# The outflow of the protostar in B335: I

Klaus W. Hodapp  University of Hawaii, Institute for Astronomy, 640 N. Aohoku
Place, Hilo, HI 96720, USA  Laurie L. Chu  IPAC, California Institute of
Technology, 1200 E California Boulevard, Pasadena, CA 91125  Thomas Greene
NASA Ames Research Center Space Science and Astrobiology Division M.S. 245-6
Moffett Field, CA 94035, USA  Michael R. Meyer  Department of Astronomy,
University of Michigan, 1085 S. University, Ann Arbor, MI 48109, USA  Doug
Johnstone  NRC Herzberg Astronomy and Astrophysics, 5071 West Saanich Rd,
Victoria, BC, V9E 2E7, Canada  Department of Physics and Astronomy, University
of Victoria, Victoria, BC, V8P 5C2, Canada  Marcia J. Rieke  Steward
Observatory, University of Arizona, Tucson, AZ 85721, USA  John Stansberry
STScI, Steven Muller Building, 3700 San Martin Drive, Baltimore, MD 21218, USA
Martha Boyer  STScI, Steven Muller Building, 3700 San Martin Drive, Baltimore,
MD 21218, USA  Charles Beichman  IPAC, California Institute of Technology,
1200 E California Boulevard, Pasadena, CA 91125  Scott Horner  NASA Ames
Research Center Space Science and Astrobiology Division M.S. 245-6 Moffett
Field, CA 94035, USA  Tom Roellig  NASA Ames Research Center - Ames Center for
Exoplanet Studies (ACES), CA, USA  George Rieke  Steward Observatory,
University of Arizona, Tucson, AZ 85721, USA  Eric T. Young  Universities
Space Research Association, 425 3rd Street SW, Suite 950, Washington DC 20024

###### Abstract

The isolated globule B335 contains a single, low luminosity Class 0 protostar
associated with a bipolar nebula and outflow system seen nearly perpendicular
to its axis. We observed the innermost regions of this outflow as part of
JWST/NIRCam GTO program 1187, primarily intended for wide-field slitless
spectroscopy of background stars behind the globule. We find a system of
expanding shock fronts with kinematic ages of only a few decades emerging
symmetrically from the position of the embedded protostar, which is not
directly detected at NIRCam wavelengths. The innermost and youngest of the
shock fronts studied here shows strong emission from CO. The next older shock
front shows less CO and the third shock front shows only H2 emission in our
data. This third and most distant of these inner shock fronts shows
substantial evolution of its shape since it was last observed with high
spatial resolution in 1996 with Keck/NIRC. This may be evidence of a faster
internal shock catching up with a slower one and of the two shocks merging.

Young stellar objects (1834) — Protostars (1302) — Cometary globules (276) —
Stellar jets (1607) —

††facilities: JWST, Keck:I

##  1 Introduction

Stars form out of dense cores of molecular gas by accretion of material. In
almost all circumstances, the gas accreted in this process has much larger
angular momentum than the newly formed star will have. As a way of shedding
the excess angular momentum, mass accretion onto a protostar or young star is
associated with outflow activity. For the youngest stars in their main
accretion phase, the outflow is in the form of a collimated jet with internal
shock fronts radiating primarily in the emission lines of H2. This fast jet
interacts turbulently with the ambient molecular material and entrains it into
a broader, slower molecular outflow that is best studied by radio emission of
CO. In later phases of mass accretion onto almost completely formed young
stellar objects (YSO), well collimated, very high velocity jets radiating in
atomic emission lines are commonly found. Outflow and jet phenomena in young
stars were recently reviewed by Bally (2016) and we refer to the references
therein for an outline of the history of this field.

The isolated globule Barnard 335 in Aquila (Barnard et al., 1927) was observed
as part of JWST program 1187, primarily to map the broadband extinction and to
measure ice absorption column densities with NIRCam Wide Field Slitless
Spectroscopy (WFSS). Details of our sample selection and preliminary ground-
based results on the onset of ice formation in those cores was presented by
Chu & Hodapp (2021). B335 is the only molecular core in our sample of three
cores for program 1187 that has an embedded protostar, discovered by Keene et
al. (1983).

The distance to B335 was first measured by Tomita et al. (1979) at 250 pc and
this was long used as the canonical distance. Using similar purely photometric
methods, Olofsson & Olofsson (2009) measured a distance in the range of 90 -
120 pc. They had already mentioned a possible association of the B335 cloud
with the star HD 184982 on the basis that this star appears to be illuminating
the cloud in addition to the general interstellar radiation field. Watson
(2020) obtained new images of this reflection nebulosity and used the Gaia DR2
distance to HD 184982 of 164.5 pc as a proxy for the distance of the B335
molecular core. For this paper, we adopt this distance value of 164.5 pc.

The larger molecular cloud surrounding the Bok globule B335 was found by
Frerking & Langer (1982). The molecular outflow associated with the embedded
protostar was discovered and mapped by Hirano et al. (1988) who also
determined that the outflow axis is inclined only 10$\arcdeg$ from the plane
of the sky, i.e., the bipolar outflow is seen nearly perpendicular to its
axis. The molecular core B335 is the densest part of a larger cometary
globule, suggesting that an external wind shaped the globule and triggered the
star formation in it, as was discussed in the case of the globules associated
with the Gum nebula by Reipurth (1983). The pattern of background star
polarization, first mapped by Hodapp (1987) is also consistent with the
globule having been formed by an external wind.

