Resolving Twin Jets and Twin Disks with JWST and ALMA: The Young WL 20 Multiple System
Mary Barsony Michael E. Ressler Valentin J. M. Le Gouellec Łukasz Tychoniec Martijn L. van Gelder





















































Mary Barsony
marybarsony60@gmail.com

0009-0003-2041-7911]Mary Barsony
13115 Dupont Road, 

Sebastopol, CA 95472, USA

0000-0001-5644-8830]Michael E. Ressler
Jet Propulsion Laboratory, California Institute of Technology, 

4800 Oak Grove Drive, 

Pasadena, CA 91109, USA

0000-0002-5714-799X]Valentin J.M. Le Gouellec
NASA Postdoctoral Fellow
NASA Ames Research Center,

Space Science and Astrobiology Division, 

M.S. 245-6, 

Moffett Field, CA  94035, USA

0000-0002-9470-2358]Łukasz Tychoniec
Leiden Observatory, 

Leiden University, 

P.O. Box 9513,

2300RA Leiden, The Netherlands

0000-0002-6312-8525]Martijn L. van Gelder
Leiden Observatory, 

Leiden University, 

P.O. Box 9513, 

2300RA Leiden, The Netherlands














§ ABSTRACT 



We report the discovery of jets emanating from pre-main-sequence objects exclusively at mid-infrared wavelengths,
enabled by the superb sensitivity of JWST's Mid-InfraRed Medium-Resolution Spectrometer (MIRI MRS) instrument. These jets are observed only in
lines of [NiII], [FeII], [ArII], and [NeII]. The H_2 emission, imaged in eight distinct transitions, has a completely 
different morphology, exhibiting a wide-angled, biconical shape, symmetrically distributed about the 
jet axes.  Synergistic high-resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations
resolve a pair of side-by-side edge-on accretion disks lying at the origin of the twin mid-infrared jets.
Assuming coevality of the components of the young multiple system under investigation,
the system age is at least (2 - 2.5) × 10^6 yr, despite the discrepantly
younger age inferred from the spectral energy distribution of the combined edge-on disk sources.
The later system evolutionary stage is corroborated by ALMA observations of CO(2-1), ^13CO(2-1), and 
C^18O(2-1), which show no traces of molecular outflows or remnant cavity walls.
Consequently, the observed H_2 structures must have their origins in wide-angled disk winds,
in the absence of any ambient, swept-up gas. 
In the context of recent studies of protostars, we propose an outflow evolutionary scenario
in which the molecular gas component dominates in the youngest sources,
whereas the fast, ionized jets dominate in the oldest sources, as is the case for the twin jets discovered in the WL 20 system.





























§ INTRODUCTION

Most stars form in multiple systems <cit.>, defying simple theoretical collapse models <cit.>.
It is now recognized that in addition to gravity and thermal pressure, turbulence, magnetic fields, and
interactions between the members of multiple systems all play a role in their formation <cit.>.
Observational studies of multiple formation are, therefore, crucial for further progress.
How do the individual members of a multiple system coevolve? 

The WL 20 triple system in Ophiuchus (Oph) is of great interest in this context, since it is a member of a rare class of multiple systems known as InfraRed Companion (IRC) systems, 
in which one member has the appearance of being significantly younger than its companions, despite the fact that they have all formed from a single core e.g.,
T Tau, Glass I, Haro 6-10, Z CMa, XZ Tau, DoAr24E
<cit.>.
Such systems are particularly interesting for pre-main-sequence evolutionary studies, since they pose
a puzzle as to why one of two or more coeval components appears significantly redder, and, in this case,
more luminous than its companions.

WL 20 was discovered in the course of a 2 μm bolometer survey, carried out with a 12^'' beam,
of the 10.5^'× 10.5^' region exhibiting the the strongest C^18O emission in the Rho Ophiuchi (ρ Oph) cloud 
<cit.>. When near-infrared arrays became available, WL 20 was resolved first into a binary
and, eventually, into a triple system, at 2.2 μm  <cit.>.
WL 20E (K=10.13) and WL 20W (K=10.40) have an angular separation of 3.17^'' at P.A. 270^∘ E of N,
whereas the IRC, WL 20S, lies 2.26^'' from its nearest neighbor, WL 20W, at a P.A. of 173^∘ <cit.>.

The first near-infrared spectra of the two brighter components, obtained with R ≡λ/δλ≤ 1000, 
determined spectral types consistent with K-M for WL 20E through an extinction of
A_V=15.4 and K7-M0 through A_V=18.1 for WL 20W <cit.>. Subsequent higher-resolution (R ∼ 1200) near-infrared
spectra established spectral types of K6 for WL 20E (GY 240B) and M0 for WL 20W (GY 240A), both seen through A_V = 16.3 <cit.>. 
It took NIRSPEC on the Keck II 10.4-meter telescope to finally obtain a spectrum of the fainter (K=12.6)
IRC, WL 20S. The R = 2200 spectra of each component refined the spectral types of WL 20E to K7 IV/V and WL 20W to M0 IV/V,
using veiling independent line ratios, whereas the spectrum of WL 20S is so heavily veiled that no absorption lines could be detected in its spectrum. 
Nevertheless, the spectral shape and δ K = 2.2 mag brightness difference between WL 20S
and WL 20W constrains WL 20S to have an infrared excess r_K < 0.9, but with an additional A_V=25, relative to its neighbors <cit.>.

Mid-infrared observations of WL 20 were first acquired with a 6^'' (or 8^'')
aperture at 10 μm, which did not resolve the multiple system <cit.>. 
In the morphological classification scheme of pre-main-sequence spectral energy
distributions (SEDs) devised by Lada and coworkers (e.g., <cit.>),
WL 20 was classified as a Class I object <cit.>. 
Diffraction-limited mid-infrared imaging on the Keck II telescope allowed 
spatially resolved determination of each component's SED, confirming the previous Class II
spectroscopic classifications of WL 20E and WL 20W <cit.>, but demonstrating the 
Class I SED of WL 20S <cit.>. 

The first millimeter continuum detection of WL 20 was with the IRAM 30-meter telescope at 1.3 mm
with an 11^'' beam <cit.>. Interferometric observations
with the six-element Owens Valley Radio Observatory (OVRO) array were required to identify WL 20S as the
source of the millimeter dust continuum emission associated with the system <cit.>.
The single-telescope and interferometric flux measurements were consistent, implying a compact source
structure origin, with no emission from any envelope component.  
This conclusion is further corroborated
by comparison of HCO+ J = 4 → 3 and 850 μm dust maps of WL 20S: Since a hallmark of the earlier
evolutionary stage is the presence of a centrally condensed envelope, such sources should exhibit 
HCO+ J = 4 → 3 emission coincident with the dust continuum peak, since
this transition has a high critical density (> 10^6 cm^-3), unique to
the dense gas located in the inner regions of protostellar envelopes. Such a spatial coincidence was not
found for WL 20S, leading to the conclusion that it lacks an infall envelope component, and 
sports a Class I SED due to its edge-on orientation <cit.>.

Intriguingly, the 12.81 [NeII] line was detected in WL 20 in a 4.7^'' slit at R=600 by Spitzer's IRS,
but was undetected in a much narrower, 0.4^'' slit by the Very Large Telescope's (VLT's) VISIR at R=30,000, 
implying a spatially extended origin for the emission <cit.>. 

Thermal radio jets are typically associated with Class 0/I protostars <cit.>. A 6 cm source detected with the Very Large Array (VLA) in an 
11^''× 5^'' beam was first associated with WL 20 in
the survey of two dense cores, A and E/F, in the ρ Oph star-forming cloud <cit.>. 
Higher-angular-resolution JVLA 3.0 cm maps (5.1^''× 2.4^'' beam at P.A.= -5^∘) 
associated the radio jet with WL 20S <cit.>.

To further delve into the mysteries posed by this system, we have acquired JWST MIRI MRS
integral field unit imaging spectroscopy encompassing all three components, 
covering the 5 - 28 μm wavelength range, supplemented  by high-resolution ALMA data.

