# Illuminating evaporating protostellar outflows: ERIS/SPIFFIER reveals the
dissociation and ionization of HH 900††thanks: Based on observations collected
at the European Southern Observatory under ESO programmes 110.257T.001 and
0101.C-0391(A).

Megan Reiter,1 Thomas J. Haworth,2 Carlo F. Manara,3 Suzanne Ramsay,3 Pamela
D. Klaassen,4 Dominika Itrich,3 and Anna F. McLeod5,6
1Department of Physics and Astronomy, Rice University, 6100 Main St - MS 108,
Houston, TX 77005, USA
2Astronomy Unit, School of Physics and Astronomy, Queen Mary University of
London, London E1 4NS, UK
3European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching
bei München, Germany
4UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill,
Edinburgh, EH9 3HJ, UK
5Centre for Extragalactic Astronomy, Department of Physics, Durham University,
South Road, Durham DH1 3LE, UK
6Institute for Computational Cosmology, Department of Physics, University of
Durham, South Road, Durham DH1 3LE, UK E-mail<EMAIL_ADDRESS>(MR)

(Accepted 2023 October 24. Received 2023 October 13; in original form 2023
July 14)

###### Abstract

Protostellar jets and outflows are signposts of active star formation. In H ii
regions, molecular tracers like CO only reveal embedded portions of the
outflow. Outside the natal cloud, outflows are dissociated, ionized, and
eventually completely ablated, leaving behind only the high-density jet core.
Before this process is complete, there should be a phase where the outflow is
partially molecular and partially ionized. In this paper, we capture the HH
900 outflow while this process is in action. New observations from the
ERIS/SPIFFIER near-IR integral field unit (IFU) spectrograph using the
K-middle filter ($\lambda$=2.06–2.34 µm) reveal $\text{H}_{2}$ emission from
the dissociating outflow and Br-$\gamma$ tracing its ionized skin. Both lines
trace the wide-angle outflow morphology but $\text{H}_{2}$ only extends
$\sim$5000 au into the H ii region while Br-$\gamma$ extends the full length
of the outflow ($\sim$12,650 au), indicating rapid dissociation of the
molecules. $\text{H}_{2}$ has higher velocities further from the driving
source, consistent with a jet-driven outflow. Diagnostic line ratios indicate
that photoexcitation, not just shocks, contributes to the excitation in the
outflow. We argue that HH 900 is the first clear example of an evaporating
molecular outflow and predict that a large column of neutral material that may
be detectable with ALMA accompanies the dissociating molecules. Results from
this study will help guide the interpretation of near-IR images of externally
irradiated jets and outflows such as those obtained with the _James Webb Space
Telescope (JWST)_ in high-mass star-forming regions where these conditions may
be common.

###### keywords:

Herbig-Haro objects – ISM: jets and outflows – photodissociation region (PDR)
– Infrared: ISM – stars: formation – stars: protostars

††pubyear: 2023††pagerange: Illuminating evaporating protostellar outflows:
ERIS/SPIFFIER reveals the dissociation and ionization of HH 900††thanks: Based
on observations collected at the European Southern Observatory under ESO
programmes 110.257T.001 and 0101.C-0391(A).–C

##  1 Introduction

Protostellar jets and outflows are produced by actively accreting young
stellar objects (YSOs). Fast, collimated jets launched close to the YSO are
often seen at visual wavelengths in hydrogen recombination and forbidden
emission lines that are excited in shocks. Molecular emission lines in the
millimetre (mm) and infrared (IR) trace outflows that may be launched directly
from the circumstellar disk or entrained from the surrounding medium by an
underlying jet. Jets and outflows may coexist and which component is more
prominent / visible depends on the environment and evolutionary stage of the
source (see Bally, 2016, for a recent review).

A few well-studied examples like HH 46/47 have played an important role in
understanding the relationship between jets and outflows. HH 46/47 is a
parsec-scale bipolar outflow (Stanke et al., 1999). The fast, collimated jet
that emerges from an embedded young stellar object (YSO) is well-studied in
bright recombination and forbidden lines that trace shock-excited gas in the
jet body (e.g., Heathcote et al., 1996; Hartigan et al., 2005; Hartigan et
al., 2011; Erkal et al., 2021). On the opposite side of the embedded driving
source, a classic molecular outflow propagates into the natal cloud with a
wide-angle morphology traced by both the _Spitzer Space Telescope_ (Noriega-
Crespo et al., 2004) and the Atacama Large Millimeter Array (ALMA; Arce et
al., 2013; Zhang et al., 2016).

Most of our knowledge about jets and outflows comes from sources like HH 46/47
– those located in nearby star-forming regions that are forming primarily or
exclusively low-mass stars. In regions with high-mass stars, the observational
picture may look quite different. Copious UV photons from the most massive
stars illuminate the surrounding cloud, including jets and outflows that
emerge into the H ii region. External irradiation renders the entire jet body
visible (Bally et al., 2006a; Smith et al., 2010), unlike jets in more
quiescent regions where the only material emitting at visual and infrared
wavelengths has been heated and excited in shock fronts (e.g., Bally &
Reipurth, 2001). This is a distinct advantage for measuring the mass-loss rate
(and thus accretion history of the driving source) because it can be
determined using the physics of photoionized gas rather than complex, non-
linear, and time-dependent shock models (Reipurth et al., 1998; Bally et al.,
2006b).

Once jets and outflows emerge into the H ii region, molecules will quickly be
dissociated. Traditional molecular outflow tracers, like CO, will only be seen
in portions of the flow that remain embedded in the cloud (e.g., Cortes-Rangel
et al., 2020). Outside the cloud, the outflow will be dissociated, ionized,
and finally, completely ablated, leaving behind only the high-density jet
core. Before this process is complete, there should be a phase where the
molecular outflow is only partially dissociated with an ionized skin. The idea
of an ionized outflow has been proposed for a few sources in the Carina Nebula
(HH 666 and HH 900; Smith et al., 2004; Hartigan et al., 2015; Reiter et al.,
2015b, a). So far, there have been no unambiguous detections of a molecular
outflow that extends into the H ii region.