The bipolar outflow emerging from the single protostar in B335 was studied by
Stutz et al. (2008) based on Spitzer Space Telescope imaging data. Out to a
wavelength of 8 $\mu$m the object appears as a bipolar nebula. Only at
wavelengths of 24 $\mu$m and longer does the object appear as an unresolved
point source, probably direct light from the central embedded protostar. The
ALMA-based studies of B335 by Evans et al. (2015) and Evans et al. (2023)
found clear evidence for infall from the redshifted absorption seen in HCN and
HCO+, estimated the age of the accreting protostar at 5.0 $\times$ 104 yrs,
and found evidence for variability and episodic accretion. Based on ALMA data
Bjerkeli et al. (2019) found evidence for the recent (around 2015) ejection of
a molecular bullet and Bjerkeli et al. (2023) presented evidence for infall
faster than free-fall velocity, interpreted as evidence for variable infall
and accretion. WISE W2 (4.6 $\mu$m) scattered light from the embedded
protostar showed an outburst between 2015 and 2022 (Kim et al., 2023), again
supporting variable accretion.

Optical Herbig-Haro shock fronts emerging from the B335 globule were found
first by Vrba et al. (1986) and then confirmed and added to by Reipurth et al.
(1992) who labelled them as HH 119 A-C. In the infrared, the first major
imaging study was done by Hodapp (1998) who labelled shock fronts found in the
H${}_{2}1–0$ S$(1)$ emission line, but not confirmed by optical spectroscopy
as HH 119 IR1-5. This naming scheme was expanded to include newly identified
shocks by Gâlfalk & Olofsson (2007). The strong extinction by the nearly edge-
on disk and the dense inner envelope around the protostar in B335 has
prevented any studies of infrared shock fronts in the immediate vicinity of
the protostar prior to JWST.

Our JWST observing program 1187 on B335 was cut short by a guide star
acquisition failure on the second of two different telescope pointings. The
data presented here are about half of the originally intended data set, which
explains the incomplete spatial coverage.

In this paper, we present our imaging and wide-field slitless spectroscopy
(WFSS) data in section 2. We discuss the outflow cavity in section 3.1, the
position and proper motion of the protostar in section 3.2, the shock proper
motions in 3.3, and the emission spectrum of some of the shocks in section
3.4.

##  2 Observations and Results

The results reported here are based on guaranteed time observations (GTO) with
the James Webb Space Telescope (JWST) (Gardner et al., 2023). The imaging
observations were obtained as part of the JWST Near-Infrared Camera (NIRCam)
(Rieke et al., 2023) Wide Field Slitless Spectroscopy (WFSS) of background
stars behind B335 aimed at mapping the column density of H2O, CO2, and CO ice
(program 1187). The use of the NIRCam grisms for slitless spectroscopy was
described by Greene et al. (2017). The observations were carried out on 2023
April 21 UTC (MJD 60059). WFSS data were obtained through the four medium
width filters F410M, F430M, F460M, and F480M, whose bandpasses cover the CO2
and CO ice features The DOI 10.17909/p188-aj80 (catalog 10.17909/p188-aj80)
contains all the spectra extracted by the stage 3 pipeline, which were not
used in this study. However, the WFSS data were used in stage 2 calibration
form and the filenames used are listed in Table 1. They can be filtered from
the dataset pointed to by the DOI. Direct images in the F277W, F300M, F356W,
and F444W filters were obtained as part of the source registration and
wavelength calibration procedure for the slitless spectroscopy and these data
is available at MAST: 10.17909/qa3f-5703 (catalog 10.17909/qa3f-5703).

Due to the strong extinction near the B335 protostar, only the long-wavelength
module data at wavelengths longer than 2.5 $\mu$m proved useful for this
paper, and are summarized in Table 1. The imaging data were processed by STScI
using the JWST Science Calibration Pipeline (10.5281/zenodo.8067394 (catalog
10.5281/zenodo.8067394)) version 1.9.6 and calibration file $jwst\\_1084.pmap$
with astrometric calibration based on the Gaia DR3 catalog. Figure 1 shows an
overview color image of B335, composed of the images in the F277W (blue
channel), F356W (green channel), and F444W (red channel), in logarithmic
scaling to emphasize low flux regions.

Table 1: Observations Details
Filter | grism | Total Integration | NGROUPS | NINTS | Date [MJD] | MAST File Name
---|---|---|---|---|---|---
F277W | clear | 773 | 4 | 2 | 60055 | jw01187-o011_t001_nircam_clear-f277w_i2d.fits
F356W | clear | 344 | 4 | 2 | 60059 | jw01187-o014_t001_nircam_clear-f356w_i2d.fits
F444W | clear | 1374 | 4 | 2 | 60059 | jw01187-o015_t001_nircam_clear-f444w_i2d.fits
F410M | GRISMC | 172 | 8 | 1 | 60059 | jw01187015002_2101_0001_nrcalong_rate.fits
F410M | GRISMC | 172 | 8 | 1 | 60059 | jw01187015002_210c_0001_nrcalong_rate.fits
F430M | GRISMC | 172 | 8 | 1 | 60059 | jw01187015002_2103_0001_nrcalong_rate.fits
F430M | GRISMC | 172 | 8 | 1 | 60059 | jw01187015002_210e_0001_nrcalong_rate.fits
F460M | GRISMC | 172 | 8 | 1 | 60059 | jw01187015002_2105_0001_nrcalong_rate.fits
F460M | GRISMC | 172 | 8 | 1 | 60059 | jw01187015002_210g_0001_nrcalong_rate.fits
F480M | GRISMC | 172 | 8 | 1 | 60059 | jw01187015002_2107_0001_nrcalong_rate.fits
F480M | GRISMC | 172 | 8 | 1 | 60059 | jw01187015002_210i_0001_nrcalong_rate.fits