The paper is structured as follows:  Observations and Data Reduction for MIRI MRS and for the ALMA data are presented in 2.1
and 2.2, respectively. MIRI MRS results are provided in 3.1 as follows:  3.1.1 covers continuum images,
3.1.2 shows the on-source spectra,  3.1.3 highlights jet images and spectra in both low- and high-excitation lines,
and 3.1.4 features the molecular hydrogen line images.
ALMA results are presented in 3.2:   3.2.1 features Band 4 (1.9 mm) continuum images and 3.2.2 both Band 6 (1.3 mm)
continuum and CO(2-1), ^13CO(2-1), and C^18O(2-1) line maps.
Discussion is contained in 4 and Conclusions in 5.



§ OBSERVATIONS AND DATA REDUCTION



 §.§ MIRI MRS


WL 20 was observed during UTC 12-13 April 2023 
as part of Guaranteed Time Observations (PID 01236; P.I. Ressler) with JWST's
MIRI <cit.>
in its MRS mode <cit.>.
Observations were acquired with a two-point dither pattern.
The pointing center for all observations was α_2000 = 16h 27m 15.77s, δ_2000 -24^∘ 38^' 44.3^''.
To maximize on-source integration time,
no dedicated background observations were undertaken. Instead,
background observations acquired earlier under PID 01236 are used to subtract the telescope
background and detector artifacts during the data processing.
All three gratings (A, B, and C) were employed through all four MIRI Channels,
using the FASTR1 read mode, thereby covering the entire 4.9 - 28.6 μm spectral range at resolutions ranging from 3500 ≥ R ≥ 1500.
Integration times were 15 minutes 55 seconds through the short grating, and 15 minutes 52 seconds in each of the medium and long gratings. 
In addition, the F1500W filter was chosen for parallel off-source imaging for the duration of the MIRI MRS observations.

There are three necessary data-processing steps for producing usable science data from
the MIRI MRS raw data.
These were performed using the JWST pipeline version 1.11.4 <cit.> using reference context
jwst_1118.pmap of the JWST Calibration Reference Data System (CRDS; <cit.>).
Level 1 processing was performed with the default settings in
 the Detector1Pipeline. 
 Level 2 processing was performed using the Spec2Pipeline.  During this step, the dedicated background of Program 01236,
 obtained at a different time and region of sky, was subtracted on the detector level in order to subtract the telescope background and detector artifacts.  
 Additionally, fringe corrections were performed using the fringe flat for extended sources (Mueller, M. et al., in prep.) 
 and detector level residual fringe corrections were applied (Kavanagh, P.  et al., in prep.). 
 Finally, the Spec3Pipeline was run with both the outlier rejection
 and master background subtraction steps switched off. This processing resulted in 12 calibrated data cubes,
 one for each combination of channel and grating settings.



 §.§  ALMA


Band 6 (1.3 mm) ALMA observations of the WL 20 system were obtained as part of program 2019.1.01792.S (PI: D. Mardones). Two datasets are used, one high-angular-resolution dust continuum observation reaching ∼ 0.11^'' with 36 sec of integration time, and a lower-angular-resolution observation reaching ∼ 1.1^'' with 194 sec of integration time. The longer-integration time, lower-angular-resolution data target various molecular lines with high spectral resolution (i.e., 0.08 km s^-1). In this work we will focus on the CO(2→1), ^13CO(2→ 1), C^18O(2→ 1) transitions. We used the  task of CASA version 6.5.2 to produce the dust continuum image and molecular line channel maps, with Briggs weighting and the auto-multithresh option for the masking operations. Robust parameters of 0.0 and -0.5 were used for the dust continuum and molecular line maps, respectively. We reached noise levels of 0.27 mJy beam^-1 and 25 mJy beam^-1 per 0.125 km s^-1, for the continuum and spectral line observations, respectively,
calculated using the root-mean-square (rms) flux of an emission-free region in the image plane in each instance.

Band 4 (1.9 mm) 155 GHz ALMA observations were obtained on 2023 June 12 within program 2022.1.01734.S (PI: Ł. Tychoniec) with 30 sec integration time.
The observations were self-calibrated with the  package[https://github.com/jjtobin/auto_selfcal]. 
The resulting measurement set was imaged with the  procedure within CASA version 6.5.2 <cit.>. A robust parameter of 0.5 was used, and automasking with standard parameters was applied.
The resulting image has a resolution of 0.096^'' × 0.16^'' and sensitivity of 0.075 mJy beam^-1 measured from the rms signal in an emission-free part of the continuum image.




§ RESULTS
 


 §.§ MIRI MRS




  §.§.§ Line-free Continuum Images:  Newly Discovered Source in WL 20S


Figure <ref> 
shows the appearance of the WL 20 system at four different line-free continuum
wavelengths through the SHORT (A) grating in each of MIRI's four MRS Channels (see Table 1 of <cit.>).
Surprisingly, a faint, new source, WL 20SE, was discovered next to WL 20S, evident at the shorter MIRI MRS wavelengths (left panel of 
Figure <ref>), with a separation of 
∼ 0.58^'' ± 0.03^'' at PA 76.1^∘ ± 0.5^∘ (measured E from N),   
relative to the brighter component, WL 20S. In the 8.1 μm continuum image and at longer wavelengths, however, 
this new source can no longer be separated from its brighter
companion, their fluxes being blended together due to the increasing point-spread-function (PSF) with wavelength.  
Consequently, when these two objects are spatially resolved, the IRC source, previously designated as WL 20S,
will now be referred to as WL 20SW, and its newly discovered neighbor will be called WL 20SE.

Table <ref> lists the detected sources and source coordinates, as determined
from the WCS header coordinates produced by the JWST pipeline for MIRI MRS and refer to 
the source centroid coordinates at 5.3 μm. A second set of coordinates, derived
from the ALMA continuum observations as described in <ref>  ALMA Band 4 Data,  is also listed.

In order to determine whether or not any of the continuum sources are extended, we
used the PSF standard observations of 10 Lac (PID 3779;  PI:  D. Gasman) with which to compare
radial profiles of WL 20E, WL 20W, and WL 20SW. 
WL 20SE has too low signal-to-noise and is too confused
with WL 20SW to state one way or the other as to whether or not it is extended.
The azimuthally averaged cross-cuts at 6.2 μm and 16.6 μm of
10 Lac, WL 20E, WL 20W, and WL 20SW, each scaled to their individual peak pixel value,
are shown in Figure <ref>. Examination of this figure
demonstrates that whilst WL 20E and WL 20W are consistent with being point sources,
WL 20SW is definitely extended at these wavelengths.

Continuum fluxes for each component of the WL 20 quadruple system are listed in Table <ref>.
The MIRI MRS fluxes are measured at the median wavelengths of the SHORT grating in each of the four MIRI MRS channels.
The apertures through which the fluxes are measured have diameters that vary with wavelength as 
2.0 × FWHM_PSF for all sources except for WL 20E, for which the aperture diameter was varied as  3.0 × FWHM_PSF,
where FWHM_PSF is given by 0.033 (λ / μ m)  +  0.106^''  <cit.>. 
The larger apertures were necessary for WL 20E in order to minimize fringing at the shortest wavelengths, since WL 20E was very close to the edge of the detector for Channel 1. 
 ALMA Band 6 (1.3 mm) and  Band 4 (1.9 mm) continuum fluxes, and the derived dust disk masses, are also tabulated in Table  <ref>; how these were arrived at
 is detailed in <ref>.





l   c  c  c  c  c  c   |c c c c c c

7
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The WL 20 System:  Coordinates  
 
Source      6cSource Coordinates (MIRI MRS)       6cSource Coordinates (ALMA Bands 4 & 6) 
   
Name        3c α (2000)      3c δ (2000)   3c α (2000)    3c δ (2000) 
  
                 h     min      sec       ^∘           ^'         ^''      h     min      sec       ^∘           ^'         ^''  

