#  _JWST_ MIRI MRS Images Disk Winds, Water, and CO in an Edge-On
Protoplanetary Disk

Nicole Arulanantham Space Telescope Science Institute, 3700 San Martin Drive,
Baltimore, MD 21218, USA  M.K. McClure Leiden Observatory, Leiden University,
PO Box 9513, NL–2300 RA Leiden, The Netherlands  Klaus Pontoppidan Jet
Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove
Drive, Pasadena, CA 91109, USA  Tracy L. Beck  Space Telescope Science
Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA  J.A. Sturm  Leiden
Observatory, Leiden University, PO Box 9513, NL–2300 RA Leiden, The
Netherlands  D. Harsono Institute of Astronomy, Department of Physics,
National Tsing Hua University, Hsinchu, Taiwan  A.C.A. Boogert Institute for
Astronomy, University of Hawai’i at Manoa, 2680 Woodlawn Drive, Honolulu, HI
96822, USA  M. Cordiner  NASA Goddard Space Flight Center  E. Dartois Institut
des Sciences Moléculaires d’Orsay, CNRS, Univ. Paris-Saclay, 91405 Orsay,
France  M.N. Drozdovskaya  Center for Space and Habitability, Universita ̵̈t
Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland  C. Espaillat Institute
for Astrophysical Research, Department of Astronomy, Boston University, 725
Commonwealth Avenue, Boston, MA 02215, USA  G.J. Melnick  Center for
Astrophysics — Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
J.A. Noble Physique des Interactions Ioniques et Moléculaires, CNRS, Aix
Marseille Univ., 13397 Marseille, France  M.E. Palumbo INAF - Osservatorio
Astrofisico di Catania, via Santa Sofia 78, 95123 Catania, Italy  Y.J.
Pendleton  Department of Physics, University of Central Florida, Orlando, FL
32816, USA  H. Terada  National Astronomical Observatory of Japan (NAOJ)  E.F.
van Dishoeck  Leiden Observatory, Leiden University, PO Box 9513, NL–2300 RA
Leiden, The Netherlands

###### Abstract

We present JWST MIRI MRS observations of the edge-on protoplanetary disk
around the young sub-solar mass star Tau 042021, acquired as part of the Cycle
1 GO program “Mapping Inclined Disk Astrochemical Signatures (MIDAS).” These
data resolve the mid-IR spatial distributions of H2, revealing X-shaped
emission extending to $\sim$200 au above the disk midplane with a semi-opening
angle of $35\pm 5^{\circ}$. We do not velocity-resolve the gas in the spectral
images, but the measured semi-opening angle of the H2 is consistent with an
MHD wind origin. A collimated, bipolar jet is seen in forbidden emission lines
from [Ne II], [Ne III], [Ni II], [Fe II], [Ar II], and [S III]. Extended H2O
and CO emission lines are also detected, reaching diameters between $\sim 90$
and 190 au, respectively. Hot molecular emission is not expected at such
radii, and we interpret its extended spatial distribution as scattering of
inner disk molecular emission by dust grains in the outer disk surface. H I
recombination lines, characteristic of inner disk accretion shocks, are
similarly extended, and are likely also scattered light from the innermost
star-disk interface. Finally, we detect extended PAH emission at 11.3$\mu$m
co-spatial with the scattered light continuum, making this the first low-mass
T Tauri star around which extended PAHs have been confirmed, to our knowledge.
MIRI MRS line images of edge-on disks provide an unprecedented window into the
outflow, accretion, and scattering processes within protoplanetary disks,
allowing us to constrain the disk lifetimes and accretion and mass loss
mechanisms.

planetary system formation; James Webb Space Telescope; infrared spectroscopy

##  1 Introduction

Planets form within circumstellar disks of gas, dust, and ice, where they must
accumulate material before it is removed via outflow mechanisms and accretion
onto the central stars. Observations suggest that this process takes at most a
few Myr, as determined by the age at which the sub-mm dust and gas content in
disks decreases significantly (see e.g., Pascucci et al. 2016; Barenfeld et
al. 2016; Simon et al. 2019). Models of viscously evolving disks predict a
corresponding decline in mass accretion rates, as the gas reservoir is also
depleted over time (Lynden-Bell & Pringle, 1974; Hartmann et al., 1998).
However, this picture is complicated by significant scatter in the observed
relationship between mass accretion rates $\left(\dot{M}_{\rm{acc}}\right)$
and disk dust masses $\left(M_{\rm{disk}}\right)$ (Hartmann et al., 1998;
Manara et al., 2023; Betti et al., 2023), which indicates that additional
physical mechanisms are required to drive mass loss and set the maximum
lifetimes of protoplanetary disks (Pascucci et al., 2023).

Recent work demonstrates that the relationship between $\dot{M}_{\rm{acc}}$
and $M_{\rm{disk}}$, including the scatter, can be reproduced by models where
magnetohydrodynamic (MHD) winds are the primary drivers of angular momentum
transfer that enables ongoing accretion (Tabone et al., 2022b; Somigliana et
al., 2023). In this framework, a disk population begins with a distribution of
initial disk sizes and viscosities (Tabone et al., 2021). A magnetized disk
wind, removing mass at approximately the same rate as accretion, can then
drive disk dispersal over a timescale consistent with observations (Blandford
& Payne, 1982; Cabrit et al., 1999; Bai & Stone, 2013; Lesur, 2021).

Although MHD wind models can successfully reproduce the decline in
$\dot{M}_{\rm{acc}}$ and $M_{\rm{disk}}$ over time, unambiguous distinction
between outflow-launching mechanisms in the inner disk remains elusive (jets,
MHD and photoevaporative winds; see e.g., Pascucci et al. 2023 for a recent
review). Outflow signatures are readily identified in high-resolution
spectroscopy, which reveals blue-shifted emission and absorption lines from
atomic transitions, including [O I], [Ne II], [N II], [S II], He I, and C II
(Edwards et al., 1987; Kwan & Tademaru, 1995; Pascucci & Sterzik, 2009;
Banzatti et al., 2019; Haffert et al., 2020; Xu et al., 2021; Fang et al.,
2018, 2023a; Campbell-White et al., 2023). The central velocities and FWHM of
the line profiles, which can be interpreted as average outflow velocities and
radial locations within a Keplerian disk, are typically used to distinguish
between an origin in photoevaporative and/or MHD winds. For example, a more
compact emitting region ($r\sim 0.1-0.2$ au) is assumed to be characteristic
of an MHD wind (Campbell-White et al., 2023). Particularly the line flux
ratios of [Ne II] 12.814 $\mu$m and [O I] 6300 Å in low-velocity
photoevaporative winds increase as emission from warm, inner disk dust
declines (Pascucci et al., 2020). A similar correlation is detected between
the inner disk dust content and the morphology of accretion-generated
Ly$\alpha$ (Arulanantham et al., 2023) and H$\alpha$ emission lines (Fang et
al., 2023a), indicating that the relative contributions of MHD and
photoevaporative winds and accretion to the total mass loss rate may evolve
over time.