Figure 1: RGB color composite of the central 118$\arcsec\times$ 81$\arcsec$ of
the B335 outflow in canonical orientation with North up and East left. The
NIRCam filter F444W is red, F356W is green, and F277W is blue, the flux
scaling is logarithmic. Bright stars that saturated the detector pixels, most
notably the bright background star 2MASS J19365867+0733595 at the western edge
of the image, are displayed with black centers. The previously known infrared
shock fronts IR3 - IR5 (Hodapp, 1998) are labelled. Figure 2: The left panel
is a direct image in the F444W filter. The four middle panels are cutouts of
the WFSS images through the F410M, F430M, F460M, and F480M filters. The right
panel is a magnified cutout of the F430M grism image showing the CO emission
from shock fronts 3E and 4E. All images are in the detector coordinate system,
the grism (GRISMC) dispersion direction is vertical. Each filter transmits a
wavelength range of the dispersed image of the scene, with spatial and
spectral information intermixed. In each of central four grism images, we
indicate the position of one star with an asterix symbol $\approx$ 5″to the
right on the side of the frame, and indicate the transmission region of each
filter (50% point) and the direction of increasing wavelengths. In the
spectrum of the star indicated by this symbol, absorption by CO2 ice is
clearly seen in the F430M filter, and absorption by CO is seen in the F460M
and F480M images. We have labelled the positions of the images of shock fronts
4E, 5E, and 6E in the H2 0-0 S(11) line in the F410M and F430M panels, and in
the S(9) line in the F460M and F480M panels. In the F460M panel, CO emission
from shock 3E and some from shock 4E are seen, and shown magnified in the
right panel for clarity. The very bright star near the top of the images is
saturated in the direct F444W image and the F410M WFSS image.

For the WFSS observations, a grism, in our case the GRISMC in the NIRCam pupil
wheel, disperses the light from every object in the field into a spectrum.
Individual medium band filters were used to record sections of this spectrum.
When using medium bandpass filters, the spectrum section in each of the WFSS
images only 200 - 300 pixels long, so that the problem of spectra of different
objects overlapping is mitigated. As it turned out, without this sub-division
of the spectral range, the CO emission from the bright compact shock front
would have been masked by flux from the reflection nebula and bright stars.
Figure 2 shows cutouts of the inner regions of the B335 bipolar nebula in the
orientation of the NIRCam detectors, where the dispersion of the GRISMC is
along the column axis. The left panel is the direct F444W image for
orientation. The other panels show the same range of columns and a wavelength
dependent shift in the vertical (dispersion) direction.

The WFSS spectra of the extended shock fronts were extracted with custom
python routines from the the flux calibrated “rate” files produced by the
STScI pipeline. The wavelength calibration was done using the H2 emission
lines in the object spectra themselves. Slitless spectroscopy data in general
intermix spatial and spectral information and the spectral resolution depends
on the spatial extent of the object. Since the shock fronts studied here are
extended and of complex morphology, our data are suitable for detecting the
presence of emission lines, but not for studies of the radial velocity or its
line structure. In extracted spectra, the lines for one object are recorded at
the proper wavelengths, but other objects, spatially offset, will also
contribute to the spectra and their lines are recorded at correspondingly
offset wavelengths.

We had two visits scheduled for WFSS observations through the four medium
width filters. Visit 15:1, covering the western part of the globule, was not
successful due to guide star acquisition problems, and a repeat request was
approved. This will give more imaging and spectroscopy in the western part of
the globule, and hopefully allow better proper motion studies. The repeat
observations, now labelled visit 65:1, are expected to be scheduled in April
or May of 2024, and will be published separately.

##  3 Discussion

###  3.1 The outflow cavity

The central region of the B335 outflow appears as a bipolar nebula in our
images. Other studies have shown that we see this outflow nearly edge-on and
that the extinction towards the protostar must be extremely high. Chandler et
al. (1990) have measured an AV $>$ 320 mag. and Keene et al. (1983) have shown
that the protostar is only detectable at far infrared and radio wavelengths.
Stutz et al. (2008) demonstrated that the embedded protostar only becomes
directly visible at 24$\mu$m in Spitzer Space Telescope images.

Indeed, even our longest wavelength image, in the JWST F444W filter, does not
show the protostar itself. Recently, Evans et al. (2023) and Kim et al. (2023)
pointed out that the WISE/NEOWISE mission (Wright et al., 2010), recorded a
pronouced outburst of B335 in the W2 (4.6$\mu$m). The outburst lasted about 12
years and is just now ending in 2023, with some uncertainty since there is
only one data point to define the quiescent baseline. Using a sophisticated
radiative transfer model of the B335 protostar, Evans et al. (2023) have
interpreted the W2 flux maximum as evidence for a temporary increase in the
protostar luminosity up to $\approx$ 22 L⊙. This near infrared variability
coincides with an increase in radio line flux observed in 2018 with ALMA by
Evans et al. (2023) and is therefore indicative of an increase in accretion
activity on the B335 protostar.

Due to the r = 5″photometry aperture and the poor spatial resolution of the
WISE telescope, these measurements covered essentially all the scattered light
from the bright parts of the B335 outflow cavities. At the time of our
$Spitzer$ observations, the B335 protostar had a luminosity of 16 L⊙ (Kim et
al., 2023), a factor of 4 above the 2010 quiescent value.

Archival $SpitzerSpaceTelescope$ Infrared Array Camera (IRAC) (Fazio et al.,
2004) images (10.26131/IRSA413 (catalog 10.26131/IRSA413)) from 2004 and
images taken with the same instrument by Chu & Hodapp (2021) in 2016 during
the outburst are shown in Fig. 3. In both IRAC channels 1 (3.55 $\mu$m) and 2
(4.49 $\mu$m), the bipolar nebula was substantially brighter in 2016 than in
2004, confirming the brightness outburst. The Spitzer photometry shown by (Kim
et al., 2023) based on the 2004 data shows the object above the quiescent
brightness, but this data point was taken with an aperture of 12″, so is not
directly comparable to the WISE data points. For our discussion, the only
important point is that the difference images in Fig. 3 do not show the full
amplitude of the outburst of the protostar.