Figure 1: An H$\alpha$ image from _HST_ showing the HH 900 jet+outflow. HH 900
emerges from a tadpole-shaped globule that is seen in silhouette against the
bright background of the H ii region. The jet-driving source is unseen inside
the opaque globule. Two additional candidate YSOs are seen outside the
globule, PCYC 838 and PCYC 842.

In this paper, we present the first clear case of an evaporating molecular
outflow. The HH 900 jet+outflow, emerges from a small, tadpole-shaped globule
located in the heart of the Carina Nebula (see Figure 1). Dozens of O- and
B-type stars in the nearby Trumpler 16 (Tr16) star cluster illuminate the
system and may have triggered its formation (Reiter et al., 2020b). H$\alpha$
emission traces a wide-angle bipolar outflow that emerges from the opaque
globule (Smith et al., 2010). Unusually for an outflow, the H$\alpha$ emission
appears to taper as it gets further from the driving source. This is the
morphology expected for a jet-driven outflow (see, e.g., Arce et al., 2007),
consistent with H$\alpha$ tracing the ionized skin of the outflow driven by an
underlying jet (seen in [Fe ii]; Reiter et al. 2015a, 2019).

CO observations with ALMA reveal a bipolar molecular outflow that extends only
to the edge of the globule where it abruptly ends (Reiter et al., 2020a). In
seeing-limited near-IR narrowband images, H2 emission extends from the globule
edge along the outflow axis (Hartigan et al., 2015). Optical integral field
unit spectroscopy from the Multi-Unit Spectroscopic Explorer (MUSE) revealed
extended [C i] emission from the same region as the $\text{H}_{2}$ (Reiter et
al., 2019). [C i] is often observed to be coincident with H2 in partially
molecular gas (e.g., Escalante et al., 1991), suggesting that the region where
both lines are detected traces the transition between the fully molecular and
fully ionized portions of the outflow. Together, these observations strongly
suggest that HH 900 has a dissociating molecular outflow.

To test this hypothesis, we obtained new near-IR integral field unit
spectroscopic observations from the Enhanced Resolution Imager and
Spectrograph (ERIS) on the Very Large Telescope (VLT). These high spatial and
spectral resolution observations allow us to probe the three key features we
expect if HH 900 has a dissociating molecular outflow, namely: (1) the same
morphology in molecular ($\text{H}_{2}$) and ionized (Br-$\gamma$) gas; (2)
similar kinematics in molecular ($\text{H}_{2}$) and ionized (Br-$\gamma$) gas
that are distinct from the fast, collimated jet (Reiter et al., 2015a); and
(3) gas that is primarily photoexcited. These data provide comparable angular
resolution and spatial coverage to previous observations obtained with the
_Hubble Space Telescope (HST)_ /ACS, VLT/MUSE, and ALMA, providing a
comprehensive view of the outflow.

Figure 2: Integrated intensity (moment 0) maps of the lines detected in the
ERIS SV data (see Table 3): (a) $\text{H}_{2}$ 1-0 S(1); (b) Br-$\gamma$; (c)
$\text{H}_{2}$ 1-0 S(0); and (d) $\text{H}_{2}$ 2-1 S(1).

##  2 Observations

###  2.1 SPIFFIER/ERIS

We observed HH 900 (RA=10:45:19.3, Dec=$-$59:44:23) using the recently-
commissioned SPIFFIER near-IR integral field unit spectrograph of the VLT/ERIS
instrument (Davies et al., 2018, 2023). SPIFFIER is the refurbished integral
field unit spectrograph SPIFFI (SPectrometer for Infrared Faint Field Imaging)
that was previously part of the Spectrograph for INtegral Field Observations
in the Near Infrared (SINFONI). Data were obtained as part of the ERIS Science
Verification 111https://www.eso.org/sci/activities/vltsv/erissv.html for Pr.
Id. 110.257T (PI: M. Reiter) on the night of 05 December 2022.

We used the lowest resolution plate scale which provides $125\times 250$ mas
spatial pixels (spaxels) over an $8^{\prime\prime}\times 8^{\prime\prime}$
field-of-view. Two overlapping pointings capture the entire extent of the HH
900 jet+outflow (excluding distant bow shocks). Observations were obtained in
good weather with seeing $\lesssim$0.8″. We also utilized laser guide star
(LGS) adaptive optics (AO) correction to further improve the image quality.

We used the high-resolution K-middle filter ($\lambda$=2.06–2.34 $\mu$m) which
covers the H2 line at 2.12 $\mu$m and Br$\gamma$ at 2.16 $\mu$m simultaneously
with spectral resolution R$\sim$11,200. The corresponding velocity resolution
is $\sim$25 km s-1.

Data were reduced using a beta version of the ERIS pipeline provided by ESO at
the time of the Science Verification run and executed through the ESO Reflex
workflow (Freudling et al., 2013). The pipeline corrects the data from the
instrumental signatures, i.e. darks and flats, calibrates a wavelength
solution using the associated arc lamps, applies a field distortion mapping,
computes and substracts the sky contribution, and generates a resampled 3D
data cube. This is then used in the following analysis.

The systemic velocity of HH 900 was measured by Reiter et al. (2020a) who
found $v_{\mathrm{LSR}}=-33.5$ km s-1. For the coordinates of Carina,
$v_{\mathrm{helio}}\approx v_{\mathrm{LSR}}+11.6$ km s-1 (Kiminki & Smith,
2018). Using this, we compute a heliocentric velocity of
$v_{\mathrm{helio}}=-21.9$ km s-1 for HH 900. All velocities in this paper are
reported relative to the heliocentric velocity of HH 900.

###  2.2 Archival data

To align the ERIS/SPIFFIER data with the archival images, we use header
astrometry for preliminary registration, then apply additional linear offsets
to align the point sources near the globule and outflow. The spatially-
resolved globule and tadpole tail provide a second check for data with few
point sources (i.e. from ALMA). Data were additionally rotated and shifted to
minimize subtraction residuals. Typical alignment uncertainties are on the
order of a pixel ($\sim$0.15″).