Searches for spatially extended gas from protoplanetary disk winds have
recently detected [O I] and Ly$\alpha$ emission from the face-on system TW Hya
(Fang et al., 2023b; Chang et al., 2023). The [O I] morphology is reproduced
by models of a radially confined MHD wind launched from within the dust
sublimation zone, while the Ly$\alpha$ distribution is consistent with
resonant scattering from the accretion shocks through H I within both a wind
and the disk itself. Spatially extended near-infrared emission from ro-
vibrational H2 lines has been identified from T Tauri systems (Eislöffel et
al., 2000; Duchêne et al., 2005; Beck et al., 2008; Beck & Bary, 2019), as
expected from high-resolution 1-D spectroscopy that showed blue-shifted
emission line profiles (Beckwith et al., 1978; Bary et al., 2003; Gangi et
al., 2020). The emission lines originate in gas with temperatures between
$T\sim 1800-2300$ K and yield line flux ratios consistent with shock
excitation (Eislöffel et al., 2000; Beck et al., 2008). Mid-infrared emission
from rotational quadrupole transitions of H2 has been detected at temperatures
between $T\sim 100-200$ K, showing line profiles consistent with extended
emission (Thi et al., 2001; Bitner et al., 2008). Still, it remains
observationally challenging to distinguish whether MHD or photoevaporative
winds are responsible for driving the detected outflows, as the wind launching
radii and velocity structure remain spatially and spectrally unresolved at the
distance to nearby star-forming regions (see e.g., Beck et al. 2008; Coleman
et al. 2024). Clues may come from spatially resolved observations of [Ne II],
where the emission line luminosity increases as dust cavities are cleared in
the disk and the contribution from photoevaporative winds becomes more
prominent (Pascucci et al., 2020).

In this paper, we report the detection of spatially extended rotational H2,
H2O, and CO emission from an edge-on protoplanetary disk with _JWST_
-MIRI/MRS, using Integral Field Unit (IFU) observations. We describe the
target, observations, and our method for constructing line and continuum
images in Section 2. In Section 3, we compare the morphology of the molecular
lines to emission lines from forbidden atomic transitions and H I Humphreys
$\alpha$ emission, which allows us to consider winds, jets, and accretion as
drivers of mass loss within the same system. We also compare the line images
to the spatial distribution of mid-infrared scattered light to constrain the
origin of the extended molecular gas emission. The kinematics of the extended
emission are discussed in Section 4, and we conclude in Section 5.

##  2 Target and Observations

###  2.1 Description of Observing Program

The target object, 2MASS J04202144+2813491 (hereinafter Tau 042021; RA
04:20:21.44, Dec +28:13:49.17) surrounds a low-mass star (M1, 0.27
$M_{\odot}$; Simon et al., 2019) star at a distance of $\sim$140 pc surrounded
by a 5${}^{\prime\prime}\mkern-1.2mu$ edge-on disk ($>$85°) first seen at
optical wavelengths with the Advanced Camera for Surveys (ACS) onboard the
_Hubble Space Telescope_ (Stapelfeldt et al., 2014; Villenave et al., 2020).
It is also one of the few edge-on disks for which both the radial and vertical
structure have been resolved in ALMA observations (Band 4,
3.67${}^{\prime\prime}\mkern-1.2mu$ by 0.2${}^{\prime\prime}\mkern-1.2mu$,
Villenave et al., 2020, 2023). New _JWST_ images of the system acquired with
NIRCam and MIRI reveal a well-mixed population of 10 $\mu$m dust grains, which
are coupled to the gas in surface layers down to the midplane; the same images
also show an X-shape in the F770W and F1280W filters, which may be associated
with a disk wind (Duchêne et al., 2024). A bipolar, collimated jet is detected
as well, using the F606W filter on _HST_ -ACS (Duchêne et al., 2014) and
_JWST_ -MIRI imaging at 12.8 $\mu$m (Duchêne et al., 2024). We selected Tau
042021 for the MIDAS program on the basis of its 6 and 6.8 $\mu$m ice
features, along with strong H2 and [Ne II] 12.8 $\mu$m gas emission lines and
broad 11.3 $\mu$m PAH emission that were visible in its Spitzer IRS spectrum
(PI: J.R. Houck; PID: 50053; see e.g., Furlan et al. 2011; Sturm et al. 2023;
Duchêne et al. 2024).

Our JWST MIRI MRS observations were obtained on February 13th, 2023 as part of
PID 1751 (PI: M. McClure). Since the target is extended and has HST
coordinates, we did not use target acquisition. Although the disk diameter is
5${}^{\prime\prime}\mkern-1.2mu$ at optical wavelengths, which is larger than
the MIRI detector between 4.9-7.65 $\mu$m, we modeled its spatial extent using
radiative transfer models fit to the HST image and Spitzer spectrum and
determined that the mid-infrared scattered light image would fit within MIRI’s
field-of-view (FOV). We used a 4-point dither pattern optimized for an
extended source, with each dither exposure consisting of one integration of 69
groups, read using the FASTR1 readout pattern, for total on-source exposure
times of 6594 s in each of the MIRI channels. This integration time was
selected to get a S/N of 10 per spaxel on the central 3 $\times$ 5 spaxel
region of our simulated disk model, which would enable detection of CH4 and
other weak ices after rebinning. Since the source is extended and large, a
dedicated sky background was obtained with dithers of the same number of
groups, but a 2-point dither pattern to reduce the non-science time. The final
3D IFU cube was produced using version 11.17.1 of the Calibration References
Data System (CRDS) file selection software in the _JWST_ Science Calibration
Pipeline (CRDS context “jwst_1174.pmap”), with Stage 3 processing utilized to
combine data from all dither positions (Bushouse et al., 2023). 1D spectra
were extracted from the eastern and western lobes of the disk and calibrated
using the JDISCS extractor (Pontoppidan et al., 2023).

The spectral resolution of the MRS changes as a function of wavelength. While
in-flight observations during the instrument commissioning were used to derive
resolving powers between $R\sim 1500-3500$ (Jones et al., 2023), calibration
against high-resolution ground-based spectroscopy of the protoplanetary disk
system FZ Tau are consistent with $R\sim 2000-3000$ (Pontoppidan et al.,
2023). Using the coefficients for each sub-band provided in Pontoppidan et al.
2023, we expect $\Delta v\sim 106$ km s-1 at 5 $\mu$m and $\Delta v\sim 123$
km s-1 at 28 $\mu$m.

Observations are acquired in four different channels: 4.9-7.65 $\mu$m (Channel
1), 7.51-11.7 $\mu$m (Channel 2), 11.55-17.98 $\mu$m (Channel 3), and
17.7-27.9 $\mu$m (Channel 4). The smallest FOV and highest spatial resolution
are available at shorter wavelengths in Channel 1 (3.2”x3.7”, 0.196” pixel-1).
The FOV increases to 6.6”x7.7” in Channel 4, and the spatial resolution
decreases to 0.273” pixel-1. Characterization of the MIRI PSF for extended
sources in the along-slice and across-slice directions is currently ongoing
and will be required to compare spatially resolved emission between channels
with different pixel scales. For point sources, the average PSF FWHMs increase
with wavelength, such that
$\theta=0.033\left(\lambda\,\mu\rm{m}\right)+0.106^{\prime\prime}$ (Law et
al., 2023; Argyriou et al., 2023).

###  2.2 Construction of Line and Continuum Images

To explore the spatial extents of the gas emission lines, we produced
continuum-subtracted spectral images for the detected transitions. The 2-D
local dust continuum was measured as the median surface brightness per spaxel
in frames selected from emission line-free wavelengths along the spectral axis
(see e.g., Pontoppidan et al. (2023); Worthen et al. (2024)). Approximately 10
frames were used to estimate the continuum, spanning $\pm 0.007$ $\mu$m on
either side of the emission line. This background image was subtracted from
each of the 4-5 frames containing the line emission, which were then stacked
to produce the final image. For both the CO and H2O line series, all detected
features were co-added to produce the final line surface brightness maps. The
heights vertical to the disk and diameters along the midplane of each emitting
region were measured as the area within which the surface brightness is
greater than $40\%\pm 2.5\%$ of the peak surface brightness in the spectral
images (see Table 1). The 40% threshold was chosen instead of a FWHM
measurement, as this is the limit where emission from both sides of the disk
can be distinguished from the residual background in the line images (see
e.g., Beck et al. 2008). The uncertainties are derived from the surface
brightness limit where the emitting region shifts into the next adjacent
spaxels. We discuss the observed gas-phase morphologies in the following
sections.