WL 20E                 16     27     15.9075       -24     38    43.8180      16     27     15.889       -24      38     43.977   

   WL 20W                 16     27     15.666        -24     38    43.930        16     27     15.652        -24     38      43.982    

   WL 20SW               16     27     15.686        -24     38    46.219       16      27     15.674        -24     38      46.260    

   WL 20SE               16    27      15.728         -24      38    46.141      16      27     15.713        -24     38      46.133   



l c c c c c c c    

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The WL 20 System:  Continuum Fluxes and Disk Dust Masses  
 
Source     Flux at       Flux at           Flux at                       Flux at              Flux at      Flux at       Dust mass   
 
Name      5.3 μm    8.1 μm   12.5 μm       19.3 μm       1.3 mm     1.9 mm          

                 (mJy)                  (mJy)                  (mJy)                  (mJy)       (mJy)      (mJy)    M_⊕   

WL 20E                 81± 0.5                 27 ± 1             85  ± 0.5          95 ± 2          2.1 ± 0.2      1.3 ± 0.1      3.3 ±  0.4           

  WL 20W                 34 ±  2                 42 ± 1             50  ± 1             133  ± 1         2.8 ± 0.3       1.4 ± 0.2    3.6 ± 0.5           

  WL 20SW                 14 ± 2                 89 ±1             47.5  ± 5       2300 ± 5          36.1 ± 0.6    16.2 ± 0.3     42 ± 2          

  WL 20SE                3.7± 0.5              -                           -                                       -           20.1 ± 0.6      9.0 ± 0.3      24 ± 4       





  §.§.§ On-source Spectra


On-source MIRI MRS spectra of WL 20E, WL 20W, and WL 20S,
are presented in the top, middle, and bottom panels of Figure <ref>, respectively. 
Spectra were extracted through apertures scaling as  2.0 × FWHM_PSF for WL 20W and WL 20S,
and as 3.0 × FWHM_PSF for WL 20E. The larger aperture was used for WL 20E in order to minimize fringing at the shortest wavelengths,
since WL 20E was close to the edge of the detector for Channel 1.
Although the aperture through which the WL 20S spectrum was extracted is centered on the coordinates of WL 20SW, 
the spectra of WL 20SW and WL 20SE are blended longwards of 6.0 μm.

Spectra of the faint new source, WL 20SE and its bright companion, WL 20SW, are presented in
Figure <ref>, in the  wavelength region where the two sources can be spatially resolved.
Note the bright emission lines of H_2 and [FeII].
The 5.3 μm continuum flux level of WL 20SE is a factor
of 3.8 times weaker than that of its neighbor, WL 20SW (see Table <ref>).
In the resolved spectra of Figure <ref>, WL 20SW displays a steeply rising continuum in contrast
to the relatively flat continuum of its newly discovered neighbor, WL 20SE.

Figure <ref> shows the
spectra of WL 20E and WL 20W over this same wavelength interval.  
The spectral types and temperatures shown on their spectra are from <cit.>.
Note the much higher continuum fluxes of WL 20E at 81 mJy  and WL 20W 
at 34 mJy in this wavelength region relative to their southern neighbors,
WL 20SW at 14 mJy and WL 20SE at 3.7 mJy, emphasizing the large extinction difference between them.

Also notable is the complete absence of the  [FeII] and H_2 emission
lines in the spectra of WL 20E and WL 20W,
in striking contrast to their appearance in the spectra of the WL 20SE/WL 20SW pair.  

In comparison with recent MIRI/MRS spectra of Class 0 and Class I protostars,
what is remarkable in these spectra are the features which are lacking:
namely, the missing broad, deep absorption features associated with various ices, including
H_2O, CO, CH_3OH,  CH_4, NH_3, to name just a few <cit.>.
This is yet another indicator of the relatively advanced, Class II
stage, of the WL 20 multiple system. 

Although there are a wealth of spectral features in each component of the WL 20 system,
the focus of this work is on the [NeII] emission from each, and on the
emission lines found in the WL 20S pair.
Identifying and analyzing all of the spectral lines and solid state features detected in these spectra is left for future investigation.









  §.§.§ Jet Images and Spectra  in Low- and High-excitation Ionic Lines


A completely unexpected discovery from the MIRI MRS imaging data was that of twin, parallel
jets, powered by the sources WL 20SW and WL 20SE in both low- and high-excitation ionic lines.
Table <ref> lists the ionic transitions, excitation potentials, wavelengths, spectral resolutions at these wavelengths, and velocity
extents of the jets.  Continuum-subtracted, emission-line images of these twin jets, each powered by one component of the WL 20S binary, are
presented in Figure <ref> for the [FeII] lines, in Figure <ref> for the [NiII] lines,
and Figure <ref> in the [ArII] and [NeII] lines.  

Surprisingly, WL 20SE, the faint, newly discovered source, drives jets that have stronger emission in the
highly excited [ArII] and [NeII] lines than the jets in these lines emanating from its brighter neighbor, WL 20SW.
The situation is reversed in the lower-excitation [FeII] and [NiII] lines, in which the WL 20SE jets 
are fainter than the corresponding jets seen in these lines driven by WL 20SW.

Since these results are difficult to see from the jet images of Figures <ref>, <ref>, and <ref>, 
in order to best distinguish the separate jets driven by WL 20SE and WL 20SW,
we present zoomed-in views of the twin jets in Figure <ref>.
The top panel shows the unprocessed, continuum-subtracted line images
of [FeII]  (5.340 μm), [NiII] (6.636 μm), and  [ArII] (6.985 μm).
These transitions were chosen because they are at the shortest wavelengths in the  low- and high-excitation jets,  
where the angular resolutions are best. The bottom panel of Figure <ref> shows the 
corresponding deconvolved images, in which the faint southern jet, driven by WL 20SE,
is clearly seen and is indicated by the white arrows.

To further highlight the differences in excitation of the jets, additional jet spectra were obtained. The jet spectra
were extracted from carefully
sized and placed apertures to minimize cross-contamination between the jets.  The locations and sizes of these apertures are depicted by white circles
on the continuum-subtracted 6.985 μm [Ar II] line image in the top-right panel of Figure <ref>.
The central coordinates of the circular, 1.00^'' diameter apertures used to extract the jet spectra
are listed in Table <ref>.
For the WL 20SW jet, the chosen aperture is to the northwest of WL 20SW, to best isolate its jet emission
from the WL 20SE jet's emission. For the WL 20SE jet, the chosen aperture is located to the southeast of WL 20SE, 
as far as
possible from the southern jet driven by WL 20SW, to avoid contamination by WL 20SW's southern jet, but still capturing as 
much as possible of the emission from WL 20SE's jet.
These apertures were 
chosen so that they are separated by the FWHM_PSF even at emission lines detected at MIRI's longest wavelengths.
Nevertheless, there will be the inevitable contamination of the continuum levels of the extracted jet spectra, which increases
with wavelength. 
 