####  2.2.1  _HST_ H$\alpha$ and [Fe ii]

A narrowband H$\alpha$ image was obtained with the F658N filter on the
Advanced Camera for Surveys (ACS) on 04 August 2014 (programme GO-13390, PI:
N. Smith). The narrowband [Fe ii] 1.64 µm (F164N) and offline continuum
(F167N) images were obtained with the infrared channel of the Wide-Field
Camera 3 (WFC3-IR) on 28 December 2013 (programme GO-13391, PI: N. Smith).
Details of these data and their reduction are presented in Reiter et al.
(2015a).

####  2.2.2 MUSE

The HH 900 system was observed with the Multi-Unit Spectroscopic Explorer
(MUSE) integral field unit spectrograph on the VLT 03 April 2018 (programme ID
0101.C-0391(A); PI: M. Reiter). These observations utilised the GALACSI
Adaptive Optics module in Wide Field Mode (WFM) to provide $\sim$0.8″ angular
resolution over the 1′$\times$ 1′ field-of-view. MUSE provides spectral
coverage from 4650–9300Å with a gap between $\sim$5800–5950Å for the laser
guide stars with spectral resolution $R=2000-4000$. Details of those
observations may be found in Reiter et al. (2019).

####  2.2.3 ALMA

ALMA Band 6 observations of the HH 900 system were obtained on 08 May 2017 and
25 September 2017 using medium and long baseline configurations (programme ID
2016.1.01537.S, PI: A. Guzmán). The maximum angular resolution is 0.02″,
comparable to the resolution of H$\alpha$ images from _HST_. CO lines were
observed with a velocity resolution of 0.08 km s-1. Anaylsis and details may
be found in Reiter et al. (2020a).

##  3 Results

Table 1: Primary emission lines detected in the HH 900 jet+outflow. Wavelengths for the $\text{H}_{2}$ lines are from Levenson et al. (2000); Br-$\gamma$ wavelength is from Chang & Deming (1996) via NIST333 https://www.nist.gov/ . All wavelengths are in vacuum.  Line | wavelength
---|---
| [µm]
$\text{H}_{2}$ 1-0 S(1) | 2.12183
Br-$\gamma$ | 2.16612
$\text{H}_{2}$ 1-0 S(0) | 2.22329
$\text{H}_{2}$ 2-1 S(1) | 2.24772

We detected Br-$\gamma$ and 3 bright $\text{H}_{2}$ lines with ERIS/SPIFFIER
(see Table 3 and Figure 2). All of the detected lines are spatially extended,
tracing the bipolar HH 900 outflow.

Bright emission from all lines also trace the ionization front on the globule
surface. The tadpole tail is prominent in $\text{H}_{2}$ images, tracing the
same morphology seen in silhouette in H$\alpha$ (Smith et al., 2010) and in
emission in CO (Reiter et al., 2020a).

Both Br-$\gamma$ and $\text{H}_{2}$ emission from HH 900 trace the wide-angle
outflow, extending smoothly from where the CO outflow ends at the edge of the
globule (see Figure 3 and Reiter et al., 2020a). Br-$\gamma$ emission traces
the same morphology as H$\alpha$, as expected. In the H ii region,
$\text{H}_{2}$ traces a broadly similar morphology to the ionized outflow
traced by H$\alpha$ and Br-$\gamma$ but the surface brightness is less
uniform. $\text{H}_{2}$ appears limb-brightened with a measurable dip in the
intensity at the mid-point of the outflow lobe (see Figure 4). $\text{H}_{2}$
emission is less extended, reaching only $\sim 2.2$″ (0.02 pc) from the
globule, compared to $\sim 5.5$″ (0.06 pc) for H$\alpha$ and Br-$\gamma$. This
is the same extent seen in seeing-limited $\text{H}_{2}$ images from Hartigan
et al. (2015) and coincides with the [C i] emission seen with MUSE (see Figure
3 and Reiter et al., 2019).

In the following analysis, we refer to the $\text{H}_{2}$ 2.12 µm line simply
as ‘$\text{H}_{2}$’ and specify the transition of the other $\text{H}_{2}$
lines where used.

Figure 3: Top: Grayscale image of $\text{H}_{2}$ from ERIS; red and blue
contours show CO J=2-1 from ALMA (emission is integrated from $-$30 km s-1 to
$-$11 km s-1 and $-$57 km s-1 to $-$35.75 km s-1, respectively); white
contours show [Fe ii] 1.64 µm from _HST_. Bottom: $\text{H}_{2}$ contours
(red; 10–50$\sigma$ in steps of $5\sigma$) on a [C i] 8727 Å image from MUSE.
Figure 4: Top: Intensity tracings showing $\text{H}_{2}$ (red), Br-$\gamma$
(blue), and [Fe ii] (black dashed line) along the length of the outflow (a).
$\text{H}_{2}$ extends a small distance from the globule but tapers off before
[Fe ii] from the jet is first detected. Br-$\gamma$ is bright throughout the
length of the outflow. Middle: $\text{H}_{2}$ image with lines showing the
location of the hortizontal (red line) and vertical (black dotted lines)
intensity tracing locations (b). Bottom: Vertical intensity tracings ordered
from east to west. In the eastern-most slice (c), both lines trace the outflow
but $\text{H}_{2}$ is dominated by emission from the tadpole tail. Closer to
the globule (d), $\text{H}_{2}$ and Br-$\gamma$ trace the wide-angle outflow.
A tracing through the globule itself (e) reveals $\text{H}_{2}$ offset inside
Br-$\gamma$, consistent with a steady-state photodissociation region on the
surface of a post-collapse globule (see Reiter et al., 2020b). The western-
most slice through the HH 900 outflow (f) shows the wide opening angle traced
by both components and illustrates the limb-brightening of the $\text{H}_{2}$
flow.