With the better spatial resolution of $Spitzer$, the difference images
(2016-2004) show that the change in brightness was not entirely uniform across
the eastern lobe of the bipolar reflection nebula, but that changes in the
local illumination conditions contributed to the brightness changes. Such
spatial variations are common in bipolar reflection nebulae, e.g., in Hubble’s
Variable Nebula NGC 2261 (Hubble, 1916), and also observed in several younger
and more deeply embedded bipolar such as Cep A by Hodapp & Bressert (2009) and
L483 by Connelley et al. (2009). However, in B335, the latest outburst was
dominated by an overall increase in the protostar luminosity, and extinction
and variations along the scattering light path are a minor contribution. Our
JWST imaging data were taken close to the end of this outburst, but with the
luminosity still about 50% above the quiescent value of 4 L⊙.

Figure 3: All panels are $40\arcsec\times 40\arcsec$ cutouts of Spitzer mosaic
images. The top panels are IRAC images in channel 1 (3.55 $\mu$m) and the
lower row is in channel 2 (4.49 $\mu$m). Each row shows the 2004 image in the
left, the 2016 image in the center, and the difference, scale for average zero
signal at right. In addition to the overall change of brightness, changes in
the spatial flux distribution are apparent.

The eastern outflow cavity is outlined in the wide-field view of B335 in the
RGB composite in Fig. 1. The two lobes of the bipolar nebula are of different
surface brightness, with the eastern lobe being brighter, indicating that this
eastern lobe suffers less extinction and is on the front side of the
equatorial disk around the protostar. This essentially confirms the finding by
Hirano et al. (1988) that the eastern outflow is slightly inclined towards the
observer. The outflow cavity walls, seen in CO velocity-integrated line
emission (Kim et al., 2023) or even better in the CS line (Evans et al., 2023)
closely resemble the outflow cavity in our images (Fig. 1), including the fact
that the eastern outflow lobe appears narrower than the western lobe.

Beyond the outflow cavity that is illuminated by the embedded protostar, the
B335 molecular core exhibits coreshine (Steinacker et al., 2010), i.e. its
edges are outlined at infrared wavelength by the scattered interstellar
radiation field. The central area of the core appears darker than the edges in
Fig. 1 and the dark area is extended roughly in north-south direction,
consistent with a large thick disk seen edge-on. This same phenomenon has been
described as a “shadow” minimum in the extended flux at 3.6 $\mu$m and 8.0
$\mu$m approximately 10″ to the south of the protostar by Stutz et al. (2008)
based on $Spitzer$ images. The extended flux minimum is clearly associated
with the nearly complete absence of background stars seen in this area of the
cloud. This qualitatively confirms the interpretation by Stutz et al. (2008)
that this depression in the extended emission is due to the flattened
structure of the B335 molecular core. A detailed mapping of the extinction in
B335 was the original purpose of our JWST observing program and will be
published separately from this paper on the outflow.

###  3.2 Position of the embedded protostar

The protostar in B335 has been observed at radio frequencies numerous times,
at different wavelengths and with basically every suitable telescope system
available. Since there is only one protostellar source in B335, the centroid
of the continuum flux distribution measures the position of the protostellar
object irrespective of wavelength. For the purpose of determining the proper
motion of the protostar, only interferometric measurements have sufficient
spatial resolution and precision of the position. In the lower panel of Fig.
4, we have indicated the position at 3.6 cm wavelength reported by Reipurth et
al. (2002) and measured in 2001. More recently, the best position measurements
are from ALMA at 1.2 or 1.3 mm, and we have included the measurements from
Maury et al. (2018) with epoch 2015, Imai et al. (2019) with epoch 2017 and
that of Okoda et al. (2022) with epoch 2019 in Fig. 4. For clarity, it should
be mentioned that these radio positions are measured against the celestial
reference coordinate system, and not relative to the B335 molecular cloud. The
protostar has always been located at the center of the bipolar nebula and
plotting those older radio positions on a contemporary image does not suggest
otherwise. Rather, the whole bipolar nebula, and likely, the whole B335
globule, has the same proper motion as the protostar radio source.

Figure 4: The top three panels are $90\arcsec\times 25\arcsec$ cutouts of the
F444W image in the canonical orientation with North up and East left. The top
three panels are different flux scalings to show all the symmetric shock front
despite the difference in extinction towards the two outflow lobes. The bottom
panel is zoomed by a factor 16 into the central region near the protostar. The
faint, innermost features in the eastern outflow lobe are identified. We have
also indicated the radio positions reported by Reipurth et al. (2002) with
epoch 2001, Maury et al. (2018) with epoch 2015, Imai et al. (2019) with epoch
2017 and that of Okoda et al. (2022) with epoch 2019. From those, we have
extrapolated the position of the protostar at the epoch of our JWST NIRcam
image, which is indicated by the red box in the bottom panel.