##  3 Results

Figure 1: _Left:_ 1-D MIRI MRS spectrum of Tau 042021, extracted from the
eastern (top) and western (bottom) reflection nebulae. We detect H2 emission
lines and forbidden emission lines from [Ne II], [Ne III], and [Fe II] on both
sides of the disk. H2O and CO2 ice absorptions are also identified, along with
PAH emission near 11 $\mu$m. _Right:_ Zoomed in spectra of ro-vibrational CO,
ro-vibrational H2O, and rotational H2O emission lines from the eastern
reflection nebula, with LTE slab models overlaid in cyan (Salyk, 2022). The CO
model has a temperature and column density of $T=1500$ K, $\log N=17.5$ cm-2,
and the rotational H2O emission lines are consistent with $T=850$ K, $\log
N=15.0$. The ro-vibrational lines are shown with the rotational model scaled
down by a factor of 20 in flux. Figure 2: MIRI MRS images of Tau 042021,
showing the ALMA 0.89 mm dust continuum (top left; Villenave et al. 2020) and
emission lines from H2 S(2) 12.278 $\mu$m (top right), [Ne II] 12.814 $\mu$m
(lower left), and H I Humphreys $\alpha$ 12.372 $\mu$m (bottom right) detected
with the MIRI MRS. White contours in each image show the mid-IR scattered
light continuum at 12.2 $\mu$m. Images have been scaled using an asinh
stretch, to maximize the contrast between the background noise and the high
S/N emission from the source. Beige circles in the bottom left corner of each
panel represent the average theoretical FWHM of the PSF, calculated from the
relationship reported in Law et al. (2023). The observed spatial distributions
are consistent with $\mu$m-size grains in the eastern and western reflection
nebulae (12 $\mu$m continuum), a disk wind (H2 S(2)), bipolar jet ([Ne II])
and scattered light from accretion shocks or the base of the jet (H I). Figure
3: MIRI MRS line images of Tau 042021, showing stacked emission lines from CO
$v=1-0$ (left) and H2O $v_{2}=1-0$ transitions, detected with the MIRI MRS.
White contours in each image show the mid-IR scattered light continuum at 5.0
$\mu$m. Images have been scaled using an asinh stretch, to maximize the
contrast between the background noise and the high S/N emission from the
source. Beige circles in the bottom left corner of each panel represent the
average theoretical FWHM of the PSF, calculated from the relationship reported
in Law et al. (2023). The emission is spatially extended beyond the expected
size of the inner disk $r<1$ au, consistent with scattering by $\mu$m-sized
dust grains in disk surface layers (see e.g., Duchêne et al. 2024). Figure 4:
MIRI MRS image of 11.3 $\mu$m PAH emission, which is spatially co-located with
the scattered light continuum from Tau 042021. While PAH emission is readily
detected in disks around more massive stars and in more externally irradiated
environments (see e.g., Geers et al. 2006, 2007), it is rare to detect the
band within disks around M-type stars. We note that the continuum on the red
side of the band is beyond the edge of the Channel 2 detector, making it
challenging to extract the total surface brightness profile. The image shown
here is from a single frame where the feature peaks in emission.

In the MIRI MRS spectrum of Tau 042021, we detect the mid-infrared dust
continuum and gas-phase emission lines from H2 and [Ne II] that were first
observed in the Spitzer spectrum (see Figure 1, Duchêne et al. 2024).
Additionally, we detect the 11.3 $\mu$m PAH band and gas-phase emission lines
from CO (ro-vibrational), H2O (rotational and ro-vibrational), OH
(rotational), H I, [Ne III], [Fe II], [Ni II], [Ar II], and [S II]. The full
list of detected transitions and emission line fluxes is provided in Table 1.

The 3-D spectral cube from Tau 042021 reveals spatially extended emission at
wavelengths corresponding to the atomic and molecular gas-phase transitions
detected in the 1-D spectra (see Figure 2, 3). At the target distance of
$d\sim 140$ pc, the pixel scales of the MIRI IFUs increase from $\sim$27 au
(0.196”) in Channels 1 and 2 to $\sim$38 au (0.273”) in Channel 4 (Argyriou et
al., 2023). Molecular gas in disks is not expected to produce strong mid-
infrared emission lines at such large radial separations from the host stars
(see e.g., Salyk et al. 2011). However, recent MIRI MRS line images of FZ Tau
revealed a spatially extended ring of H2 emission (Pontoppidan et al., 2023).
A ring of [Ar II] emission was also detected in the Beta Pictoris system,
spatially coincident with its dusty debris disk (Worthen et al., 2024). In the
following sections, we present the continuum-subtracted line images of atomic
and molecular lines in Tau 042021 and discuss the observed gas-phase
morphologies. We find that the western nebula is brighter than the eastern
nebula in all tracers reported in Table 1 and Table 2, as expected if the
western nebula is tilted slightly towards the observer (Villenave et al.,
2020; Simon et al., 2019).

###  3.1 Scattered and Thermal Dust Continuum Emission

We observe a strong difference between the mid-infrared and millimeter
emission of the disk. The scattered light lobes in Tau 042021 are similar to
those seen with the F606W ($\lambda_{0}=0.5888\,\mu$m) and F814W
($\lambda_{0}=0.8115\,\mu$m) filters of the Advanced Camera for Surveys (ACS)
onboard the _Hubble Space Telescope_ (Stapelfeldt et al., 2014), but require
the presence of $10\,\mu$m grains at high disk altitudes (Duchêne et al.,
2024). Conversely, the millimeter emission traces thermal emission from cold,
large grains in the disk midplane. Each panel in Figure 2 shows the 12 $\mu$m
dust continuum from Tau 042021, drawn as white contours over the images. The
upper left panel depicts the ALMA Band 7 (0.89 mm) emission from Villenave et
al. (2020).

###  3.2 Spatially Resolved Mid-IR H2 Emission in a Protoplanetary Disk

We detect 8 H2 emission lines from the rotational ladder that fall within the
MIRI MRS spectral range (see Table 1). The upper right panel of Figure 2 shows
the image of the H2 S(2) ($v=0-0$) emission line at 12.278 $\mu$m, compared to
the scattered light dust continuum at 12.2 $\mu$m. We observe H2 emission
across the entire radial extent of the dust disk, spanning a diameter of
$220^{+8}_{-7}$ au in the western nebula. The H2 S(2) emission is
significantly more vertically extended than the dust scattered light, which
only reaches $95^{+4}_{-3}$ au above the disk midplane. By contrast, the H2
S(2) emerging from the western nebula extends to $171\pm 7$ au.

The vertically extended H2 S(2) emission forms an X-shaped distribution,
centered on the darker disk midplane. This is consistent with the _JWST_
NIRCam and MIRI images of Tau 042021, which revealed an X-shaped structure in
the 7.7 and 12.8 $\mu$m filters that was attributed to a disk wind (Duchêne et
al., 2024). We measured the semi-opening angle of each half of the X in the
MIRI MRS line images, relative to the vertical disk axis (aperture position
angle of 81.896∘), finding $\theta=35^{\circ}\pm 5^{\circ}$. The angle with
respect to the disk midplane is then $\theta\sim 55^{\circ}$, which is steeper
than the $36^{\circ}$ angle between the midplane and the X in the 7.7 $\mu$m
MIRI image (Duchêne et al., 2024). A similar “onion-like” wind structure has
previously been reported for DG Tau B, where velocity-resolved images show
increasingly wide opening angles in emission lines tracing larger launching
radii (Pascucci et al., 2023). This includes the 45∘ semi-opening angle of the
wide molecular wind traced by ro-vibrational H2 $v=1-0$ S(1) emission (Beck et
al., 2008; Agra-Amboage et al., 2014) and the 90∘ cone measured from UV-
fluorescent H2 emission in the same system (Schneider et al., 2013).