###  3.1 Velocity structure

Figure 5: Top: $\text{H}_{2}$ image showing the location and width of the
slice used to make the P-V diagrams. Middle: $\text{H}_{2}$ and Br-$\gamma$
P-V diagrams and a third P-V diagram showing $\text{H}_{2}$ contours (red) on
the Br-$\gamma$ P-V diagram. Bottom: The best-fit velocity derived from
fitting a Gaussian to velocity slices along the jet axis (see Section 3.1).

The HH 900 outflow lies close to the plane of the sky, with a tilt angle
$\lesssim 10^{\circ}$ (Reiter et al., 2015a). The bulk of the outflow velocity
is therefore captured in the tangential motions in the plane of the sky. We
benefit from the high velocity resolution of SPIFFIER/ERIS to measure the more
modest radial velocities from the outflow. To measure the velocity of the
outflow components traced by $\text{H}_{2}$ and Br-$\gamma$, we construct
position-velocity (P-V) diagrams of each line (see Figure 5). We extract a
slice 5 pixels wide through the center of the jet+outflow. Both H2 and
Br-$\gamma$ show blueshifted emission from the western limb of the jet and
redshifted emission to the east, consistent with other velocity measurements
of the jet+outflow (Reiter et al., 2015a, 2020a).

To obtain a more precise estimate, we fit a Gaussian to the velocity profile
in different slices across the outflow. We take the average emission in three-
pixel-wide slices (the equivalent of three columns in the P-V diagrams). Line
profiles are largely single-peaked. Low $\text{H}_{2}$ velocities between
$-3$″ and $-4$″ (on the eastern side of the globule) are contaminated by the
blueshifted tadpole tail (see Appendix B and Figure 10). Note that the tadpole
tail is blueshifted on the same (eastern) side of the globule where the
outflow is redshifted. Overall, $\text{H}_{2}$ velocities in the outflow are
faster than Br-$\gamma$.

Velocity measurements for the HH 900 jet+outflow were previous reported by
Reiter et al. (2015a). Br-$\gamma$ velocities are consistent with the
marginally-resolved blueshifted velocities seen in the western limb of the
outflow in H$\alpha$ (which reach a maximum blueshifted velocity of $\sim$-16
km s-1). All outflow velocities traced by $\text{H}_{2}$ and Br-$\gamma$ are
slower than the [Fe ii] emission from the jet ($\pm\sim$30 km s-1; see
Appendix A).

###  3.2 The YSOs

The HH 900 driving source remains unseen at near-IR wavelengths as it is
deeply embedded inside the tadpole-shaped globule. However, we detect two
point sources that are visible outside the globule (see Figure 1). Both were
identified as candidate young stellar objects (YSOs) in the Pan Carina YSO
Catalog (PCYC) by Povich et al. (2011). We use aperture extraction to isolate
the stellar spectrum and remove the sky background. For both objects, the
‘sky’ background includes nebular emission from the H ii region and structured
emission from either the edge of the globule or the limb-brightened HH 900
outflow. As a result, the Br-$\gamma$ and $\text{H}_{2}$ line profiles may be
contaminated with residual nebular emission. Spectra of both sources are shown
in Appendix C.

The first source, PCYC 838, lies directly on top of the western limb of the HH
900 jet+outflow. We detect Br-$\gamma$ in emission in the PCYC 838 YSO
spectrum but no $\text{H}_{2}$ lines. Br-$\gamma$ emission may indicate active
accretion (e.g., Fairlamb et al., 2017). However, we do not detect other
indicators of the circumstellar disk such as the CO bandhead in emission or
absorption (e.g., Carr, 1989; Calvet et al., 1991). The absence of the
molecular bands may point to a slightly higher mass for this object,
consistent with the spectral type (roughly G-type) estimated from the optical
spectrum (Reiter et al., 2019) and the mass estimated from the best-fitting
model of the spectral energy distribution ($2.5\pm 1.2$ M⊙; Povich et al.,
2011).

A second star, identified as candidate YSO PCYC 842 by Povich et al. (2011),
lies at the bottom of the tadpole-shaped globule. No prominent emission lines
(i.e. Br-$\gamma$) are detected in this source. No millimeter continuum or
molecular line emission associated with this source was detected with ALMA
(Reiter et al., 2020a), suggesting a lack of circumstellar material. Indeed,
Reiter et al. (2015a) questioned whether this source is a YSO based on its
motion toward the globule and the low resolution of the data used for its
original classification. The relatively featureless spectrum of this source
does not provide evidence that this source is young or actively accreting.

##  4 HH 900 as an evaporating molecular outflow

We argue that the morphology, velocity, and excitation of the extended
$\text{H}_{2}$ and Br-$\gamma$ emission in HH 900 trace an ionized and
evaporating outflow. In this picture, the molecular outflow is rapidly but not
instantaneously dissociated once it emerges into the H ii region. Extended
$\text{H}_{2}$ emission traces the extent (and therefore the time) that
molecules survive in the H ii region. Spatially coincident [C i] emission seen
with MUSE supports this interpretation (see Figure 3). The ionization
potential of carbon is lower than hydrogen (11.26 eV compared to 13.6 eV), so
[C i] is expected to coexist with partially dissociated $\text{H}_{2}$ (see,
e.g., Osterbrock & Ferland, 2006).

High spatial and spectral resolution integral field unit spectroscopy with
ERIS allows us to perform three key tests of this hypothesis. If HH 900 is an
evaporating molecular outflow we expect: (1) the same morphology in molecules
($\text{H}_{2}$) and recombination lines (Br-$\gamma$); (2) similar outflow-
like kinematics in molecular ($\text{H}_{2}$) and recombination lines
(Br-$\gamma$) that are distinct from the fast, collimated jet traced by [Fe
ii]; and (3) photoexcitation dominating over shock excitation in the outflow.
We discuss each test individually below.

###  4.1 Morphology

Figure 6:  $\text{H}_{2}$ contours (black) on a Br-$\gamma$ grayscale image
(a). [Fe ii] 1.64 µm contours on an $\text{H}_{2}$ image (b) and Br-$\gamma$
image (c). $\text{H}_{2}$ contours on an H$\alpha$ image from _HST_ (d).
$\text{H}_{2}$ contours are 10–50$\sigma$ in steps of 5$\sigma$; [Fe ii]
contours are 3–30$\sigma$ in steps of 3$\sigma$.