 The extracted spectra from these two apertures are presented in Figure <ref>. 
  At the shortest wavelengths, where the spatial resolution allows the best separation of the jet apertures from the continuum emission
 of WL 20SE and WL 20SW, the spectra exhibit a suppressed continuum dominated by emission lines from shocked gas.
 As we progress to longer wavelengths, the inevitable contamination of the continuum levels of the jet spectra increases
 in Figure <ref>. 
Examination of the jet spectra of WL 20SE and WL 20SW in Figure <ref> shows the presence of both low- and high-excitation
emission lines in the jets powered by each source – a result that is difficult to see from the jet images 
of Figures <ref> - <ref> alone.  The jet spectra also show the much stronger [ArII] and [NeII] line strengths in the jet driven by
WL 20SE compared with those of the WL 20SW jet.

lcccccc

7
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Low- and High-excitation Jet Emission Lines from WL 20SW and WL 20SE  
 
 Line     Wavelength     Transition     Excitation     Ionization     Spectral     Velocity
  
               (μm)                           Potential       Potential        Resolution     Extent

                                                          (eV)             (eV)                  km s^-1a       km s^-1  
 
[FeII]          5.340169      a4F9/2-a6D9/2      7.90     16.19       ∼ 83      -76.8→ +102.8   

 [FeII]          6.721283      a4F9/2-a6D7/2      7.90      16.19      ∼ 83       -3.7→ + 67.7 

 [FeII]        17.935950      a4F7/2-a4F9/2      7.90     16.19        ∼ 150     -183.0→ + 117.9 

 [FeII]        24.519250      a4F5/2-a4F7/2      7.90     16.19        ∼ 180     -149.8→ -3.0  

[FeII]        25.988290      a6D7/2-a6D9/2     7.90     16.19         ∼ 200      -130.2→ +77.4 

[NiII]           6.636000      2D3/2-2D5/2         7.64     18.17        ∼ 84       -90.4→ +126.5    

 [NiII]         10.682200      4F7/2-4F9/2          7.64     18.27       ∼ 92      -60.3→ +49.1    

 [ArII]          6.985274     2P1/2-2P3/2      15.76     27.63       ∼ 84      -71.8→ +65.5   

 [NeII]          12.813550      2P1/2-2P3/2     21.56      40.96      ∼ 100       -53.8→ +121.7 

aUsing R_MRS(max) value from Table 3, Column 6 for the central wavelength of each subband from Labiano et al. 2021, except for Channel 4,
where we have used the resolving power listed in the last column of Table 1 in <cit.>.

Line data are from https://www.mpe.mpg.de/ir/ISO/linelists










l c c c c c c

7
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Center Coordinates for Extracted Jet Spectra  
 
Jet       6cJet Aperture Coordinates 
  
Aperture        3c α (2000)      3c δ (2000) 
  
Name                 h     min      sec       ^∘           ^'         ^'' 

WL 20SW Jet                 16     27     15.6686         -24     38    45.0694    

   WL 20SE Jet               16    27      15.8045         -24      38    47.0638     







  §.§.§ Molecular Hydrogen Line Images


Figure <ref> displays the distribution of molecular hydrogen line emission in the eight transitions listed in Table <ref>, continuum-subtracted and 
integrated over all channels in which each line is detected.
Note the stark contrast between the appearance of the gas traced via the molecular hydrogen transitions and the jet-like structures evident in the ionized lines
of Figures <ref> through <ref>.



rcccccccc

9
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Molecular Hydrogen Lines Detected in the WL 20 System 

Wavelength     
Transition     
J     
J^'    
 g_J    
 E_u   
 A   
 Spectral Resolutiona           
 Velocity Extent

  (μm)                                                                               (K)        s^-1     (km s^-1)      (km s^-1) 

17.0348       0-0 S(1)     3     1     21       1015    4.761 × 10^-10         ∼ 121       -63.2  → +24.8

 12.2785        0-0 S(2)     4     2       9      1682     2.755 × 10^-9           ∼ 100       -57.6 → +3.4  

 9.66491        0-0 S(3)     5     3     33      2504     9.836 × 10^-9           ∼ 91         -5.3 → +75.4 

  8.02505       0-0 S(4)     6     4     13      3475     2.643 ×  10^-8          ∼ 85         -82.2 → +15.0 

  6.90952       0-0 S(5)     7     5     45      4586     5.879 ×  10^-8           ∼ 84       -83.3 → +55.5  

  6.10856       0-0 S(6)     8     6     17      5830     1.142 ×  10^-7           ∼ 83       -86.9 → +30.9  

  5.51116       0-0 S(7)     9     7      57     7197      2.001 ×  10^-7          ∼ 83       -64.2 → +66.4

  5.05311      0-0 S(8)    10    8       21     8677      3.236 ×  10^-7          ∼ 83       -42.7 → +4.8 

aUsing R_MRS(max) value from Table 3, Column 6 for the central wavelength of each subband from Labiano et al. 2021


In order to examine the physical conditions of the molecular hydrogen gas, we have produced an excitation
diagram obtained at the apex of the brightest H_2 S(5) emission north of WL 20E,
at J(2000) 16h 27m 15.85s, -24^∘ 38^' 41.89^'', simultaneously fitting for 
A_V and T_ex.  The results are displayed in Figure <ref>. The derived, best-fit
parameters at this position yield T_ex = 1161K  ± 70K, N_H2 = 7.98 ± 1.77 × 10^21 cm^-2,
and A_V = 12 ± 1.




 §.§ ALMA




  §.§.§ ALMA Band 4 Data


Figure <ref> shows Band 4 (1.9 mm) images of the WL 20 system obtained with ALMA. 
The WL 20S source is resolved into a binary, with the eastern component spatially coinciding with the newly discovered companion
evident at the shortest MIRI MRS wavelengths seen in Figure <ref>.

The millimeter source positions and continuum fluxes from 
ALMA Band 6 (1.3 mm) and  Band 4 (1.9 mm) are obtained by using the  procedure in CASA <cit.>. 
For WL 20E and WL 20W, each of which remains unresolved by ALMA, a single Gaussian fit was
used for both flux and positional determinations.  By contrast, WL 20SE and WL 20SW are both resolved by ALMA (see Figure <ref>),
and their elongated morphologies are best fit by a combination of two Gaussian components.
The tabulated positions for WL 20SE and WL 20SW are simply the mean value of the two Gaussian fits, whereas the reported fluxes are the sum of the
two Gaussian components fitted to each source. The resulting positions and fluxes are reported in Tables <ref> and <ref>, respectively.

The continuum fluxes can be used to infer disk dust masses, assuming optically thin dust emission, using the equation from <cit.>:

    M=D^2F_ν/κ_ν B_ν(T_ dust)

with D - the distance to the source, B_ν - the Planck function for a temperature T_ dust, κ_ν - the dust opacity. The temperature of the dust is assumed to be
30 K, typical for young protostellar envelopes <cit.>. A value of κ_1.9  mm= 0.6   cm^2 g^-1 was used, scaled from κ_1.3  mm= 0.9   cm^2 g^-1 provided in <cit.> using β = 1. <cit.>.  The resulting dust masses are reported
in the last column of Table <ref> in units of Earth masses.  





  §.§.§ ALMA Band 6 Data






Band 6 (1.3 mm) continuum ALMA data were acquired at similar resolution to the Band 4 (1.9 mm) data, and are 
represented by gray contours appearing in Figure <ref>.
Positions and fluxes of the sources detected in the  Band 6 continuum are presented in Tables <ref> and <ref>, respectively.

Figure <ref> shows the appearance of the WL 20 system in ^12CO J=2→1 (left panel),
^13CO J=2→1 (middle panel), and C^18O J=2→1 (right panel), all shown in color,
with their respective flux scales indicated by the color bars  in Jy/beam km s^-1 units. 
The FWHM beam size used for the CO isotopologue observations was roughly a factor of 10 larger in each dimension
than the beam size used for the continuum observations, as indicated in the Figure <ref> caption.
The ^12CO J=2→1 emission appears to peak near the elongated continuum structures associated with WL 20SW and its newly discovered companion, WL 20SE.
The gray ellipse drawn in each panel of Figure <ref> indicates the location and extent of this CO J=2→1  peak,
for direct comparison with the gas morphologies  evident in the ^13CO J=2→1
and  C^18O J=2→1 maps, as well as with the Moment 1 maps presented in Figure <ref>.

Although the ALMA CO observations do not have enough angular resolution to resolve
the twin disks of WL 20SE and WL 20SW individually, we can, nevertheless, determine velocity-integrated 3σ line fluxes from the ^13CO(2-1) and C^18O(2-1)
moment-zero maps for later use in determining the total gas mass associated with both disks together.
When measured within two beam areas (0.46^'' × 0.92^'' at P.A. = 345^∘),
centered on the two edge-on disks detected in the continuum,
the ^13CO(2-1) and C^18O(2-1) line fluxes
are 0.6869 Jy km s^-1 and 0.09979 Jy km s^-1, respectively.