The 7.7 $\mu$m filter on MIRI spans wavelengths between 6.581-8.687 $\mu$m
(Wright et al., 2023), which includes the H2 S(5) and H2 S(4) transitions
detected in our MIRI MRS spectrum (6.909 $\mu$m, 8.025 $\mu$m, respectively).
In the line images, the H2 S(5), S(6), S(7), and S(8) emission lines trace an
identical spatial region as the dust at 5 $\mu$m, while the H2 S(1), S(3), and
S(4) emission lines are also vertically extended. It is likely then that the
X-shaped feature imaged at 7.7 $\mu$m by Duchêne et al. (2024) includes a
superposition of the vertically extended H2 S(4) and the less extended H2 S(5)
emission. We find that the sizes of both nebuluae decrease monotonically as a
function of wavelength, with the H2 S(7) emission line at 5.511 $\mu$m showing
a height of just $94^{+5}_{-4}$ au and a diameter of $177\pm 4$ au across the
brighter, western lobe of the disk. This trend cannot be attributed to the
increase in PSF size toward longer wavelengths, since the disk extends well
beyond the the average PSF FWHM at the corresponding wavelengths listed in
Table 1. The spatially extended emitting regions we measure are also
consistent with the 1-D spectroscopy results of Richter et al. (2002), who
reported non-detections of H2 S(1)-S(8) rotational transitions within the
small 2${}^{\prime\prime}\mkern-1.2mu$x2${}^{\prime\prime}\mkern-1.2mu$
aperture on the Texas Echelon Cross Echelle Spectrograph (TEXES) in a small
sample of protoplanetary disks, despite clear detections with the larger
aperture on the _Infrared Space Observatory_ that could better capture
extended emission (Thi et al., 2001).

We note that the H2 emission lines in which the extended emission is most
prominent, S(1) and S(2) at 17.036 and 12.278 $\mu$m, respectively, are both
observed at wavelengths longer than the limit below which the MIRI cruciform
artifact appears ($\lambda<10-12$ $\mu$m; Gáspár et al. 2021). The feature is
a result of lower quantum efficiency at shorter wavelengths in the MIRI
detectors, which leaves the remaining photons to scatter internally. Since the
extended H2 emission is observed at longer wavelengths, where the quantum
efficiency of the detectors is higher, the images are not contaminated by the
artifact.