The proper motion of the B335 protostar is 19 mas yr-1 with an estimated
uncertainty of $\pm$ 1 mas yr-1 based on the positions by Reipurth et al.
(2002) and Okoda et al. (2022). Extrapolating from the Reipurth et al. (2002)
and Okoda et al. (2022) measurements, we get the extrapolated position of the
protostar at the epoch of the JWST imaging observations as 19 37 0.9025 +7 34
09.42 (J2000.0) with an uncertainty of $\approx$ 20 mas and have indicated
this by the red box in Fig. 4. This position places the protostar not in the
center of the high-extinction region separating the two outflow lobes, but
close to its eastern edge, consistent with the eastern lobe being oriented
slightly towards the observer, as was already noted by Hirano et al. (1988)
and discussed in the subsection above. The protostar is located close to what
appears to be the first emission knot in the string of shock fronts, and that
we therefore label HH119 JWST 0E. At present, we do not have spectroscopic
data to prove that this is indeed a shock front, rather than scattered
continuum light.

###  3.3 Shock proper motion

Numerous outflow shock fronts are found within the outflow cavity by Vrba et
al. (1986), Reipurth et al. (1992), Hodapp (1998), and Gâlfalk & Olofsson
(2007). The newly discovered shock fronts described here are named HH 119 JWST
with a numbering scheme acknowledging the symmetric nature of these shocks:
for example, the closest of the newly discovered shock fronts east of the
protostar is HH 119 JWST 0E, labelled 0E in Fig. 4. In this paper, in
particular for labels in figures, we will abbreviate the shock front names to
just the last two characters, e.g., 0E. Of these shock fronts, only HH 119
JWST 6E has previously been labelled as IR2. Figure 1 shows three large shock
fronts in the eastern half of the bipolar outflow and outside of the more
detailed view in Fig. 4: IR3, 4 and 5 in the nomenclature of Hodapp (1998).

The proper motion measurements of Gâlfalk & Olofsson (2007) showed that shock
front IR2, the only one of the inner shock fronts that was found by ground-
based observations (Hodapp, 1998), and D = IR5, the large thin bow shock
front, have large proper motions around 200 mas yr-1. Both are located on the
central axis of the outflow cavity, defined by a string of smaller shock
fronts closer to the protostar, shown in Fig. 4. We will refer to these shocks
as axial shocks.

In contrast, IR3 and IR4a are located farther away from that central axis and
have an order of magnitude smaller proper motions of around 20 mas yr-1. We
will refer to these and similar shocks as peripheral shocks. This distinction
is also seen in the morphology of these shocks: The axial shock IR5, in
particular, is clearly a large shock front of the outflow from the protostar
into ambient gas, with the bow of the shock front pointing away from the
protostar. In contrast, the bows of the peripheral shock fronts IR3 and IR4a
appear to point towards the protostar, indicating a shock of the outflow on a
stationary denser clump of material, with a wake of outflow gas flowing around
the obstacle. This morphology and their location away from the outflow axis
suggest that IR3 and IR4a may be shocks formed by the interaction of the
outflow with the cavity walls.

To generalize this finding: High proper motion shock fronts are found mostly
along a fairly narrow jet near the center of the outflow cavity, while low
proper motion shock fronts are less concentrated and may be formed by
interaction of the outflow with the cavity walls.

To illustrate the proper motion of shock fronts, we compare the JWST results
with the $K$-band image of B335 obtained by Hodapp (1998) on 1996 Oct. 1 (MJD
= 50357 ) with the Keck I NIRC camera. This image is the deepest ground-based
image of B335 and it shows part of the reflection nebulosity in the eastern
outflow lobe, and, in particular, the shock front named IR2. Fig. 5 shows
cutouts of the 2023 JWST F277W image and the 1996 Keck NIRC $K$-band image.
The images were aligned to optimally match the background stars within the
limits imposed by their random proper motions. The rms shift of individual
stars on the 1996 Keck images relative to their position in the 2023 JWST
images was 221 mas or 8 mas yr-1.

The axial shock front 6E = IR2, shows a dramatic position shift and major
morphological changes. In contrast, the peripheral shock front IR 3 (lower row
in Fig. 5) that was just recorded in the south-eastern corner of the $K$-band
image, shows very little motion.

Figure 5: Comparison of shock position and morphology between 1996 (Keck NIRC
$K$-band image) and 2023 (JWST NIRCam F277W image). The top row shows the 1996
K-band image of the inner shock fronts on the left, the 2023 F277W image in
the center, and the difference on the right. Shock front 6E has moved
substantially and has changed shape from two distinct shocks to one merged
shock. The lower row shows the same for shock front IR 3 that shows minimal
morphological change and only a very small (compared to 6E) change in
position. The brightest star (anonymous) in this cutout shows high proper
motion.

The shock front 6E = IR2 was measured by Gâlfalk & Olofsson (2007) to be 183
mas yr-1. We essentially confirm this proper motion with a measurement of the
proper motion from the epoch 1996 Keck image of Hodapp (1998) and our JWST
F277W image, the closest equivalent to the $K$-band in our data set. Our
measurement of the proper motion is 166 mas yr-1 or 129.4 km s-1 transverse
velocity. The accuracy of this value is limited by the morphological changes
that this shock front has undergone between the two images, making a clear
association of features in the two images impossible. Whereas the 1996 image
(Fig. 5 top left panel) shows two distinct shock fronts closely together along
the outflow axis, the younger of those shock fronts has apparently caught up
with the older one and by 2023, they formed one merged shock. From its proper
motion, we derive the kinematic age of shock front 6E as 27784 d = 76 yrs and
the ejection year at 1947. Due to the morphological changes, we estimate the
accuracy of the proper motion and the kinematic age to be $\pm$10% and the
accuracy of the ejection data as $\pm$8 yrs.