In contrast to the CO J=2→1 peak associated with the twin disks of WL 20SW + WL 20SE,
there is a remarkable lack of gas emission toward either of the sources, WL 20E or WL 20W.  Quantitative upper limits
on this emission are derived by measuring the 3 σ rms in a line-free region of the ^13CO(2-1) moment-zero
map, which yields a value of ≤ 1.11 × 10^-3 Jy km s^-1,  and a corresponding 3 σ upper limit for the C^18O(2-1) emission ≤ 5.1 × 10^-4 Jy km s^-1.



§ DISCUSSION




 §.§ MIRI MRS Continuum Sources


lccccccc

8
0pt
  Continuum Flux Comparison of the WL 20 Components 
 
                              3c —— Keck II  ——–             3c  ——  MIRI MRS ——                
  [-10pt] 
            E                     W      SW + SE          E     W       SW  + SE       
  [-10pt]  
                                             Combined                             Combined          
 [-10pt]
Wavelength                                 6c—————————— Flux —————————————      
 [-10pt]
(microns)                             (mJy)     (mJy)      (mJy)                                    (mJy)     (mJy)       (mJy)        
 [-10pt]

7.9                                  121                  38.4                        123.0                            88          35        78.8                       

 10.3                                   72.6                49.6                        345.0                            76.9       71.3     178                         

  10.8                                 79.0                 51.5                        281.0                            85          73.4      249                          

 12.5                                  86.8                44.3                         610.0                            85          49.7      475                        

 17.9                                 78.0                93.9                         2720.0                           87         117.5      1955.7      Channel 3 LONGa 

 17.9                                 …    …    …    86          113        1822.5       Channel 4 SHORTa   

 20.8                                 109.0            117.0                         3700.0                           99.3       154.3     2856.4                      

 24.5                                  <155.0           <155.0                   6600.0                    116.0       241.6     4217.5             
         
 aFor 17.9 μm, we list two MIRI MRS measurements since this wavelength is covered at the edge of  Channel 3 LONG and in Channel 4 SHORT. 



The biggest surprise from the MIRI MRS continuum images presented in Figure <ref> is the discovery of a new member of the WL 20 system, WL 20SE,
adjacent to the previously known IRC source, which we are now calling WL 20SW.
The continuum mid-infrared appearance of the WL 20 system has previously been presented at 7.9, 10.3,  12.5, 17.9, 20.8, and 24.5 μm from Keck II imaging observations
with sufficient resolution at 7.9 and 10.3 μm to have resolved WL 20SE from WL 20SW  <cit.>.
The MIRI observations show WL 20SE to emit about 26.5% of the flux of  WL 20SW at 5.3 μm, a wavelength at which both sources are well resolved by MIRI (see Figure 
<ref>). 
If the 5.3 μm flux ratio between WL 20SE and WL 20SW  were constant out to 7.9 μm, and the source fluxes had not varied between the time of the MIRI and the Keck II observations, 
the contribution of WL 20SE to the previously reported 7.9 μm flux would have been 33 mJy, a level near the detection limit, judging from the lowest level contour of 40 mJy
in the 7.9 μm plot of the WL 20 system (Figure 1 d) of <cit.>).

In fact, <cit.> did
report that WL 20S showed extended  structure, beyond the point-source appearances of WL 20E and WL 20W (see their Figure 7). Furthermore, it was noted that the extended structure
did not vary with wavelength, in contrast to other Class I objects, which generally exhibit increasing size with wavelength.

The combined fluxes from WL 20SW and WL 20SE had exhibited mid-infrared variability in the past on timescales of a few years <cit.>; thus, it is 
of interest to compare the newly acquired MIRI MRS flux measurements with previously published ones. These measurements are presented in
Table <ref>, from which it is clear that the combined fluxes from WL 20SW and WL 20SE are consistently lower in 2023 
than they were in 1998.



 §.§ Gas Emission from Jets and Disk Winds


The most spectacular discovery of the MIRI MRS observations of the WL 20 multiple system
is that of the parallel, twin, bipolar jets powered by WL 20SE and WL 20SW, in multiple transitions of 
[FeII] and [NiII], as well as in the higher-excitation [ArII] and [NeII] ionic lines 
(see Figures <ref> - <ref> and Table <ref>).

This is the first young system, lacking any associated, cold molecular outflow,
as traced by, for example, CO 1-0 or CO 2-1 emission, 
to exhibit ionized jets, discovered via centimeter radio observations <cit.>, but now resolved and imaged in the mid-infrared.
Recent JWST studies of Class 0 and Class I outflows point to a unified
picture of nested structures, the three outermost of which have wide opening angles, with the outermost layer (if detected) seen in scattered
near-infrared light, the next inner layer mapped in millimeter CO lines, within which the mid-infrared hydrogen emission is found.  
Collimated jets seen via their ionic emission lines are found along the symmetry axis of the wide-angled, outer layers
 <cit.>.  In the case of WL 20,
the scattered light <cit.> 
and CO outflow components are missing, since, at this evolutionary stage, there is no infall envelope left.

None of the detected molecular hydrogen
line maps of Figure <ref> show evidence of jets emanating directly from WL 20SE and WL 20SW.
Instead, the molecular hydrogen maps display a distinct biconical morphology with the apex coinciding with the
locations of WL 20SE and WL 20SW.  The ionized, parallel jets propagate along the symmetry axis of the molecular hydrogen cone.
At first glance, one might be tempted to interpret the H_2 structure as outlining biconical cavity walls. However,
examination of the gas distribution in Figure <ref> shows a lack of surrounding gas within which to form a cavity.
Rather, there is a clear spatial anticorrelation of the cold, CO molecular gas and the warm H_2 gas, strongly
supporting the interpretation that the H_2 structure originates from wide-angled disk winds.

These observations can be understood in the context of the coevolution of the protostellar envelope
and the jet sources it surrounds.  For the youngest, Class 0 protostars, the majority of the mass is
still in the envelope, and, as jets emerge from the central accreting sources, there is plenty of molecular material to entrain, 
resulting in molecular jets and outflows. As the sources evolve through the Class I stage, most of the mass is now 
concentrated in the protostars, with some remnant molecular infall envelope material, allowing for the appearance of molecular outflows,
wide-angled disk winds, and the mid-infrared ionized jets.  Finally, by the Class II stage, the molecular material from the original
protostellar envelope is gone (see Figures <ref> and <ref>), so, if enough disk accretion activity is still ongoing, the jets
will appear as purely ionic, since there is now no ambient molecular material to entrain.  Furthermore, this circumstance 
strengthens the case for the interpretation of the mid-infrared molecular hydrogen structures of Figure <ref> as resulting
from disk winds, which must be the origin of the observed H_2, since the envelope material is long gone.



 §.§ Disks in the WL 20 System




  §.§.§ The Dust Disks Detected by ALMA


Both the 1.3 mm and the 1.9 mm ALMA continuum observations, acquired at similarly high resolutions (0.14^'' × 0.11^''
and 0.096^'' ×  0.16^'',  respectively) resolve the newly discovered WL 20SE source and the previously known WL 20SW into twin, edge-on disk structures
(see Figures <ref> and <ref>, respectively). 
The total 1.3 mm flux from the entire WL 20 system is 61.1 mJy (see Table <ref>),
to be compared with the previously published 95 mJy acquired with the 12^'' beam of the IRAM 30-meter, with a stated ∼ 20% flux calibration uncertainty
 <cit.>.  Thus, within the stated uncertainties, 
  the high-resolution ALMA observations are consistent with having detected all of the continuum emission from the system.