=3mm

Table 1: Detected Gas Phase Emission Lines

Species | Transition | Wavelength | $E_{\rm{upper}}$ | $F_{\rm{int,west}}$ | $F_{\rm{int,east}}$ | PSF | $h_{\rm{west}}$ | $d_{\rm{west}}$ | $h_{\rm{east}}$ | $d_{\rm{east}}$
---|---|---|---|---|---|---|---|---|---|---
|  | ($\mu$m) | (K) | (W m-2) | (W m-2) | (au) | (au) | (au) | (au) | (au)
H2aaUpper level energies (Eupper) for H2 from Roueff et al. (2019), | S(8) | 5.053 | 8677.1 | $7.3\times 10^{-20}$ | $3.3\times 10^{-20}$ | 38 | $94\pm 4$ | $138\pm 5$ | $49^{+6}_{-30}$ | $84^{+20}_{-30}$
$v=0-0$ | S(7) | 5.511 | 7196.7 | $2.6\times 10^{-19}$ | $1.2\times 10^{-19}$ | 40 | $94^{+5}_{-4}$ | $177\pm 4$ | $36^{+19}_{-24}$ | $24^{+68}_{-16}$
| S(6) | 6.108 | 5829.8 | $1.3\times 10^{-19}$ | $5.9\times 10^{-20}$ | 43 | $97\pm 4$ | $179\pm 4$ | $46^{+26}_{-12}$ | $65^{+46}_{-29}$
| S(5) | 6.909 | 4586.1 | $7.1\times 10^{-19}$ | $3.6\times 10^{-19}$ | 47 | $113\pm 4$ | $182\pm 4$ | $82^{+8}_{-7}$ | $151^{+13}_{-20}$
| S(4) | 8.025 | 3474.5 | $3.5\times 10^{-19}$ | $2.1\times 10^{-19}$ | 52 | $151\pm 7$ | $190\pm 6$ | $125\pm 6$ | $200^{+16}_{-15}$
| S(3) | 9.667 | 2503.7 | $1.0\times 10^{-18}$ | $6.6\times 10^{-19}$ | 60 | $156^{+8}_{-7}$ | $207\pm 6$ | $142\pm 9$ | $277^{+10}_{-8}$
| S(2) | 12.278 | 1681.6 | $4.6\times 10^{-19}$ | $3.2\times 10^{-19}$ | 72 | $171\pm 7$ | $220^{+8}_{-7}$ | $150^{+13}_{-12}$ | $284\pm 11$
| S(1) | 17.036 | 1015.1 | $9.3\times 10^{-19}$ | $6.6\times 10^{-19}$ | 94 | $190^{+22}_{-10}$ | $250\pm 8$ | $127^{+16}_{-15}$ | $237^{+25}_{-41}$
CO |  $v=1-0$, P(25)-P(40) | 4.909-5.092 | 4725.1-7338.1 | $\left(1.5-7.3\right)\times 10^{-20}$ | $\left(1.3-3.3\right)\times 10^{-20}$ | 38 | $94\pm 3$ | $187\pm 6$ | $73^{+5}_{-13}$ | $150^{+22}_{-20}$
H2O, | $11_{7\,5}\rightarrow 10_{4\,6}$ | 14.1780 | 3340.5 | $1.7\times 10^{-20}$ | $1.3\times 10^{-20}$ | 80 | $70^{+9}_{-10}$ | $113^{+13}_{-12}$ | $92^{+9}_{-7}$ | $142^{+10}_{-12}$
Rotational | $15_{6\,10}\rightarrow 14_{3\,11}$ | 14.2093 | 3443.2 | $3.4\times 10^{-20}$ | $2.6\times 10^{-20}$ | 80 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
| $14_{5\,10}\rightarrow 13_{2\,11}$ | 14.8951 | 4198.7 | $1.5\times 10^{-20}$ | $1.4\times 10^{-20}$ | 84 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
| $14_{6\,9}\rightarrow 13_{3\,10}$ | 14.9229 | 4438.4 | $1.6\times 10^{-20}$ | $1.6\times 10^{-20}$ | 84 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
| $13_{3\,10}\rightarrow 12_{2\,11}$ | 15.6246 | 3474.2 | $1.9\times 10^{-20}$ | $1.8\times 10^{-20}$ | 87 | $280\pm 5$ | $255^{+8}_{-9}$ |  $\cdots$ |  $\cdots$
| $12_{6\,7}\rightarrow 11_{3\,8}$ | 16.1148 | 3501.7 | $2.5\times 10^{-20}$ | $2.5\times 10^{-20}$ | 89 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
| $12_{5\,8}\rightarrow 11_{2\,9}$ | 17.1025 | 3273.7 | $1.2\times 10^{-20}$ | $1.5\times 10^{-20}$ | 94 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
| $13_{4\,9}\rightarrow 12_{3\,10}$ | 17.5043 | 3645.6 | $1.5\times 10^{-20}$ | $1.7\times 10^{-20}$ | 96 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
H2ObbMedium Sub-Band: $\lambda=5.66-6.63$ $\mu$m; Long Sub-Band: $\lambda=6.53-7.65$ $\mu$m, | $7_{5\,3}\rightarrow 7_{4\,3}$ | 5.6564 | 9058.9 | $2.6\times 10^{-20}$ | $2.6\times 10^{-20}$ | 41 | $61\pm 3$ | $90\pm 6$ | $40^{+6}_{-11}$ | $33^{+14}_{-7}$
Bend Mode | $8_{4\,4}\rightarrow 8_{3\,5}$ | 5.8801 | 9129.8 | $2.2\times 10^{-20}$ | $1.5\times 10^{-20}$ | 42 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
$v_{2}=1-0$ | $6_{0\,6}\rightarrow 5_{1\,5}$ | 5.8964 | 8106.2 | $1.3\times 10^{-20}$ | $9.7\times 10^{-21}$ | 42 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
| $8_{1\,8}\rightarrow 8_{2\,7}$ | 6.9612 | 8661.5 | $2.5\times 10^{-20}$ | $2.3\times 10^{-20}$ | 47 | $<25$ | $<34$ |  $\cdots$ |  $\cdots$
| $3_{2\,1}\rightarrow 4_{3\,2}$ | 7.0228 | 7793.6 | $2.3\times 10^{-20}$ | $9.6\times 10^{-21}$ | 47 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
| $4_{3\,2}\rightarrow 5_{3\,3}$ | 7.1892 | 8051.2 | $1.1\times 10^{-20}$ | $3.8\times 10^{-21}$ | 48 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
OH, |  $v=1-1$ R(18.5) | 14.650 | 10760 | $1.2\times 10^{-20}$ | $8.2\times 10^{-21}$ | 83 | $106\pm 6$ | $148\pm 7$ |  $\cdots$ |  $\cdots$
Rotational |  $v=0-0$ R(20.5) | 13.54 | 7494.7 | $3.0\times 10^{-21}$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
|  $v=1-1$ R(22.5) | 17.769 | 7082 | $1.8\times 10^{-20}$ |  $\cdots$ | 97 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
H I | Pf$\alpha$ (6-5) | 7.456 | 153420.9 | $6.5\times 10^{-20}$ | $4.6\times 10^{-20}$ | 50 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
| Hu$\beta$ (8-6) | 7.503 | 155338.6 |  $\cdots$ |  $\cdots$ | 50 | $12\pm 7$ | $14\pm 8$ |  $\cdots$ |  $\cdots$
| Hu$\alpha$ (7-6) | 12.372 | 154583.8 | $2.6\times 10^{-20}$ | $1.9\times 10^{-20}$ | 72 | $92\pm 7$ | $113\pm 13$ | $134\pm 2$ | $121\pm 3$
[Ne II] |  ${}^{2}P_{1/2}$–${}^{2}P_{3/2}$ | 12.814 | 1122.9 | $1.3\times 10^{-18}$ | $6.6\times 10^{-19}$ | 74 | $145^{+8}_{-9}$ | $103^{+5}_{-3}$ | ¡110 | ¡66
$\left[\rm{Ne\,III}\right]$ |  ${}^{3}P_{1}$–${}^{3}P_{2}$ | 15.555 | 924.9 | $8.8\times 10^{-20}$ | $8.2\times 10^{-20}$ | 87 | $167^{+13}_{-10}$ | $120\pm 4$ | $152^{+13}_{-12}$ | $114^{+20}_{-5}$
[Fe II]ccUpper level energies (Eupper) for [Fe II] from Bautista et al. (2015) |  ${}^{4}F_{9/2}$–${}^{6}D_{9/2}$ | 5.340 | 2694.2 | $1.3\times 10^{-19}$ | $6.7\times 10^{-20}$ | 40 | $150\pm 15$ | $49\pm 5$ | $150\pm 15$ | $47\pm 3$
|  ${}^{4}F_{7/2}$–${}^{4}F_{9/2}$ | 17.936 | 3496.4 | $6.2\times 10^{-19}$ | $4.7\times 10^{-19}$ | 98 | $<132$ |  $\cdots$ | $<96$ |  $\cdots$
|  ${}^{4}F_{5/2}$–${}^{4}F_{7/2}$ | 24.519 | 4083.2 | $2.1\times 10^{-19}$ | $2.0\times 10^{-19}$ | 128 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$
[Ni II] |  ${}^{2}D_{3/2}$–${}^{2}D_{5/2}$ | 6.636 | 2168.2 | $1.2\times 10^{-19}$ | $4.5\times 10^{-20}$ | 45 | $85\pm 3$ | $68\pm 5$ |  $\cdots$ |  $\cdots$
|  ${}^{4}F_{7/2}$–${}^{4}F_{9/2}$ | 10.682 | 13423.9 | $3.9\times 10^{-20}$ | $2.2\times 10^{-20}$ | 64 | $167\pm 15$ | $<73$ | $107^{+13}_{-5}$ | $75^{+2}_{-3}$
|  ${}^{2}F_{5/2}$–${}^{2}F_{7/2}$ | 12.729 | 14554.3 | $1.1\times 10^{-20}$ | $8.8\times 10^{-21}$ | 74 | $44\pm 15$ | $24\pm 20$ |  $\cdots$ |  $\cdots$
[Ar II] |  ${}^{2}P^{0}_{1/2}$–${}^{2}P^{0}_{3/2}$ | 6.985 | 2059.8 | $2.1\times 10^{-19}$ | $8.5\times 10^{-20}$ | 47 | $84^{+3}_{-4}$ | $72^{+3}_{-4}$ |  $\cdots$ |  $\cdots$
[S III] |  ${}^{3}P_{2}$–${}^{3}P_{1}$ | 18.713 | 1198.6 | $3.6\times 10^{-20}$ | $4.4\times 10^{-20}$ | 101 |  $\cdots$ |  $\cdots$ |  $\cdots$ |  $\cdots$

bbMedium Sub-Band: $\lambda=5.66-6.63$ $\mu$m; Long Sub-Band:
$\lambda=6.53-7.65$ $\mu$m

Upper level energies (Eupper) for H2O (rotational and bend mode) from Barber
et al. (2006)

Table 2: Measured Spatial Extents of Dust Emission  Tracer | PSF FWHM | $h_{\rm{west}}$ | $d_{\rm{west}}$ | $h_{\rm{east}}$ | $d_{\rm{east}}$
---|---|---|---|---|---
| (au) | Height | Diameter | Height | Diameter
Dust, Channel 1aaChannel 1: $\lambda=4.9-7.65$ $\mu$m; Channel 2: $\lambda=7.51-11.7$ $\mu$m; Channel 3: $\lambda=11.55-17.98$ $\mu$m; Channel 4: $\lambda=17.7-27.9$ $\mu$m | 38 | $95^{+4}_{-3}$ | $181^{+11}_{-10}$ | $67\pm 7$ | $135\pm 13$
Dust, Channel 2bbMedium Sub-Band: $\lambda=5.66-6.63$ $\mu$m; Long Sub-Band: $\lambda=6.53-7.65$ $\mu$m | 60 | $139\pm 6$ | $233^{+11}_{-10}$ | $121\pm 8$ | $219^{+15}_{-20}$
Dust, Channel 3**For the forbidden emission lines, $h$ represents the vertical extent of the knots closest to the midplane and $d$ represents the jet widths.ccMedium Sub-Band: $\lambda=13.34-15.57$ $\mu$m; Long Sub-Band: $\lambda=15.41-17.98$ $\mu$m | 94 | $302^{+10}_{-9}$ | $301^{+15}_{-16}$ |  $\cdots$ |  $\cdots$
Dust, Channel 4**No resolved separation between eastern and western lobes | 98 | $321\pm 10$ | $304^{+15}_{-14}$ |  $\cdots$ |  $\cdots$