Without second epoch data, we do not know the proper motion of the other axial
shock fronts closer to the protostar that form a string of individual shocks
tracing the central axis of the flow. Assuming, for lack of better
information, that their proper motion is the same as that of 6E = IR2, their
distance from the extrapolated present position of the protostar leads to the
shock front ages and ejection dates listed in Table 2. The ages of those axial
shocks close to the protostar are only of order of decades, similar to what
was found in adaptive optics studies of other protostellar outflows, e.g., SVS
13 (Hodapp & Chini, 2014), and MHO3252Y3 (Hodapp & Chini, 2018). We compare
our result to those of Evans et al. (2023) who interpret the start of the W2
light curve outburst as the time of launch of a new molecular “bullet”. This
point in time closely matches our estimate for the launch time of the rather
dim shock 2E, but the light curve does not show a similar maximum coinciding
with the launch of the much brighter shock 3E.

We have tentatively included the feature 0E in this list, assuming that this
is the youngest shock front that is just emerging at present. Since this
object is so faint and does not have much contrast with the surrounding
emission, we could not obtain a spectrum of it. However, Rubinstein et al.
(2023) have published a JWST NIRSpec IFU spectrum of this feature, which will
be discussed in section 3.4. We expect that with a second epoch NIRCam image
expected in 2024, the other axial shocks close to the protostar should also
have measurable proper motions.

Table 2: B335 Shock Fronts

Identification | RA | Dec | Distance | Ejection Age [d] | Ejection JD | Ejection Date
---|---|---|---|---|---|---
HH119 JWST 0E | 294.25380 | 7.56928 | 0.12 | 265 | 2459794 | 2022 Aug.
HH119 JWST 1E | 294.25397 | 7.56927 | 0.75 | 1654 | 2458405 | 2018 Oct.
HH119 JWST 2E | 294.25414 | 7.56927 | 1.34 | 2954 | 2457105 | 2015 Mar.
HH119 JWST 3E | 294.25434 | 7.56928 | 2.08 | 4587 | 2455472 | 2010
HH119 JWST 3W | 294.25307 | 7.56921 |  |  |  |
HH119 JWST 4E | 294.25550 | 7.56930 | 4.65 | 10254 | 2449805 | 1995
HH119 JWST 4W | 294.25222 | 7.56904 |  |  |  |
HH119 JWST 5E | 294.25667 | 7.56887 | 10.5 | 23153 | 2436906 | 1959
HH119 JWST 6E | 294.25723 | 7.56897 | 12.6 | 27784 | 2432275 | 1947
HH119 JWST 6W | 294.24943 | 7.56784 |  |  |  |
HH119 JWST 7E | 294.26113 | 7.56870 | 27.0 | 59537 | 2400522 | 1860
HH119 JWST 8E | 294.26367 | 7.56848 | 35.6 | 78501 | 2381558 | 1808

Note. — The distance in arcsec is measured from the extrapolated position of
the protostar. The ejection ages assume that all shock fronts listed here have
the same proper motion as 6E

###  3.4 Emission from the youngest shock fronts

The marginally resolved 13 year old bright emission knot labelled 3E in Fig. 4
is indeed a shock front. One NIRCam long-wave module pixel subtends $\approx$
10 AU at the distance of B335, so this is the approximate linear size of the
marginally resolved knots. The axial shock front 3E is bright enough relative
to the surrounding emission in the eastern outflow lobe that in the F460M WFSS
frame in Fig. 2, it shows a series of emission lines that are identified as
ro-vibrational transitions of CO in the extracted spectrum in Fig. 6 (top
panel). CO bandhead emission is frequently seen in the spectra of protostars
and young star themselves, a recent example being the work on the B335
protostar and its disk, seen in scattered light in the innermost parts of the
outflow cavity by Rubinstein et al. (2023). Their NIRSpec IFU pointing
contained object 0E in our nomenclature, which they name B335-E, and the
reflection nebulosity immediately west of the protostar that they call B335-W
and we have not individually labelled. They detect strong emission from the CO
ro-vibrational transitions in the range from 4.4 - 5.2 $\mu$m. While
Rubinstein et al. (2023) work under the assumption that this emission is
scattered light from the protostar and its disk, their result leaves the
question open whether 0E (their B335-E) is actually an emerging shock front.
Recently, JWST observations of HH211 by Ray et al. (2023) have shown CO
emission around 4.6 $\mu$m not only from scattered light from the immediate
vicinity of the protostar, but also in the bow shocks of that outflow.

Figure 6:  Extracted WFSS spectra of the three brightest (3E, 4E, 6E) of the
young axial shock fronts in the eastern lobe of the B335 outflow. For
orientation, small postage stamp cutouts of the direct image of each of the
three shock fronts are included with North up and East left. The WFSS spectra
extracted in each filter are color-coded: F410M (blue), F430M (green), F460M
(orange), and F480M (red). All spectra shown here were divided by the
transmission profile of filter, and spectral regions recorded with less than
10% filter transmission were excluded. The spectra of shock regions 3E and 4E
are sky-subtracted. For the most extended of these three shock fronts, 6E, we
did not subtract the signal from nearby sky positions, since there was no
”sky” without H2 emission available. Also, this spectrum records the H2
emission of the fainter shock region 5E that, due to the degeneracy between
spatial and spectral information in slitless spectroscopy data, are recorded
at the wrong, longer wavelength.

The bright, extended scattered light in the eastern outflow lobe shows deep
absorption in the WFSS frame through the F430M filter in Fig. 2. This
absorption feature, broadened by the extended flux distribution of the
scattered light in the outflow cavity, is indicative of absorption by CO2 ice
in the B335 molecular core. Our WFSS spectroscopy of background stars behind
B335, the original purpose of our program 1187, shows saturated CO2 ice
absorption near the center of the B335 core, which will be published in a
separate paper.