$\text{H}_{2}$ emission extends beyond the edge of the globule, tracing the
same wide-angle flow as Br-$\gamma$ (see Figure 6). The $\text{H}_{2}$ appears
limb-brightened, as though tracing the edges of an outflow cavity. The width
of the $\text{H}_{2}$ and Br$\gamma$ profiles are comparable for intensity
tracings through the outflow (see Figure 4). This is in contrast to tracings
through the globule itself that show an offset of $\sim$0.5″ between the
Br-$\gamma$ and $\text{H}_{2}$ emission peaks.

The key morphological difference between $\text{H}_{2}$ and Br-$\gamma$ is
their extent. $\text{H}_{2}$ reaches $\sim$2.2″ (0.02 pc) from the globule
edge into the H ii region. The terminal edge of the limb-brightened
$\text{H}_{2}$ outflow coincides with the onset of [Fe ii] emission from the
jet (see Figure 6). Br-$\gamma$ extends $\sim$5.5″ (0.06 pc) tracing the full
length of the jet seen in H$\alpha$ and [Fe ii]. This is consistent with an
evaporating molecular outflow if $\text{H}_{2}$ disappears at the point where
molecules are completely dissociated.

###  4.2 Outflow velocities

Outflow velocities traced by $\text{H}_{2}$ increase with distance from the
driving source. This Hubble-like flow is characteristic of jet-driven outflows
(see Figure 2 from Arce et al., 2007). The highest $\text{H}_{2}$ velocities
are $\pm\sim 20$ km s-1, remarkably similar to the highest velocities seen
from the embedded CO outflow (Reiter et al., 2020a). $\text{H}_{2}$ velocities
are $\sim$10 km s-1 faster than Br-$\gamma$ in both limbs of the outflow (see
Figure 5) and for all slices across the globule (see Appendix B). This
velocity difference is comparable to the sound speed in ionized gas
($c_{s}\sim 11$ km s-1), the expected velocity of a photoevaporative flow.

The fastest velocities measured in $\text{H}_{2}$ and Br-$\gamma$ are both
lower than in the collimated [Fe ii] jet where Reiter et al. (2015a, 2019)
found fast, steady velocities up to $\pm$30 km s-1. Br-$\gamma$ outflow
velocities are consistent with zero while the fastest $\text{H}_{2}$
velocities are closer to 30 km s-1. Several other outflows have been observed
to have a nested or ‘onion-layer’ structure with fast, highly collimated
components on the jet axis surrounded by slower, wider angle layers (e.g.
Lavalley-Fouquet et al., 2000; Bacciotti et al., 2000; Pyo et al., 2003;
Coffey et al., 2008). This same structure is seen in HH 900 with [Fe ii]
emission tracing the fast core of the jet while $\text{H}_{2}$ traces
molecular layers either lifted from the disk or entrained from the
envelope/globule. On the surface is the ionized Br-$\gamma$ emission where the
strong external radiation field first encounters the slowest, widest layers of
the outflow.

###  4.3 Excitation

Figure 7: Top: Intensity map showing the $\text{H}_{2}$ 1-0 S(1) / Br-$\gamma$
line ratio. Values $<$1 indicate that photoexcitation dominates. Bottom:
Intensity maps of the $\text{H}_{2}$ 1-0 S(1) / $\text{H}_{2}$ 2-1 S(1) line
ratio. The expected ratio is $\sim$3 in photoexcited gas, $\sim$10 in shock-
excited gas.

We measure two different emission line ratios in the spatially-resolved
outflow to determine the excitation of the gas. All four emission lines
detected with these ERIS observations are within a narrow wavelength range so
differences in wavelength-dependent extinction are negligible. We expect
$\text{H}_{2}$/Br-$\gamma$$<$1 for a dissociating outflow where
photoexcitation will play a dominant role (the expected ratio in shock-excited
gas is $>$1; see discussion in e.g., Yeh et al., 2015). The $\text{H}_{2}$ 1-0
S(1) / $\text{H}_{2}$ 2-1 S(1) ratio provides an additional diagnostic. For
shock-excited gas, models predict a line ratio $\gtrsim 10$ (Shull &
Hollenbach, 1978); for photo-excited gas, the expected line ratio is $\sim
1.5-3$ (Black & van Dishoeck, 1987).

Line ratio maps are shown in Figure 7. We take the ratio of the integrated
intensity (moment 0) obtained by integrating each line over an interval
$\pm\sim$4 Å from the line center. Ambient nebular emission is estimated from
the median value of the sky above and below HH 900.

The $\text{H}_{2}$/Br-$\gamma$ ratio is $\geq 1$ throughout the HH 900 system.
The highest values are seen on the eastern edge of the globule and in a few
knots in the tadpole tail indicating that shock-excitation contributes in
these portions of the system. Ratios in the outflow are lower but increase to
higher values toward the outflow edges. No part of the system has
$\text{H}_{2}$/Br-$\gamma$ $<1$.

The interpretation of the $\text{H}_{2}$ 1-0 S(1) / $\text{H}_{2}$ 2-1 S(1)
ratio map is less clear. All values in the $\text{H}_{2}$ 1-0 S(1) /
$\text{H}_{2}$ 2-1 S(1) ratio map are $<$10, below the range expected for
shock-excited gas ($\gtrsim$10). However, throughout the map, the flux ratio
is $>$3, larger than expected for pure photoexcitation. Unlike the
$\text{H}_{2}$/Br-$\gamma$ ratio map, the globule edge is not prominent in the
$\text{H}_{2}$ 1-0 S(1) / $\text{H}_{2}$ 2-1 S(1) ratio map and there are no
prominent bright spots in the tadpole tail.

Based on these ratio maps, the simplest interpretation is that a combination
of shocks and photoexcitation contribute to the observed emission. Note that
the typical $\text{H}_{2}$ knot sizes seen in the spectacular HH 212 jet are
$\sim$250$-$500 au (Zinnecker et al., 1998). At the distance of Carina (2.35
kpc; Göppl & Preibisch, 2022), this corresponds to $<0.25^{\prime\prime}$ –
too small to be resolved with ERIS.