###  3.3 Bipolar Jets Detected in Forbidden Emission

We report the detection of spatially extended [Ne II] (12.814 $\mu$m) emission
from Tau 042021 (see Figure 2). Emission lines from [Ne III] (15.555 $\mu$m),
[Fe II] (5.34, 17.936 $\mu$m), [Ni II] (6.636, 10.682, 12.729, 18.241 $\mu$m),
[Ar II] (6.985 $\mu$m), and [S III] (18.713 $\mu$m) are also identified and
spatially resolved, with median emitting regions between $\sim 67-75$ au wide
and $\sim$150 au long (see Figure 1, Table 1). Like the H2 S(2) emission shown
in Figure 2, the [Ne II] emission extends beyond the vertical height of both
dust reflection nebulae observed at 12 $\mu$m. However, the [Ne II] appears in
4 distinct knots, with a much narrower radial extent than the H2 in the knots
that coincide with the dust continuum. The distribution is consistent with
emission from a bipolar jet (see e.g., Dougados et al. 2000), which was
discovered in the earlier _HST_ and _JWST_ images of Tau 042021 (Duchêne et
al., 2014, 2024).

High-resolution spectroscopic studies of mid-IR [Ne II] emission typically
detect either a high-velocity component (HVC) associated with collimated jets
or a low-velocity component (LVC) tracing either MHD or photoevaporative disk
winds in the 1-D emission line profiles (Herczeg et al., 2007; Güdel et al.,
2010; Pascucci et al., 2011, 2020; Alexander et al., 2014). Although the MIRI
MRS observations do not have the spectral resolution to distinguish HVC or LVC
components, the X-shape observed with the F1280W broadband filter on MIRI
(Duchêne et al., 2024) is consistent with the H2 S(2) emission that we
observe, and not with the [Ne II] jet. We do not spatially resolve changes in
the jet width as a function of distance from the star, which places an upper
limit of $<3^{\circ}$ on the jet opening angle (see e.g., Narang et al. 2023).

###  3.4 Detection of spatially extended PAH emission

The PAH emission feature seen in the Spitzer spectrum and detected in Figure 1
is also extended with approximately the same distribution as the scattered
dust continuum. The IFU imaging proves that the PAH emission is absent in the
background and localized to the disk. With a spectral type of M1$\pm$2 (Luhman
et al., 2009), this may be the first detection of PAH emission around a low
mass T Tauri star (M-K spectral type) where the PAH emission is intrinsic to
the disk itself rather than due to external irradiation (Geers et al., 2006).
It may be that the unique geometry of this system, which blocks the thermal
emission from the inner disk, allows the relatively weaker PAH emission around
T Tauri stars to be detected at mid-infrared wavelengths. The spatial
extension of the PAH feature is robust; however its location at the end of
channel 2 means that the red continuum is poorly defined. Therefore we leave
the analysis of this feature to future work.

###  3.5 Scattering of Inner Disk CO and H2O Emission

Spatially resolved emission from CO ro-vibrational lines between $4.9-5.3$
$\mu$m is detected in the MIRI MRS cube, corresponding to the transitions
$v=1-0$ P(25)-P(40). Figure 3 shows the combined emission from these features
after subtraction of the local continuum, revealing a span of $187\pm 6$ au in
diameter and $94\pm 3$ au in height above the disk midplane for the western
lobe of the disk. The fainter, eastern lobe has a diameter of
$150^{+22}_{-20}$ au and a height of $73^{+5}_{-13}$ au. By contrast, spectro-
astrometric observations of the ro-vibrational CO lines were consistent with
more compact distributions of hot gas, with LTE temperatures of 100-1000 K and
emitting areas inside $r<4$ au (Pontoppidan et al., 2011; Brown et al., 2013).
The emitting region we observe with MIRI in this edge-on disk is significantly
larger than expected for lines originating in gas within the 100-1000 K
temperature range, indicating that the extended emission is more likely
scattered light (see e.g., Pontoppidan et al. 2002).

Numerous emission lines from the rotational and ro-vibrational bending mode
transitions of H2O (5.65-6.19 $\mu$m; see Table 1) are also readily detected
in the MIRI spectral cube. Figure 3 shows the stacked images of ro-vibrational
bending mode features detected within the medium sub-band of Channel 1. The
H2O distributions extend far beyond the emitting regions derived from slab
model fits to mid-infrared spectra from less inclined disks ($r<1$ au; see
e.g., Salyk et al. 2011; Perotti et al. 2023; Banzatti et al. 2023; Gasman et
al. 2023; Muñoz-Romero et al. 2024). The image of the H2O rotational lines in
Tau 042021 shows a diameter of $113^{+13}_{-12}$ au and a vertical extent of
$70^{+9}_{-10}$ au in the western lobe of the disk. In Channel 1, the ro-
vibrational lines between 5.6-5.9 $\mu$m span a disk diameter of $90\pm 6$ au
in the bright lobe and a vertical height of $61\pm 3$ au. However, the larger
pixel scale in Channel 3 makes it challenging to directly compare the spatial
distributions of the rotational and bending mode emission lines, and the
difference in emitting areas reported here is not statistically significant.

###  3.6 Hydrogen Recombination Lines Emerging from Accretion Shocks

The bottom right panel of Figure 2 shows the 2-D spectral image of the H I
(7-6) Hu$\alpha$ emission line at 12.372 $\mu$m. We also identify H I
Pf$\alpha$ (7.456 $\mu$m) and Hu$\beta$ (7.503 $\mu$m) emission lines in the
1-D spectrum, which were tentatively detected in the _Spitzer_ -IRS spectrum
(Duchêne et al., 2024). However, the peak surface brightnesses from the
Pf$\alpha$ and Hu$\beta$ emission lines ($\sim 5\times 10^{-20}$ W m-2
arcsec-2) are close to the background noise level in the continuum-subtracted
images, making it challenging to measure the spatial extent of the emitting
regions. We constrain the stronger H I (7-6) emission to a diameter of $113\pm
13$ au and a height of $92\pm 7$ au in the brighter, western nebula. The
fainter, eastern nebula has a diameter of $121\pm 3$ au and a height of
$134\pm 2$ au.

Infrared H I recombination lines are typically correlated with accretion
luminosity in Class I (see e.g., Fiorellino et al. 2021) and Class II sources
(see e.g., Pascucci et al. 2007; Salyk et al. 2013; Rigliaco et al. 2015;
Alcalá et al. 2017), implying that the emission originates within the
accretion shocks rather than the disk surface. In particular, the Hu$\alpha$
transition was detected in 46/114 disks observed with _Spitzer_ (40%), where
it was found to trace H I number densities between $10^{10}-10^{11}$ cm-3
(Rigliaco et al., 2015). We measure the integrated flux in the H I (7-6)
emission line from Tau 042021, finding $F=2.6\times 10^{-20}$ W m-2 in the
western lobe and $1.9\times 10^{-20}$ W m-2 on the eastern side of the disk.
However, Figure 2 shows that much of the intrinsic emission line flux is
likely scattered out of the line-of-sight.