The next of the inner axial shock fronts is the more extended 28-year-old 4E
that has a clear bow shock morphology. We show the extracted spectrum in Fig.
6 (middle panel). The emission from CO is clearly detected, and the emission
lines of H2 are more prominent relative to CO than in the case of the younger,
more compact 3E. The third spectrum, the bottom panel in Fig. 6, is the
extracted spectrum of the 76-year-old axial shock 6E and is dominated by the
two H2 emission lines that fall in the transmission bands of the filters used
here. We do not detect CO emission in this shock. Similarly, the more distant
older shock fronts are emitting in the 0-0 S(9) (4.6946 $\mu$m) and S(11)
(4.1811 $\mu$m) lines of H2. The H2 0-0 S(10) line is just outside the
bandpass of filter F430M and therefore not recorded.

Assuming that the morphology, extent, and emission line characteristics of
these three shocks represent a pattern, we conclude that typical shocks emerge
from the protostar as fairly small ($\approx$ 10 AU) clumps of material,
expand over the course of a few decades, and change their density and
temperature so that CO ro-vibrational lines are no longer excited. In the
recent past, the B335 protostar appears to have undergone velocity changes in
its molecular jet, leading to the production of internal shock fronts every 3
to 5 years. The apparently much higher frequency of shock ejection in recent
decades compared to the larger separation and therefore lower shock frequency
in the older shocks. This can be understood if shock fronts are initially
ejected at a higher than average velocity and over time catch up with slower
moving outflow material ahead of them. This scenario of individual internal
work surfaces in a jet propagating into a dense medium was developed and
modeled by Raga et al. (1990), applied to HH47, and further developed for jets
of non-uniform density by Raga & Kofman (1992), applied to the case of HH34.
Their scenario is similar to what we actually observed for shock front 6E,
where two closely spaced shock fronts appear to have merged over the course of
the past 27 years. We explain the fact that only a few strong shocks remain
detectable after centuries by the combined effect of shock fronts expanding
sideways and of shock fronts merging.

##  4 Summary and Conclusions

While many of the more distant and less obscured shock fronts have been
studied before, the deep JWST images in the NIRCam long wavelength channel
presented here show the full bipolar nebula associated with the B335
protostar. With much higher spatial resolution than older $Spitzer$ images,
symmetric sets of shock fronts were found in the outflow lobes that indicate
the repeated ejection of fast-moving gas whose differential velocities lead to
the formation of these internal, relatively low excitation shock fronts. The
kinematic age of the closest shock fronts from the protostar is of the order
of a few years, if we assume the same proper motion as the closest shock with
measured proper motion (6E). The youngest shock front with a spectrum (3E) is
dominated by emission lines of gaseous CO. The next older shock front (4E)
still shows CO emission, but with diminished strength, while more distant and
older shock fronts (6E) do not show CO, but are dominated by H2 emission
lines. The string of shock fronts emerging from the B335 protostar is
indicative of repeated ejection events, probably due to some form of accretion
episodicity. Among the older shock fronts at larger distances from the
protostar, we note a distinction into two classes of proper motion, based on
the measurements by Gâlfalk & Olofsson (2007). The low proper motion
peripheral shocks in our images have their bow shocks pointing towards the
protostar, indicative of shocks on a nearly stationary clump of denser
material where the outflowing gas flows around the obstacle. By contrast, all
high proper motion axial shock fronts have their bow shocks pointing away from
the protostar, indicating shocks of the outflow wind against near stationary
and uniform ambient gas. For the shock front 6E, we document the apparent
merger of two distinct shock front seen in 1996 into one shock front in 2023.

This project was supported by NASA through the JWST /NIRCam project, contract
number NAS5-02105 (M. Rieke, University of Arizona, PI). The data were
obtained under GTO program 1187, downloaded from the Mikulski Archive for
Space Telescopes (MAST) at the Space Telescope Science Institute and are
available at MAST: 10.17909/qa3f-5703 (catalog 10.17909/qa3f-5703). The first
epoch image of B335 was obtained at the W. M. Keck Observatory, which is
operated as a scientific partnership among the California Institute of
Technology, the University of California and NASA. The Observatory was made
possible by the generous financial support of the W. M. Keck Foundation. This
work is based in part on observations made with the Spitzer Space Telescope,
which was operated by the Jet Propulsion Laboratory, California Institute of
Technology under a contract with NASA. D.J. is supported by NRC Canada and by
an NSERC Discovery Grant. We thank Bo Reipurth for helpful discussions and
comments and Thomas Dutkiewicz for help with the mining of the DOIs. We thank
the referee for constructive comments that helped improve the paper.