##  5 Discussion

Evaporating molecular outflows are the missing piece that connects (cold)
molecular outflows seen inside clouds with the hot ionized jets laid bare in H
ii regions. Once outflows leave the protection of their natal cocoons,
external irradiation rapidly heats, dissociates, and ionizes them. This
explains the abrupt end of CO outflows like HH 900 at the edge of the globule
(or dust pillar in the case of HH 901 and HH 902, see Cortes-Rangel et al.,
2020).

In HH 900, the CO outflow morphology connects smoothly with the ionized
outflow traced by H$\alpha$ and Br-$\gamma$ that is seen outside the globule.
Near-IR [Fe ii] emission traces the fast, collimated jet that bisects the
wide-angle ionized outflow. However, this component is first detected
$\gtrsim$1.5″ from the edges of the globule. Non-instantaneous dissociation of
the molecular outflow provides one explanation for this gap. In this region,
all UV photons are consumed ionizing the skin of the outflow (Br-$\gamma$;
H$\alpha$) and dissociating molecules just inside it ($\text{H}_{2}$; [C i]),
exhausting the high energy photons before they can reach the core and ionize
Fe (first ionization requires 7.6 eV). Essentially, slices along the outflow
at different distances from the globule trace the development and progression
of a photodissociation region (PDR) through the column of outflowing gas (see
Figure 9).

In a less punishing environment than Carina, HH 900 would look like a
traditional molecular outflow with CO tracing more of its extent. The HH 900
YSO remains deeply embedded in the globule with the only detection to date in
the millimeter continuum with ALMA. This suggests that the source evolutionary
stage is in the Class 0/I regime, consistent with HH 900 being in the most
active outflow phase (we discuss the mass-loss rates in Section 5.1).

One of the outstanding challenges for understanding the local and large-scale
impact of protostellar jets and outflows is obtaining a full mass census.
Typically, only one component is visible: the underlying (atomic) jet is
unseen in molecular outflows while optical and near-IR images reveal
collimated atomic jets that have largely escaped their natal clouds and may no
longer be surrounded by a molecular outflow. External illumination reveals all
of these components in the HH 900 jet+outflow. The system is younger than
other well-studied examples of externally irradiated jets (e.g., in Orion,
Bally & Reipurth, 2001; Bally et al., 2006a; Kirwan et al., 2023), capturing a
consequential but not well understood outflow phase.

External irradiation in the H ii region alters the observable diagnostics but
provides an extraordinary opportunity because it also illuminates the entire
body of the jet+outflow. Mass-loss rates can therefore be estimated using the
well-understood physics of photoionized gas instead of non-linear and time-
dependent shock models. Following Bally et al. (2006a), we can use the well-
studied environment to determine the dissociation time, and therefore the
mass-loss rate, of the dissociating molecular outflow.

###  5.1 Mass-loss rate

We make a simple estimate of the photodissociation rate of molecular hydrogen
entrained in the outflow. The free space photodissociation rate is
$D_{0}=5.18\times 10^{-11}\chi$ s-1 (Sternberg et al., 2014; Bialy, 2020)
where $\chi$ is the Draine UV radiation field strength (Draine, 1978).
Ignoring shielding, the photodissociation rate of $\text{H}_{2}$ at the
surface of a slab irradiated from one side (the outflow in our case) is

$R_{\mathrm{phot}}=\frac{1}{2}\chi 5.18\times 10^{-11}~{}\mathrm{s}^{-1}.$ (1)

From Smith (2006), the FUV luminosity of Tr16 is log(LFUV) $=6.79$ L⊙. We
convert this to the local flux assuming the median distance to the OB stars in
Tr16 from Alexander et al. (2016). This gives an incident Draine UV radiation
field of $\chi=10^{3}$, corresponding to a dissociation timescale of $\sim$1.2
years. For an outflow velocity of 30 km s-1, this gives a distance of $\sim$8
au for the extent of the $\text{H}_{2}$ outflow. This is $\sim$3 orders of
magnitude smaller than the observed $\text{H}_{2}$ extent of HH 900
($\sim$5000 au, see Section 4.1).

This simple estimate ignores the effect of shielding. However, $\text{H}_{2}$
can be shielded by dust (which may also be present in the outflow Smith et
al., 2010; Reiter et al., 2019, 2020a) or it can self-shield, provided a
sufficiently large $\text{H}_{2}$ column (Sternberg et al., 2014). Including
self-shielding, the dissociation rate becomes

$R_{\mathrm{phot}}=\frac{1}{2}\chi 5.18\times
10^{-11}f_{\mathrm{shield}}(N_{\mbox{$\text{H}_{2}$}})\exp\left(-\tau_{g}\right)\mathrm{s}^{-1}.$
(2)

where $\tau_{g}=\sigma_{g}N_{\mbox{$\text{H}_{2}$}}$ is the dust column
optical depth and $f_{\mathrm{shield}}(N_{\mbox{$\text{H}_{2}$}})$ is the H2
self-shielding parameter; both depend on the H2 column
$N_{\mbox{$\text{H}_{2}$}}$. We follow Bialy (2020) and adopt
$\sigma_{g}=1.9\times 10^{-21}$cm2. For
$f_{\mathrm{shield}}(N_{\mbox{$\text{H}_{2}$}})$ we use the Draine & Bertoldi
(1996) fit

$f_{\mathrm{shield}}(N_{\mbox{$\text{H}_{2}$}})=\frac{0.965}{\left(1+x/b_{5}\right)^{2}}+\frac{0.035}{\left(1+x\right)^{1/2}}\\\
\times\exp\left[-8.5\times 10^{-4}\left(1+x\right)^{1/2}\right]$ (3)

where $x=N_{\mbox{$\text{H}_{2}$}}/5\times 10^{14}$cm-2 and $b_{5}=2/10^{5}$cm
s-1. This gives an $\text{H}_{2}$ photodissociation timescale that depends
only on the $\text{H}_{2}$ column for a given $\chi$. We solve this expression
for the $\text{H}_{2}$ column that enables the H2 outflow to extend $\sim
5\times 10^{3}$ au with a flow velocity of $\sim 30$ km s-1. We calculate the
photodissociation timescale $t_{\mathrm{phot}}=1/R_{\mathrm{phot}}$ and
multiply by a characteristic speed (30 km s-1) to obtain the maximum
observable $\text{H}_{2}$ extent.