The edge-on disk structures  resolved by ALMA explain both the Class I SED (spectral energy 
distribution) of WL 20SW (the dominant source at near- and mid-infrared wavelengths) and the additional A_V =25 inferred from its infrared spectrum, in addition to the A_V = 16 toward its northern neighbors.
These extinction values were previously inferred from near-infrared spectroscopy.
Photospheric spectral features evident in the near-infrared (2.06-2.49 μm) spectra of WL 20E and WL 20W were used to determine their 
spectral types and veilings <cit.>. One can then apply various amounts of reddening, corresponding to known values of A_V, to match the observed spectral slopes
and fluxes to determine A_V = 16 for these sources.  Since the corresponding near-infrared spectrum of WL 20S (WL 20SW+WL 20SE) did not show any obvious 
absorption or emission features, its spectral slope and brightness were used to place constraints on its A_V value. Under the assumption that its intrinsic spectrum
and K flux were the same as that of WL 20W, its spectrum could be matched with A_V =  41 <cit.>.

The newly discovered edge-on disks of WL 20SE and WL 20SW by ALMA are well resolved, with diameters ∼ 100 au at 125 pc (see the right panel of Figure  <ref>).
The disks surrounding WL 20E and WL 20W, on the other hand, remain unresolved with ALMA, implying disk diameters less than 13 AU for each, for
an assumed distance of d= 125 pc, derived from VLBA parallax measurements <cit.>.  This finding is perfectly in line 
with the results of the ODISEA (Ophiuchus Disk Survey Employing ALMA), which found the 1.3mm continuum disk sizes in Oph heavily weighted toward compact disks
with radii < 15 AU for 85% of detected objects <cit.>.  

Disk dust masses of just 3.3 M_⊕ and 3.6 M_⊕ are derived for WL 20E and WL 20W, respectively.
These values are in line with average disk dust masses derived for Class II objects in Corona Australis <cit.>,
 IC348 <cit.>, and Ophiuchus <cit.>.
By contrast, the disk masses around both WL 20SW and WL 20SE,  42 M_⊕  and 24 M_⊕, respectively (see Table <ref>),  are in line with
the higher average disk dust masses derived for Taurus <cit.> and Lupus <cit.>.



  §.§.§ X-ray excited 12.8μm [NeII] Disk Emission


The 12.81 μm [NeII] line was detected toward each of the WL 20 components, with the observed line profiles shown in Figure <ref>.
Spectra were extracted through the same apertures as for the continuum flux measurements reported in Table <ref>.
Continuum-subtracted [NeII] line fluxes, A_12.8, the extinction at 12.81 microns, and extinction-corrected NeII line luminosities are listed in Table <ref>.
For reference, spectral types and inferred T_eff are also listed <cit.>.
The extinction at 12.81 μm was derived from the published values of A_V
of 16 for both WL 20E and WL 20W, and A_V = 41 toward WL 20S <cit.>.  We use A_J = 0.282A_V <cit.>, followed by the conversion
A_12.81 =  0.16A_J for R_V = 5.5 for ρ Oph <cit.>. Taking into account the extinction toward each source, and using a distance
of 125 pc, we arrive at the intrinsic [NeII] line luminosities in the last column of Table <ref>.

The WL 20 system was previously observed at 12.81 μm from the ground with VLT/VISIR through a 0.4^'' slit at a resolution of R = 30,000 corresponding 
to ∼ 10 km s^-1 <cit.>.
The observed coordinates were closest  to WL 20E and an upper limit of  < 0.2 × 10^-14 erg cm^-2 s^-1 was established, quite close to our clear detection of the line toward
WL 20E at 0.24 × 10^-14 erg cm^-2 s^-1. 

The observed values for the [NeII] line fluxes agree nicely with predictions from X-ray excitation of neon in the warm upper atmospheres of disks around T Tauri stars, as first proposed
by <cit.>.   These authors calculated a 12.81 μm [NeII] flux of ∼ 10^-14 erg cm^-2  s^-1 for a fiducial T Tauri disk model from <cit.>,
assuming a central star mass of 0.5 M_⊙, stellar radius of 2 R_⊙, T = 4000K, L_x  = 10^30 erg s^-1, and
accretion rate of 10^-8 M_⊙ yr^-1, for a face-on disk orientation at an assumed distance of 140 pc. 
           
We note that an X-ray flux of  L_x  = 1.16 ± 0.09 ×  10^30 erg s^-1
from the DROXO (Deep Rho-Ophiuchi XMM-Newton Observation) survey is reported by <cit.> for the WL 20 system in its entirety, without
distinguishing among its individual components. 
The coordinates listed for the origin of  the X-ray emission in WL 20, (J2000) α = 16:27:15.9, δ = -24:38:43.7 with a 1.1^'' positional error,
originate from Table A.1. of <cit.>. These coordinates, as previously noted, are closest to those of WL 20E.  Given that the FWHM of Newton/XMM is 
∼6^'', the deduced coordinates of the peak X-ray emission are biased toward the 
strongest emitter within the PSF, so we do not know the X-ray flux associated with each individual component of the WL 20 system. 



lccccc

9
0pt
 [NeII] Line Fluxes and Additional Properties of the WL 20 System Components 
6 
 
Source     Spectral     T_eff     A_12.8μ m         Ne II Line      Ne II Line

  
                Type                                                                 Flux              Luminosity

  
                                 (K)                                                          (10^-14 erg cm^-2 s^-1)        (10^28 erg s^-1) 
 
WL 20E                  K7 IV/V                         4040                          0.72                                               0.24             0.87      

  WL 20W                 M0 IV/V                         3800                           0.72                                               0.21            0.76      

  WL 20S                   …        …    1.85                                               0.36             1.31      

  





  §.§.§ ALMA Constraints on Disk Gas Masses


We can estimate disk gas masses using the models of <cit.>,
with the measured  ^13CO(2-1) and C^18O(2-1) line fluxes and flux upper limits
reported  in <ref> and a distance of 125 pc.  For the combined disks of WL 20SE and WL 20SW, these values lead to a 
combined gas mass of about 100 M⊕ (3 × 10^-4 M_⊙) for an assumed CO/C^18O ratio of 550, or just ∼1 M⊕ for the case of
a CO/C^18O ratio of 1650, meant to allow for selective photodissociation in the models. 
The combined gas mass of 100 M⊕ leads to  a gas/dust ratio of just 1.5 for the combined disks of WL 20SE and WL 20SW,
and a factor of 100 lower for the case of the higher CO/C^18O ratio.

In stark contrast to the copious CO(2-1) gas emission toward the WL 20SW/WL 20SE system,
there is a striking lack of emission toward
either of the sources WL 20E or WL 20W (see left panel of Figure <ref>). 
Multiplying the 3σ upper limits for the ^13CO(2-1) and C^18O(2-1) line fluxes reported
in <ref> by 4 π × (d =125 pc)^2 for comparison with the grid of models presented in Figure 6 of  <cit.>,
we find that our observed upper limits fall well  below the lowest model gas mass of just 0.1 M_Jup (32  M_⊕). 
This gas mass upper limit is to be compared with
the derived dust masses of 3.3 M_⊕ and 3.6 M_⊕ for WL 20E and WL 20W, respectively.
One firm conclusion, as a result of this comparison, is
that the gas to dust ratio in these disks  is at least 10 times lower than the canonical ISM (Interstellar Medium) value of 100,
since otherwise, we would have 
had clear detections of both disks in each of the CO(2-1) isotopologues.



 §.§ Gas Emission from the Surroundings




  §.§.§ ALMA


The twin disks of WL 20SW and WL 20SE lie near the peak of the flattened, elliptically shaped CO(2-1) structure evident in the left panel of Figure <ref>,
and outlined by the gray ellipses in both Figure <ref> and Figure <ref>, which shows the Moment 1 maps in all three isotopologues, CO(2-1), ^13CO(2-1), and C^18O(2-1).
A Moment 1 map is the integrated velocity-weighted intensity map divided by the integrated intensity map, and emphasizes the predominant
velocity distribution of the gas. A large-scale velocity gradient across this gaseous envelope
is most apparent in the CO(2-1) Moment 1 map in the left panel of Figure <ref>.  
Within the outlined elliptical area in this panel, the minimum and maximum velocities
are -1.41 km s^-1 and +3.62 km s^-1, respectively, relative to the cloud's +4 km s^-1 V_LSR.  