## References

  * Bally (2016) Bally, J. 2016, ARA&A, 54, 491. doi:10.1146/annurev-astro-081915-023341
  * Barnard et al. (1927) Barnard, E. E., Frost, E. B., & Calvert, M. R. 1927, [Washington] Carnegie institution of Washington, 1927.
  * Bjerkeli et al. (2023) Bjerkeli, P., Ramsey, J. P., Harsono, D., et al. 2023, A&A, 677, A62. doi:10.1051/0004-6361/202245195
  * Bjerkeli et al. (2019) Bjerkeli, P., Ramsey, J. P., Harsono, D., et al. 2019, A&A, 631, A64. doi:10.1051/0004-6361/201935948
  * Boogert et al. (2015) Boogert, A. C. A., Gerakines, P. A., & Whittet, D. C. B. 2015, ARA&A, 53, 541. doi:10.1146/annurev-astro-082214-122348
  * Chandler et al. (1990) Chandler, C. J., Gear, W. K., Sandell, G., et al. 1990, MNRAS, 243, 330
  * Chu & Hodapp (2021) Chu, L. E. U. & Hodapp, K. W. 2021, ApJ, 918, 2. doi:10.3847/1538-4357/ac0ae8
  * Connelley et al. (2009) Connelley, M. S., Hodapp, K. W., & Fuller, G. A. 2009, AJ, 137, 3494. doi:10.1088/0004-6256/137/3/3494
  * Evans et al. (2015) Evans, N. J., Di Francesco, J., Lee, J.-E., et al. 2015, ApJ, 814, 22. doi:10.1088/0004-637X/814/1/22
  * Evans et al. (2023) Evans, N. J., Yang, Y.-L., Green, J. D., et al. 2023, ApJ, 943, 90. doi:10.3847/1538-4357/acaa38
  * Fazio et al. (2004) Fazio, G. G., Hora, J. L., Allen, L. E., et al. 2004, ApJS, 154, 10. doi:10.1086/422843
  * Frerking & Langer (1982) Frerking, M. A. & Langer, W. D. 1982, ApJ, 256, 523. doi:10.1086/159928
  * Gâlfalk & Olofsson (2007) Gâlfalk, M. & Olofsson, G. 2007, A&A, 475, 281. doi:10.1051/0004-6361:20077889
  * Gardner et al. (2023) Gardner, J. P., Mather, J. C., Abbott, R., et al. 2023, PASP, 135, 068001. doi:10.1088/1538-3873/acd1b5
  * Greene et al. (2017) Greene, T. P., Kelly, D. M., Stansberry, J., et al. 2017, Journal of Astronomical Telescopes, Instruments, and Systems, 3, 035001. doi:10.1117/1.JATIS.3.3.035001
  * Hirano et al. (1988) Hirano, N., Kameya, O., Nakayama, M., et al. 1988, ApJ, 327, L69. doi:10.1086/185142
  * Hodapp (1987) Hodapp, K.-W. 1987, ApJ, 319, 842. doi:10.1086/165502
  * Hodapp (1998) Hodapp, K.-W. 1998, ApJ, 500, L183. doi:10.1086/311412
  * Hodapp & Bressert (2009) Hodapp, K. W. & Bressert, E. 2009, AJ, 137, 3501. doi:10.1088/0004-6256/137/3/3501
  * Hodapp & Chini (2014) Hodapp, K. W. & Chini, R. 2014, ApJ, 794, 169. doi:10.1088/0004-637X/794/2/169
  * Hodapp & Chini (2018) Hodapp, K. W. & Chini, R. 2018, ApJ, 864, 172. doi:10.3847/1538-4357/aad636
  * Hubble (1916) Hubble, E. P. 1916, ApJ, 44, 190. doi:10.1086/142284
  * Imai et al. (2019) Imai, M., Oya, Y., Sakai, N., et al. 2019, ApJ, 873, L21. doi:10.3847/2041-8213/ab0c20
  * Keene et al. (1983) Keene, J., Davidson, J. A., Harper, D. A., et al. 1983, ApJ, 274, L43. doi:10.1086/184147
  * Kim et al. (2023) Kim, C.-H., Lee, J.-E., Contreras Peña, C., et al. 2023, arXiv:2312.05781. doi:10.48550/arXiv.2312.05781
  * Maury et al. (2018) Maury, A. J., Girart, J. M., Zhang, Q., et al. 2018, MNRAS, 477, 2760. doi:10.1093/mnras/sty574
  * Okoda et al. (2022) Okoda, Y., Oya, Y., Imai, M., et al. 2022, ApJ, 935, 136. doi:10.3847/1538-4357/ac7ff4
  * Olofsson & Olofsson (2009) Olofsson, S. & Olofsson, G. 2009, A&A, 498, 455. doi:10.1051/0004-6361/200811574
  * Raga et al. (1990) Raga, A. C., Canto, J., Binette, L., et al. 1990, ApJ, 364, 601. doi:10.1086/169443
  * Raga & Kofman (1992) Raga, A. C. & Kofman, L. 1992, ApJ, 386, 222. doi:10.1086/171008
  * Ray et al. (2023) Ray, T. P., McCaughrean, M. J., Caratti o Garatti, A., et al. 2023, Nature, 622, 48. doi:10.1038/s41586-023-06551-1
  * Reipurth (1983) Reipurth, B. 1983, A&A, 117, 183
  * Reipurth et al. (1992) Reipurth, B., Heathcote, S., & Vrba, F. 1992, A&A, 256, 225
  * Reipurth et al. (2002) Reipurth, B., Rodríguez, L. F., Anglada, G., et al. 2002, AJ, 124, 1045. doi:10.1086/341172
  * Rieke et al. (2023) Rieke, M. J., Kelly, D. M., Misselt, K., et al. 2023, PASP, 135, 028001. doi:10.1088/1538-3873/acac53
  * Rubinstein et al. (2023) Rubinstein, A. E., Tyagi, H., Nazari, P., et al. 2023, arXiv:2312.07807. doi:10.48550/arXiv.2312.07807
  * Steinacker et al. (2010) Steinacker, J., Pagani, L., Bacmann, A., et al. 2010, A&A, 511, A9. doi:10.1051/0004-6361/200912835
  * Stutz et al. (2008) Stutz, A. M., Rubin, M., Werner, M. W., et al. 2008, ApJ, 687, 389. doi:10.1086/591789
  * Tomita et al. (1979) Tomita, Y., Saito, T., & Ohtani, H. 1979, PASJ, 31, 407
  * Vrba et al. (1986) Vrba, F. J., Luginbuhl, C. B., Strom, S. E., et al. 1986, AJ, 92, 633. doi:10.1086/114194
  * Watson (2020) Watson, D. M. 2020, Research Notes of the American Astronomical Society, 4, 88. doi:10.3847/2515-5172/ab9df4
  * Wright et al. (2010) Wright, E. L., Eisenhardt, P. R. M., Mainzer, A. K. et al., AJ, 140, 1868