Figure 8: The photodissociation timescale as a function of $\text{H}_{2}$
column (left axis). This is also presented in terms of the extent of molecular
H2 expected for a column travelling at 30 km s-1 (right axis). This is
calculated by multiplying the photodissociation timescale
$t_{\mathrm{phot}}=1/R_{\mathrm{phot}}$ by the typical flow speed. The
horizontal black line is the observed extent of $\text{H}_{2}$ in the outflow.

Results from this simple analysis are shown in Figure 8. We estimate that an
$\text{H}_{2}$ column of $\sim 1\times 10^{18}$ cm-2 is required to reproduce
the observed extent of the outflow. Assuming the outflow is a cylinder, we
convert this to a volume density by dividing by the observed width of the
outflow. For an $\text{H}_{2}$ column density of $\sim 10^{18}$ cm-2 and a jet
width of $2r_{\mathrm{out}}\sim$1.5″ (3450 au), the $\text{H}_{2}$ volume
density is $n_{\mbox{$\text{H}_{2}$}}\sim 20$ cm-3. We compute the mass-loss
rate of the $\text{H}_{2}$ outflow assuming a constant density and radius in a
cylindrical jet

$\dot{M}_{\mbox{$\text{H}_{2}$}}=n_{\mbox{$\text{H}_{2}$}}m_{\mbox{$\text{H}_{2}$}}\pi
r_{\mathrm{out}}^{2}v_{\mathrm{out}}$ (4)

where $m_{\mbox{$\text{H}_{2}$}}$ is the mass of molecular hydrogen and
$v_{\mathrm{out}}=30$ km s-1 is the characteristic velocity of the outflow.
From this, we estimate $\dot{M}_{\mbox{$\text{H}_{2}$}}\approx 1.6\times
10^{-9}$ M⊙ yr-1.

This simple estimate yields a mass-loss rate two orders of magnitude smaller
than estimated for the ionized component, $\dot{M}_{\mathrm{H}\alpha}\approx
5.7\times 10^{-7}$ M⊙ yr-1 (Smith et al., 2010). The mass-loss rate of the
molecular outflow inside the globule is an order of magnitude higher than the
ionized component with an average $\dot{M}_{\mathrm{CO}}\approx 3.5\times
10^{-6}$ M⊙ yr-1 (Reiter et al., 2020a). The highest mass-loss rate is measure
in the low-ionization jet core with $\dot{M}_{\mathrm{[FeII]}}\approx
1.7\times 10^{-5}$ M⊙ yr-1 (Reiter et al., 2016).

The large discrepancy between the mass-loss rates in the molecular outflow
measured inside and outside the globule is not entirely surprising. We ignore
any dust shielding in the outflow despite indirect evidence for dust in the
outflow. In addition, we argue that $\text{H}_{2}$ emission traces partially
dissociated molecular gas. The column of neutral material tracing the brief
phase between dissociation and ionization should increase once the outflow is
in the H ii region. Near the edge of the globule where the outflow first
emerges into the H ii region, we expect that the sum of the molecular
($\text{H}_{2}$), neutral (C i?), and ionized (H$\alpha$ or Br-$\gamma$)
components to be comparable to the CO mass-loss rate measured inside the
globule. This implies that there is a large column of neutral material in the
outflow. Future observations of neutral gas tracers like C i with ALMA may
reveal the mass and extent of this neutral component.

Finally, we note that the low volume density we estimate in $\text{H}_{2}$ is
consistent with photoexcited gas. Black & van Dishoeck (1987) predict that, in
steady state, the boundary layer should have
$n(\mbox{$\text{H}_{2}$})<<n(\mathrm{H})$. We derive an
$n(\mbox{$\text{H}_{2}$})$ that is 13$\times$ smaller than the $n_{e}$
reported by Smith et al. (2010), consistent with this prediction.

Figure 9: Cartoon depiction of the jet observables (left) and the structure of
the jet+outflow (right). Cold molecules only survive in the protection of the
globule (CO; gray). In the H ii region, molecules are hot ($\text{H}_{2}$;
red) before they are completely dissociated leaving the ionized outflow
(Br-$\gamma$; blue) surrounding the underlying jet ([Fe ii]; black).

##  6 Conclusions

In H ii regions, jets and outflows that emerge from their natal clouds will be
externally illuminated, dissociated, and ionized by UV photons from nearby
high-mass stars. In this paper, we present new near-IR integral field unit
spectroscopy from ERIS/SPIFFIER that provides the first clear evidence that HH
900 is a dissociating and evaporating molecular outflow. Extended
$\text{H}_{2}$ and Br-$\gamma$ emission trace the wide-angle outflow as it
smoothly extends from the edge of the CO outflow. $\text{H}_{2}$ emission
extends $\sim$2.2″ (0.02 pc) from the globule edge, tracing the molecule
survival time. The ionized outflow, traced by Br-$\gamma$, reveals the full
extent of the outflow as it reaches $\sim$5.5″ into the H ii region.

These new spatially and spectrally resolved observations of HH 900 allow us to
perform three tests to confirm that HH 900 is an evaporating molecular
outflow. First, we show that $\text{H}_{2}$ and Br-$\gamma$ trace the same
morphology as long as molecules survive. Second, we show that both lines trace
outflow-like kinematics. Velocities of both lines are modest where the outflow
emerges from the globule, then $\text{H}_{2}$ increases to speeds approaching
the high velocity of the underlying jet seen in [Fe ii]. Velocities between
each component differ by $\sim$10 km s-1 with [Fe ii] tracing the fastest
velocities, $\text{H}_{2}$ intermediate, and Br-$\gamma$ the slowest,
consistent with the layered velocity structure seen in other outflows. Third,
diagnostic line ratios indicate a significant contribution from
photoexcitation to the excitation in the outflow. Together, these tests
provide strong evidence that an evaporating molecular outflow connects the
cold (CO) outflow seen inside the globule with the hot ionized outflow
(H$\alpha$, Br-$\gamma$) seen in the H ii region.