It is tempting to interpret the combined morphology and velocity distribution of the flattened ^12CO(2-1) structure peaking 
on the twin disks of WL 20SE and WL 20SW as a pseudo-disk encompassing both of the smaller-scale twin disks.  
A pseudo-disk is simply a flattened  protostellar envelope first proposed as the natural consequence of including a uniform magnetic
field threading the density distribution of the singular isothermal sphere model of cloud core collapse <cit.>.
If we were to interpret the elliptical CO(2-1) structure as a pseudo-disk, then for 
a central mass of 0.5 - 1.0 M_⊙, the expected range of 
freefall velocities, (2GM_*/R_pseudo-disk), at a 310 AU radius (being the extent of the semi-major axis of the ellipse)
would be 1.1 km s^-1 to 2.4 km s^-1.
The expected velocity gradients
for infalling gas motions are, however, not observed in
the individual CO(2-1) velocity channel maps (not shown here).

Another possible interpretation of the observed velocity structure of the of the CO(2-1) gas within its emission peak is bulk rotation:
Assuming a central mass of 0.5 M_⊙ to 1 M_⊙, the corresponding range of 
Keplerian velocities expected at a radius of 310 AU
is 1.45 km s^-1 to 3.0 km s^-1, centered on the cloud's V_LSR of 4 km s^-1.  Whereas the values of the observed velocity
extrema of -1.41 km s^-1 and +3.62 km s^-1 are close to those expected for a single, spatially resolved, Keplerian disk,
their locations are not – they are not centered on WL 20SE and WL 20SW.
Close examination of the individual velocity channels reveals red- and blue-shifted gas components on each side of the twin disks,
consistent with the presence of two unresolved Keplerian disks blended together. Recall
that the large beam size of the CO(2-1) observations could not resolve the twin disks that 
were discovered via the much higher resolution continuum observations.

In the isotopologue maps of the central and right panels of Figure <ref>,
we see progressively deeper into the envelope's gas structure, given the decreasing optical depths of the
^13CO(2-1) and C^18O(2-1) emission lines. Optical depth effects may account for the differing velocity structures encountered in the
corresponding Moment 1 maps.
The ^13CO(2-1) Moment 1 map shows mostly red-shifted gas towards both disks, reaching
magnitudes of up to  + 3 km s^-1 relative to the cloud's V_LSR.
In the  C^18O(2-1) Moment 1 map, the gas surrounding WL 20SW and WL 20SE is again red-shifted, but at lower velocities,
ranging from about 0.5 - 1.5 km s^-1 relative to the V_LSR. 

An additional explanation for the gas structure in which the twin edge-on disks are embedded is that it represents
the leftover envelope gas which has mostly been obliterated by the twin bipolar jets emanating from WL 20SE and WL 20SW.
We can estimate the gas mass of this structure from the 
integrated intensity of the ^13CO(2-1) emission
within the ellipse of the central panel of Figure <ref>, under the assumption of optically thin emission.
Adjusting the formula for the gas
mass estimate derived for ^13CO(1-0) from
 <cit.> (following <cit.>) to ^13CO(2-1):





    M(H_2)   =  1.37 × 10^-5  exp(5.34/T_ex)   (T_ex + 0.88) ×
     exp(10.58/ T_ex)  (D/100  pc)^2 (10^-6/X (^13CO)) ∫  S_ν  dv M_⊙


where D is the distance to the source in units of 100 pc, X(^13CO) is the fractional abundance of ^13CO with respect to H_2, and T_ex is the excitation temperature of the gas.
Assuming T_ex = 30K at 125 pc distance, and using the measured S_ν  dv of 4.01 Jy km s^-1 in the ellipse in the central panel of Figure <ref>
yields a remnant envelope gas mass of just 4.5 × 10^-3 M_⊙.  By comparison, a core mass of 0.024 M_⊙  (or just 25 M_Jupiter) for WL 20 was
derived from
submillimeter continuum mapping of the ρ Oph cloud at 850 μm with the 13^'' beam
of the James Clerk Maxwell Telescope, adjusted for a distance of 125 pc <cit.>.

Placing this in an evolutionary context, it is useful to point out here that in a recent ALMA survey of 7 Class 0, 7 Class I, 
and 7 Flat Spectrum sources in Orion B, all sources exhibited beautiful red- and blue-shifted, bipolar CO (2-1) and 
^13CO (2-1)outflow structures <cit.>, 
in contrast with the case of WL 20S which completely lacks any trace of 
CO outflow activity (see left and middle panels of Figures <ref> and <ref>). 
Furthermore, in this same study, in which the
ambient cloud cores were mapped in  C^18 O(2-1), 
it was demonstrated that outflows remove a significant amount of gas from their parent cores. 
In the case of the WL 20 multiple system, it is amply evident that the mass already 
residing in the pre-main-sequence stars, exceeding 1 M_⊙ 
in just the WL 20E and WL 20W components alone, is far in excess of the ambient core gas mass, which is, at most, 0.024 M_⊙.



  §.§.§ Extended [NeII]






In Figure <ref>, 
we present the observed [NeII] line profiles
extracted through the jet apertures presented in Figure <ref>. 
We also extracted a spectrum completely off-source to the northwest, through
a 1.00^'' circular aperture centered at α_2000  =16:27:15.703,
δ_2000 = -24:38:31.108,
displayed in the rightmost panel of Figure <ref>.
[NeII] emission was still detected, albeit with a peak flux ∼ 20-30 times weaker than that observed through the jet apertures.

This result led us to produce the continuum-subtracted, [NeII] integrated line map  in the left panel of Figure <ref>, to examine the larger-scale spatial distribution
of the [NeII] emission.  Integrating the [NeII] line flux over the entire 5.5^'' × 6.2^''
FOV, shown in the right panel of Figure <ref>,
yields a value of 5.23 × 10^-15 erg cm^-2 s^-1, 
not corrected for extinction.  This value is to be compared with the 
6.28 ± 0.25 × 10^-14 erg cm^-2 s^-1 from the 
Spitzer/IRS c2d data  <cit.>.   
The Spitzer spectra were obtained  through a 4.7^'' × 11.1^'' aperture, with a resolution of R = 600.

As previously emphasized by <cit.>, the discrepancy between their VLT/Vizier [NeII] line flux upper limit and the 
value detected by Spitzer could be reconciled by the inferred presence of spatially extended
[NeII] emission originating from outflows. The new MIRI MRS imaging data confirm the shock-powered origin
of much of the observed [NeII] emission, in addition to the X-ray-excited [NeII] disk emission from each component of the WL 20 system, as 
can be seen in the left panel of Figure <ref>. The [NeII] clearly fills the bipolar lobes traced by the H_2 maps of Figure <ref>.



§ PUTTING IT ALL TOGETHER: CONCLUSIONS AND SUMMARY





In Figure <ref>, we present 
the combined 
discoveries of JWST MIRI MRS and ALMA in the WL 20 IRC system:
images of the twin disks, ionized twin jets, and the biconical molecular H_2 gas structure.

The four dust continuum disks detected by ALMA are represented by contours, and their locations are marked by the crosses
in each panel.
The highest-angular-resolution images showing the jets are shown in color in the left and right panels,
in the high-excitation [ArII] line at 6.985 μm,
and the low-excitation [FeII] line at 5.34 μm, respectively.
The jet driving sources, discovered with the MIRI MRS instrument in its highest angular resolution channels, coincide with the locations of the two small, edge-on disks, resolved by ALMA.

The molecular gas, in stark contrast with the ionized gas, is distributed in a double-cone-shaped structure, whose apex is centered 
on the two edge-on disks of WL 20SE and WL 20SW.
The double-cone-shaped morphology is shown in the middle panel of Figure <ref>,
which highlights the 
continuum-subtracted, H_2 0-0 S(3) 9.67 μm emission.
The parallel jets propagate along the cone's central axis.
Portions of this bipolar conical gas structure clearly continue beyond the observed MIRI MRS field-of-view.