In a less inclined disk, the infrared SED is relatively flat between
$\sim$12-100 $\mu$m (see e.g., Dullemond et al. 2007). The SED from Tau 042021
shows that the flux increases from $\lambda F_{\lambda}\sim 10^{-16}$ W m-2 at
$\sim$12 $\mu$m to $\lambda F_{\lambda}\sim 1\times 10^{-14}$ W m-2 at $\sim$
100 $\mu$m (Duchêne et al., 2024), as the contribution from thermal emission
becomes visible in the edge-on disk. If the decrease is predominantly an
inclination effect, the “true” flux at $\sim$12 $\mu$m is likely also closer
to $10^{-14}$ W m-2. We include this flux ratio of $\sim$100 as a scaling
factor to correct the H I Hu$\alpha$ emission line flux for scattering and use
the linear relationship between the H I and accretion luminosities (Rigliaco
et al., 2015) to derive $L_{\rm{acc}}=7.9\times 10^{-5}\,L_{\odot}$ for Tau
042021. The accretion rate can then be estimated as
$\dot{M}_{\rm{acc}}=L_{\rm{acc}}R_{\ast}/GM_{\ast}\left(1-R_{\ast}/R_{\rm{in}}\right)=2\times
10^{-11}\,M_{\odot}$ yr-1 (Gullbring et al., 1998), assuming
$R_{in}=5R_{\ast}$ (Rigliaco et al., 2015), $R_{\ast}=1.7\,R_{\odot}$, and
$M_{\ast}=0.27\,M_{\odot}$ (Simon et al., 2019). This value is likely a
significant underprediction, and we discuss the implications further in
Section 4.2.

##  4 Analysis

###  4.1 Extracting the Molecular Gas Thermal Structure

To explore whether the radially extended H2, CO, and H2O emission is
consistent with a distribution of scattered light, we used the spectools_ir
package (Salyk, 2022) to derive the temperature, column density, and emitting
area of the warm gas. spectools_ir produces slab models in Local Thermodynamic
Equilibrium (LTE), which we fit to the 1-D spectrum of Tau 042021 after using
the publicly available ctool routine (Pontoppidan et al., 2023) to subtract
the continuum. We fit each molecule individually, as the H2 and CO emission
lines are well separated from both the rotational and ro-vibrational H2O
transitions.

Table 3 reports the best-fit slab model parameters for all three molecules. We
find that the CO spectrum is consistent with $T=1500$ K and $\log N=17.5$
cm-2, demonstrating that the temperature required to produce the $J=25-40$
emission lines is similar to what is observed in less inclined disks (Salyk et
al., 2011; Brown et al., 2013). A similar effect is seen in the H2O emission
lines, which require $T=850$ K and $\log N=15.0$ cm-2. Again, this is roughly
consistent with _JWST_ MIRI MRS observations of water in less inclined
protoplanetary disks, which show temperatures between $\sim 600-900$ K (Kóspál
et al., 2023; Grant et al., 2023; Gasman et al., 2023; Perotti et al., 2023;
Banzatti et al., 2023; Muñoz-Romero et al., 2024). We note that Tau 042021
does not show an excess cool H2O component, which has been associated with
pebble drift in compact disks (Banzatti et al., 2023). Rather than originating
at large radial and vertical separations, the extended emission we observe is
likely scattered along the disk surface by large dust grains, an effect
uniquely detected in edge-on systems.

In the case of H2, we find that two temperature components are required to
reproduce the observed spectrum. The first, with $T=400$ K and $\log N=19.4$
cm-2 captures the bulk of the emission from the S(1)-S(4) emission lines,
which are vertically extended in the line images. However, this model under-
predicts the S(5)-S(8) emission line fluxes, which are better reproduced when
an additional gas population with $T=1500$ K and $\log N=20.1$ cm-2 is
included. The hotter temperature is consistent with the CO within the inner
disk, while the cooler temperature is closer to the 100 K wind temperature
assumed for the mid-infrared H2 emission lines measured by Thi et al. (2001).
However, we note that the emitting area we retrieve for the 1500 K H2 ($r=5$
au) is significantly larger than the intrinsic emitting area of the CO
($r=0.01$ au).

Table 3: Slab Model Retrieval Parameters

Species | T (K) |  $\log N$ (cm-2) | r (au)
---|---|---|---
CO | 1500 | 17.5 | 0.01
H2O | 850 | 15.0 | 0.8
H2, hot | 1500 | 20.1 | 5
H2, warm | 400 | 19.4 | 175aafootnotemark:
aafootnotemark:

We allowed the emitting area of the 400 K H2 component to vary between
0.01-200 au, based on the vertically extended spatial distribution in Figure
2.

###  4.2 No Velocity Structure Detected in Spatially Extended Emission Lines

Figure 5: Emission-weighted velocity maps of H2 S(2) _(left)_ and [Ne II]
emission lines from Tau 042021 _(right)_ (see e.g., Beck et al. 2008; Narang
et al. 2023). No velocity structure is detected in either of the maps, as
expected for an edge-on disk from which the maximum jet and wind velocity
components are not oriented along the line-of-sight.

We explore the velocity structure in the H2 and atomic emission lines, using
both the 1-D MIRI MRS spectrum and the spectral images. We fit each of the
emission lines listed in Table 1 with a model of a Gaussian profile,
superimposed on a linear continuum extending $\pm 0.02$ $\mu$m from the
central wavelength of the transition. The free parameters are the amplitude,
velocity centroid, and width of the Gaussian profile and the slope and
intercept of the continuum. The best-fit velocity centroids for the H2
emission lines range between $-20$ km s-1 and $2$ km s-1, with no trends as a
function of wavelength. A similar range is derived from the fits to the atomic
emission lines, which span $-25$ km s-1 to 6 km s-1. The best-fit FWHMs of the
emission lines indicate that the features are barely wider than the instrument
resolution, making it challenging to detect any kinematic signatures in the
1-D line profiles.

Figure 5 shows the emission-weighted velocity maps for the H2 S(2) and [Ne II]
emission lines (see e.g., Beck et al. 2008; Narang et al. 2023). No velocity
gradients are detected in either of the spectral images. We do not identify a
red- or blue-shifted side of the jet or wind, consistent with expectations for
outflowing material moving perpendicularly away from the edge-on disk and
therefore the line-of-sight. Observations of near-IR H2, [O I] $\lambda$6300 Å
and high-resolution [Ne II] emission lines have blue-shifted wind components
with velocity centroids $v_{c}<20$ km s-1 (Pascucci & Sterzik, 2009; Pascucci
et al., 2020; Gangi et al., 2020; Beck & Bary, 2019; Banzatti et al., 2019),
which is smaller than the spectral resolution of MIRI at 12 $\mu$m ($\Delta
v\sim 120$ km s-1). For a typical jet speed of $\sim 150-200$ km s-1 (Shang et
al., 1998; Narang et al., 2023), the line-of-sight velocity toward a disk with
$i>85^{\circ}$ (Villenave et al., 2020) would be $<20$ km s-1. This is again
smaller than the MIRI spectral resolution and consistent with the velocity
centroids in Figure 5. Rather, the images of spatially extended wind and jet
emission presented here are complementary to high spectral resolution
observations that can measure kinematics.