To the best of our knowledge, this is the first direct evidence for an
evaporating molecular outflow. As such, these observations provide an
excellent test case for models of irradiated jets and outflows (i.e.,
Estrella-Trujillo et al., 2021). Finally, near-IR observations from the _James
Webb Space Telescope_ (JWST) will soon be revealing jets and outflows in many
high-mass star-forming regions (e.g., Reiter et al., 2022). These externally
irradiated sources will likely have more in common with objects like HH 900
than the well-studied jets seen in more local, quiescent regions.

## Acknowledgements

We would like to thank the referee, Dr. William Henney, for a prompt and
thoughtful report that improved the quality of the manuscript. We would like
to thank Lowell Tacconi-Garman for support with the preparation, reduction,
and analysis of the new ERIS/SPIFFIER data. We wish to thank Nathan Smith for
productive discussion about the velocity structure of HH 900. TJH is funded by
a Royal Society Dorothy Hodgkin Fellowship and UKRI ERC guarantee funding
(EP/Y024710/1). CFM is funded by the European Union (ERC, WANDA, 101039452).
DI is funded by the European Research Council (ERC) via the ERC Synergy Grant
ECOGAL (grant 855130). Views and opinions expressed are however those of the
author(s) only and do not necessarily reflect those of the European Union or
the European Research Council Executive Agency. Neither the European Union nor
the granting authority can be held responsible for them.

Based on observations collected at the European Southern Observatory under ESO
programmes 110.257T.001 and 0101.C-0391(A). This paper makes use of the
following ALMA data: ADS/JAO.ALMA #2016.1.01537.S. ALMA is a partnership of
ESO (representing its member states), NSF (USA) and NINS (Japan), together
with NRC (Canada) and NSC and ASIAA (Taiwan) and KASI (Republic of Korea), in
cooperation with the Republic of Chile. The Joint ALMA Observatory is operated
by ESO, AUI/NRAO and NAOJ. This work uses observations made with the NASA/ESA
Hubble Space Telescope, obtained from the Data Archive at the Space Telescope
Science Institute, which is operated by the Association of Universities for
Research in Astronomy, Inc., under NASA contract NAS 5-26555. The HST
observations are associated with GO 13390 and GO 13391. This research made use
of Astropy,444http://www.astropy.org a community-developed core Python package
for Astronomy (Astropy Collaboration et al., 2013; Price-Whelan et al., 2018).
This research made use of APLpy, an open-source plotting package for Python
(Robitaille & Bressert, 2012).

## Data Availability

The ERIS data that are presented in this paper are publicly available from the
ESO archive555https://archive.eso.org/cms.html. Archival MUSE data are also
available from the ESO archive. The ALMA data used in this study are publicly
available from the ALMA
archive666https://almascience.nrao.edu/aq/?result_view=observations. Data from
_HST_ are publicly available via the MAST
archive777https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html.

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##  Appendix A Reconciling HH 900 velocities

The systematic velocity of HH 900 was not known when Reiter et al. (2015a)
analyzed the [Fe ii] emission from the jet. The derived velocity of $\approx
15$ km s-1 is much redder than Carina itself which has a heliocentric velocity
of $\approx-8.4$ km s-1 (Rebolledo et al., 2016). Re-examining the spectra in
Reiter et al. (2015a), we find that the estimated systematic velocity is
almost exactly equal to the heliocentric velocity correction for the date of
observation (15.75 km s-1).

We have remeasured the HH 900 jet velocities from the FIRE spectra presented
in Reiter et al. (2015a) including the heliocentric velocity correction and
the known velocity of the system (see Section 2 and Reiter et al., 2020a).
This centers the [Fe ii] emission symmetrically around the source velocity,
extending to $\sim\pm 30$ km s-1.

We note that the jet velocities measured from [Fe ii] emission in Reiter et
al. (2015a) and Reiter et al. (2019) were both asymmetric with the blueshifted
jet extending to $\sim-20$ km s-1 while the redshifted jet extended to $\sim
40$ km s-1. This offset corresponds to the blueshift of $\sim 10$ km s-1 of HH
900 relative to Carina.

##  Appendix B P-V slices

We show three slices through the HH 900 jet+outflow in Figure 10. As in
Section 3.1, each slice is 5 spatial pixels wide. The slice through the top
edge of HH 900 and the globule offer the most pristine measure of the outflow
kinematics as this is unaffected by the tadpole tail. The middle P-V slice is
the same as discussed in Section 3.1. The P-V slice along the bottom of the
outflow and globule are clearly affected by the blueshifted tadpole tail which
is brighter than the outflow in $\text{H}_{2}$. The tadpole globule is not
resolved in Br-$\gamma$. Based on diffraction-limited H$\alpha$ images (see
Figure 1), we expect the tail, like the globule, would be seen in silhouette,
so does not affect the Br-$\gamma$ velocities.

Figure 10: P-V slices extracted from the three positions in the tadpole shown
in the top figure. The red-blue outflow velocities are most prominent in the
top slice which is mininally affected by the blueshifted tadpole tail. Middle
panel is the same as Figure 5, reproduced here for convenience. Bottom panel
is the lowest slice with blueshifted emission from the tadpole tail clearly
affecting the $\text{H}_{2}$ velocities.

Figure 11: P-V slice that extends the entire length of the Br-$\gamma$
outflow.

##  Appendix C YSO spectra

Figure 12: Spectrum of the PCYC 838 YSO located in the western limb of the HH
900 jet+outflow. Various emission lines are detected, including the Br$\gamma$
line, tracing ongoing accretion on the target.  Figure 13: Spectrum of the
star at the bottom of the globule (PCYC 842 in Povich et al., 2011).