Based on near-infrared spectroscopy and ground-based, spatially resolved photometry,
the northern components, WL 20E and WL 20W, can be placed on model isochrones in
the Hertzsprung-Russell diagram.  Using this technique,
the most plausible system age is determined to be ∼ 2 - 2.5 × 10^6 yr
<cit.>.
Given the proximity of the sources to each other, and taking into consideration the typical dimensions of pre-stellar cores, 
it is plausible to assume that the system evolved coevally,  leading to the conclusion that both WL 20SE and WL 20SW,
their disks, and their jets are also this old.
A later evolutionary stage for WL 20SE and WL 20SW is in keeping with the 
lack of any molecular outflows associated with these objects in the ALMA observations
of CO(2-1) and ^13CO(2-1).
In addition, the structure of the relatively
optically thin C^18O(2-1) emission in the right panel of Figure <ref>
shows a pronounced anticorrelation
with the H_2 morphology of Figure <ref>, suggesting
that the combined outflows have dissipated
the original molecular gas core from which the system formed.
Such destruction of the molecular gas environment is also evident in the distribution
of [NeII] emission, which is not confined to the jets, but is distributed, albeit at a low level, 
throughout the MIRI MRS FOV (see Figures <ref> and <ref>).
Finally, the core associated with the entire WL 20 system contains
only 0.024 M_⊙ of gas, accentuating the fact that the central objects
have already attained their birth masses.

The lack of a massive molecular core surrounding the system at this Class II evolutionary stage, 
simplifies the interpretation of the origin of the biconical H_2 gas – it must originate in wide-angled disk winds, 
since there is no infalling or ambient cold gas to entrain. 
With regard to the presence of collimated, ionized jets, five have been found so far in Class 0 sources with MIRI MRS or NIRSpec
 <cit.>, whereas one Class 0 source was found to be purely molecular 
<cit.>.
However, in a comprehensive K-band spectroscopic survey of 
26 Class 0 sources carried out with the MOSFIRE instrument at Keck I, 
whereas 90% of sources showed H_2 emission, characteristic of shocks in outflows, 
only 20% showed [FeII] emission lines, presumably
associated with narrow jets <cit.>.
This percentage of [FeII] detections in Class 0 sources may be a
lower limit, given the high line-of-sight extinction toward these most embedded protostars
and the fact that in many Class 0 sources in which the 5.34 μm [FeII] lines were detected by JWST,
the corresponding H or K band [FeII] lines were not.
For five Class I systems that have been 
investigated by JWST NIRSpec or MIRI MRS, all show the presence of ionized 
jets:  DGTauB <cit.>, the TMC 1 binary <cit.>.
and the HH46 IRS binary <cit.>. In a K-band spectroscopic survey carried
out with the Keck II telescope,
of 52 Class I and Flat Spectrum sources, 23 show H_2 emission and none were reported to show [FeII] <cit.>. 

It has previously been suggested
that as the outflow evolves, the mass-loss rate decreases, velocities increase,
and the jet becomes progressively more ionized <cit.>.
In light of the information we have so far, it can be stated that the H_2 component
of the outflow activity, regardless of its origin, decreases in frequency with
evolutionary stage.

There are interesting differences amongst
the WL 20 system components worth pointing out as well: 
The WL 20SW/SE sources are both actively driving ionized jets, whereas neither WL 20E nor WL 20W are 
currently jet or outflow sources. The edge-on disks of WL 20SE and WL 20SW are well-resolved, both with 
extents of ∼ 100 AU, whereas the disks around WL 20E and WL 20W remain unresolved, suggesting
disk projected diameter upper limits of just 13 AU (see Figure <ref>).
Although small, the gas masses associated with the disks of WL 20SE and WL 20SW are measurable, whereas
any gas emission associated with the unresolved dust disks of WL 20E and WL 20W remains undetected, 
signaling a highly depleted gas to dust ratio compared with that of the ISM.

To summarize, combined JWST MIRI MRS and ALMA observations of the young, multiple, infrared companion system, WL 20, in the Ophiuchus 
star-forming region resulted in the discovery of the following:



  * A previously unknown companion to WL 20SW: WL 20SE

  * Twin, edge-on disks of ∼ 100 AU diameter,  with disk dust masses of  24 ± 4 M_⊕
and 42 ± 2 M_⊕ associated with WL 20SE and WL 20SW, respectively, and a combined gas mass of just
1 - 100 M_⊕

  * Unresolved disks with diameters < 13 AU and dust masses of 3.3 ± 0.4 M_⊕ and 3.6 ± 0.5 M_⊕ 
for WL 20E and WL 20W, respectively, directly detected for the first time, with gas/dust ratios ≤ 10

  * Parallel, ionized jets, emanating from both WL 20SE and WL 20SW, seen in five transitions of [FeII],
two transitions of [NiII], and in [ArII] and [NeII]

  * The presence of extended, low-level [NeII] emission throughout

  * A biconical H_2 structure surrounding the ionized jets, observed in eight different mid-infrared H_2 lines, originating in wide-angled disk winds


What is remarkable about these jets and the H_2 disk winds is the lack of any associated molecular line emission from cold gas  in 
the millimeter wavelength region.



§ ACKNOWLEDGMENTS

M.B. would like to thank Ewine van Dishoeck for her leadership, enthusiasm, and encouragement during the preparation of this work;
the entire MIRI JOYS+ (JWST Observations of Young protoStars+) team. Diego Mardones for obtaining the Band 6 ALMA data, and Sue Terebey for 
fruitful discussions. We thank the anonymous referee for their attentive reading and numerous suggestions for improvement of the originally submitted manuscript.
The work of M.E.R. was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract
with the National Aeronautics and Space Administration. 
V.J.M.LG.'s research was supported by an appointment to the NASA Postdoctoral Program at the NASA Ames 
Research Center, administered by the Oak Ridge Associated Universities under contract with NASA.
M.L.v.G. acknowledges support from ERC Advanced grant 101019751 MOLDISK,
TOP-1 grant 614.001.751 from the Dutch Research Council (NWO) and The Netherlands
Research School for Astronomy (NOVA). Astrochemistry in Leiden is supported by the Netherlands Research School for Astronomy (NOVA).

This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope.
The JWST data presented in this article were obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract
NAS-5-03127 for JWST.  The specific observations analyzed can be accessed via 
[DOI:10..17909/d60d-mh65]https://doi.org/10..17909/d60d-mh65 with the DataSet Title: “MIRI/MRS WL 20”,
and [DOI:10.17909/pqce-5432]https://doi.org/10.17909/pqce-5432 with the DataSet Title: “MIRI/MRS Background used for WL 20 Data.” 
The JWST MIRI data are from Program ID 01236, PI: Mike Ressler.

The following national and international funding agencies funded and supported the MIRI development:
NASA; ESA; Belgian Science Policy Office (BELSPO); 
Centre Nationale d'Études Spatiales (CNES); 
Danish National Space Center;
Deutsches Zentrum für Luft- und Raumfahrt (DLR); 
Enterprise Ireland;  
Ministerio de Economiá y Competividad;
The Netherlands Research School for Astronomy (NOVA);
The Netherlands Organization for Scientific Research (NWO); 
Science and Technology Facilities Council; 
Swiss Space Office; 
Swedish National Space Agency; and UK Space Agency.

This paper makes use of the following ALMA data: ADS/JAO.ALMA#2019.1.01792.S and
ADS/JAO.ALMA#2022.1.01734.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 South Korea), in cooperation with the
Republic of Chile. The joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ.  This research has made
use of NASA's Astrophysics Data System Bibliographic Services, as well as the SIMBAD database,
operated at CDS, Strasbourg, France.  

Software: Numpy <cit.>; Astropy, a community-developed
core Python package for Astronomy <cit.>, Matplotlib <cit.>, and SuperMongo by Robert Lupton and Patricia Monger
(https://www.astro.princeton.edu/ rhl/sm/ ).



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