###  4.3 Comparing Mass Loss Rates Derived from MIRI MRS Spectral Images

As reported in Section 3, vertically extended H2 is observed beyond the
scattered light continuum in the S(1)-S(4) emission lines. This behavior is
distinctly different from the CO and H2O emission lines, which show emitting
regions that are more consistent with the dust distributions in the MIRI MRS
wavelength range. Assuming that the observed H2 S(1) emission is optically
thin, we use the integrated line flux to estimate the mass of warm gas
contained in the wind (Thi et al., 2001):

$F_{J^{\prime}-J^{\prime\prime}}=\frac{hc}{4\pi\lambda}N\left(\rm{H}_{2}\right)A_{J^{\prime}-J^{\prime\prime}}x_{J^{\prime}}\Omega,$
(1)

where
$x_{J^{\prime}}=g_{u}\times\exp{(-E_{J^{\prime}-J^{\prime\prime}}/kT_{\rm{exc}})}/Q(T_{\rm{exc}})$
is the fraction of H2 molecules in the upper state $\left(J=3\right)$,
$Q(T_{\rm{exc}})$ is the partition function of H2 at temperature
$T_{\rm{exc}}$ (Popovas & Jørgensen, 2016), $A_{J^{\prime}-J^{\prime\prime}}$
is the Einstein coefficient describing the probability of radiative decay from
$J=3$ to $J=1$ (Roueff et al., 2019), and $\Omega$ is the solid angle spanned
by the emitting region reported in Table 2.

The H2 emission lines in the MIRI MRS spectrum are consistent with two
temperature components: $T=400$ K, which dominates the S(1)-S(4) emission
lines, and $T=1500$ K to capture the line fluxes from S(5)-S(8). Assuming the
400 K temperature component in the wind, we measure $M_{{\rm H}_{2}}\sim
5.4\times 10^{-7}\,M_{\odot}$. At the typical wind velocities of $\sim$10 km
s-1 predicted by models and the measured maximum emission height of 175 au
above the disk midplane, this corresponds to a mass flux of
$\dot{M}_{\rm{wind}}\sim 6.5\times 10^{-9}\,M_{\odot}$ yr-1. The uncertainties
in this measurement are dominated by the size of the emitting region and the
gas temperature, which is less constrained without observations of the H2 S(0)
emission line at 28.2 $\mu$m.

The measured accretion rate, $\dot{M}_{\rm{acc}}=2\times 10^{-11}\,M_{\odot}$
yr-1 is approximately two orders of magnitude lower than the wind mass loss
rate, which contradicts the ratios of
$\dot{M}_{\rm{wind}}/\dot{M}_{\rm{acc}}\sim 0.1-1$ measured from the 12CO and
[O I] $\lambda$6300 LVC emission lines (Pascucci et al., 2023). However, we
note that the measured accretion rate is also three orders of magnitude lower
than the threshold for strong accretors with HVC [Ne II] emission line
components in 1-D spectra (Pascucci et al., 2020), which originate in jets
like the one imaged with MIRI from Tau 042021. This apparent contradiction
likely takes its origin in the fact the the H I line may be strongly impacted
by scattering along the line of sight through the edge-on disk, masking the
bulk of the emission from the accretion shock itself (see e.g., Alcalá et al.
2019). Additionally, some of the Hu$\alpha$ emission may originate at the base
of the jet, and the large methodological uncertainty in deriving a stellar
mass for a target in an edge-on disk plays a role as well (see e.g., Hartmann
et al. 2016 for a recent review). It is likely that the linear relationship
between H I Hu$\alpha$ emission and accretion luminosity does not hold for
edge-on disks like Tau 042021, which will require H I and dust scattering
models to retrieve the missing line flux.

###  4.4 Distinguishing Between Outflow-Launching Mechanisms

The semi-opening angle of $35^{\circ}\pm 5^{\circ}$ measured from the H2 S(2)
emission line (Figure 2) is consistent with the range of semi-opening angles
between $\sim 20-40^{\circ}$ for low-velocity winds reported in Pascucci et
al. (2023). These values were derived from spectrally resolved [O I] $\lambda$
6300 emission lines, which can be decomposed into high-velocity ($v=50-300$ km
s-1 and low-velocity components ($v<10$ km s-1). The largest blueshifts are
detected in low-velocity components from disks with inclinations $i\sim
35^{\circ}$, where the ejected material is moving directly toward the observer
along the line-of-sight (Banzatti et al., 2019). Indeed, MHD wind models
predict that mass outflow occurs when the poloidal magnetic field lines reach
semi-opening angles of $\theta<30^{\circ}$ relative to the disk rotation axis
(Blandford & Payne, 1982). Our H2 observations are consistent with both the
wind model predictions and the [O I] low-velocity components, implying a MHD
wind origin for the vertically extended emission.

_Spitzer_ -IRS observations of Tau 042021 were used to classify the object as
a Class II disk with no remaining envelope, based on its extinction-free
infrared spectral indices ($n_{5-12}=-0.73$, $n_{12-20}=-0.75$; McClure et al.
2010; Furlan et al. 2011). Although 13CO $J=3-2$ emission is observed at large
scale heights (Simon et al., 2019), it is not coincident with the vertically
extended emission near 12 $\mu$m (Duchêne et al., 2024) and the sub-mm gas
surface density distribution does not show signs of an outflow cavity. The
mid-infrared H2 emission also does not appear similar to the spatially
resolved outflow cavities detected in younger protostellar systems (see e.g.,
Federman et al. 2023), consistent with the classification of Tau 042021 to a
later stage of disk evolution. As reported in Section 4.1, we do not resolve
velocity structure in this edge-on disk with the MIRI MRS, but the measured
semi-opening angle of the spatially extended H2 emission is most consistent
with MHD wind model predictions. This is consistent with models that require
MHD winds to reproduce the observed relationship between $\dot{M}_{\rm{acc}}$
and $M_{\rm{disk}}$ in the disk population (Tabone et al., 2022a; Somigliana
et al., 2023).

##  5 Summary & Conclusions

We present _JWST_ MIRI MRS observations of gas-phase emission in the edge-on
protoplanetary disk system Tau 042021. The data provide spatially resolved
spectral images of gas-phase atomic and molecular emission lines, revealing:

  * •

An X-shaped MHD disk wind with a semi-opening angle of $35^{\circ}\pm
5^{\circ}$, traced by vertically extended emission from rotational transitions
of H2 (see also, Duchêne et al. 2024);

  * •

A collimated, bipolar jet in emission from [Ne II], [Ne III], [Ni II], [Fe
II], [Ar II], and [S III] (see e.g., Duchêne et al. 2014, 2024), with an
opening angle of $<3^{\circ}$;

  * •

Radially extended CO and H2O consistent with line emission from the inner disk
at 1 au, scattered on large dust grains in the disk surface out to 100 au.

  * •

H I Hu$\alpha$ emission in each nebula, consistent with scattered light from
the accretion shocks or emission from the base of the jet.

Assuming the H2 emission is optically thin, the wind contains a gas mass of
$M_{H_{2}}\sim 5.4\times 10^{-7}\,M_{\odot}$, corresponding to a mass flux of
$\sim 6.5\times 10^{-9}\,M_{\odot}$ yr-1. The Hu$\alpha$ emission was used to
estimate a mass accretion rate of $\dot{M}_{\rm{acc}}=2\times
10^{-11}\,M_{\odot}$ yr-1, although we note that this is likely an
underestimate due to scattering and extinction along the line-of-sight and
uncertainties in the size of the line emitting region. This work demonstrates
the unexpected ability to spatially resolve gas-phase emission lines from
large protoplanetary disks with the MIRI MRS, which we have shown to be a
powerful tool for measuring disk mass loss in winds and jets and for exploring
inner disk sub-structure and compositions through scattered light.

##  6 Acknowledgements

A portion of this research was carried out at the Jet Propulsion Laboratory,
California Institute of Technology, under a contract with the National
Aeronautics and Space Administration (80NM0018D0004). We acknowledge support
from JWST grant GO-01751. E.D. and J.A.N. acknowledge support from French
Programme National ‘Physique et Chimie du Milieu Interstellaire’ (PCMI) of the
CNRS/INSU with the INC/INP, co-funded by the CEA and the CNES.

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