\arcsec$,2.1$\arcsec$)

##  Appendix B Jet simulation

In this section, we describe the simulation that we have performed to verify
that the narrow opening angle of the jet is not the result of interferometric
filtering. We first created rectangular structures with a fixed length and
different widths between 1/6 and 10 times the size of the synthesised beam
($\sim$0$\aas@@fstack{\prime\prime}$05–3$\aas@@fstack{\prime\prime}$2). The
interferometric response to each structure was determined by entering the
visibilities corresponding to each structure into the SiO jet uv-table. The
visibility weights were kept unchanged. To investigate the influence of
thermal noise on the image reconstruction, Gaussian noise was added to the
visibilities according to their weights. Finally, the uv-table of each
structure was Fourier-transformed to produce a dirty map, and was then
cleaned. The results are shown for a possible thermal noise model in Fig. 12.

Figure 12:  Interferometric response to each structure after running the
model. The width w of the rectangular (jet-like) structures given as input in
the model is depicted in white in the upper left corner. The synthesised beam
is depicted in white in the lower left corner. The colour scale is the same
for all the panels. The box size is 5$"\times$5$"$.

##  Appendix C Maps

In Fig. 13 we present the dust emission with superimposed contours of the
velocity-integrated emission of SiO at different velocity regimes discussed in
the main text. In Figs. 14-18 we report the channels maps of
CH3OH(5${}_{1,4}-$41,3 A), CH3OH(4${}_{-2,3}-$3-1,2 E), HC3N(3–2), C18O(2–1),
CH3OH(4${}_{2,3}-$51,4 E), CS(5–4), and SiO(6–5).

Figure 13: Continuum emission at 232 GHz in colours, with superimposed
contours of the velocity-integrated emission of SiO at different velocity
regimes. The colour images are for intensities higher than 3$\sigma$ with
$\sigma$ = 91.7 $\mu$Jy beam-1. The SiO contours are at 5$\sigma$ with
$\sigma$= 5.8 mJy beam-1 km s-1. They correspond to the blueshifted high-
velocity regime (blue), the blueshifted low-velocity regime (cyan), and the
redshifted low-velocity regime (red) (See Sect. 3.2). The different cores are
depicted with magenta stars and are labelled in black. The positions of the
SiO peaks from this study are depicted with open magenta triangles. We draw
four arrows from HOPS-370, one perpendicular to the disk, two towards the SiO
peaks, and one similar to the direction of the jet in [O I] by González-García
et al. (2016). The synthesised beam of SiO and continuum are depicted in the
lower left corner in blue and grey, respectively.

Figure 14: Channel maps of methanol at 50 K (top) and 45 K (bottom). The
contours are at [5,20]$\sigma$ (top) and [5, 20, 40, 80]$\sigma$ (bottom) with
$\sigma$= (5 mJy beam-1 km s-1 top, and 8 mJy beam-1 km s-1 bottom). The cores
are depicted with magenta stars and are labelled in black. The synthesised
beam is depicted in the lower left corner. We draw one arrow from HOPS-370
perpendicular to its disk. The beam, maximum recoverable angular scale (MRS),
and velocity resolution values are reported in the bottom right panel. Figure
15: Channel maps of HC3N. Contours start at 3$\sigma$ and increase by
5$\sigma$ with $\sigma$= 4 mJy beam-1 km s-1. The cores are depicted with
magenta stars and are labelled in black. The synthesised beam is depicted in
the lower left corner. We draw one arrow from HOPS-370 perpendicular to its
disk. The beam, MRS, and velocity resolution values are reported in the bottom
right panel.

Figure 16: Channel maps of C18O (left) and CH3OH at 61 K (right). Contours
start at 5$\sigma$ for C18O, and 3$\sigma$ for CH3OH and increase by 5$\sigma$
with $\sigma$= (4 mJy beam-1 km s-1 left, and 6 mJy beam-1 km s-1 right). The
cores are depicted with magenta stars and are labelled in black. The
synthesised beam is depicted in the lower left corner. We draw one arrow from
HOPS-370 perpendicular to its disk. The beam, MRS, and velocity resolution
values are reported in the bottom right panel. Figure 17: Channel maps of CS.
The contours are at [5, 20, 40]$\sigma$ with $\sigma$= 4 mJy beam-1 km s-1.
The cores are depicted with magenta stars and are labelled in black. The
synthesised beam is depicted in the lower left corner. We draw one arrow from
HOPS-370 perpendicular to its disk. The beam, MRS, and velocity resolution
values are reported in the bottom right panel. Figure 18: Channel maps of SiO.
The contours are at [5, 20, 40]$\sigma$ with $\sigma$= 2.5 mJy beam-1 km s-1.
The cores are depicted with magenta stars and are labelled in black. The
synthesised beam is depicted in the lower left corner. We draw one arrow from
HOPS-370 perpendicular to its disk. The beam, MRS, and velocity resolution
values are reported in the bottom right panel.