# Early Planet Formation in Embedded Disks (eDisk) X:
Compact Disks, Extended Infall, and a Fossil Outburst in the Class I Oph IRS43
Binary

Suchitra Narayanan National Science Foundation Graduate Research Fellow
Institute for Astronomy, University of Hawai‘i at Mānoa, 2680 Woodlawn Dr.,
Honolulu, HI 96822, USA  Jonathan P. Williams Institute for Astronomy,
University of Hawai‘i at Mānoa, 2680 Woodlawn Dr., Honolulu, HI 96822, USA
John J. Tobin National Radio Astronomy Observatory, 520 Edgemont Rd.,
Charlottesville, VA 22903, USA  Jes K. Jørgensen Niels Bohr Institute,
University of Copenhagen, Øster Voldgade 5-7, 1350, Copenhagen K, Denmark
Nagayoshi Ohashi Academia Sinica Institute of Astronomy & Astrophysics, 11F of
Astronomy-Mathematics Building, AS/NTU, No.1, Sec. 4, Roosevelt Rd, Taipei
10617, Taiwan, R.O.C.  Zhe-Yu Daniel Lin University of Virginia, 530 McCormick
Rd., Charlottesville, Virginia 22904, USA  Merel L.R. van ’t Hoff Department
of Astronomy, University of Michigan, 1085 S. University Ave., Ann Arbor, MI
48109-1107, USA  Zhi-Yun Li University of Virginia, 530 McCormick Rd.,
Charlottesville, Virginia 22904, USA  Adele L. Plunkett National Radio
Astronomy Observatory, 520 Edgemont Rd., Charlottesville, VA 22903, USA
Leslie W. Looney Department of Astronomy, University of Illinois, 1002 West
Green St, Urbana, IL 61801, USA  Shigehisa Takakuwa Department of Physics and
Astronomy, Graduate School of Science and Engineering, Kagoshima University,
1-21-35 Korimoto, Kagoshima,Kagoshima 890-0065, Japan  Academia Sinica
Institute of Astronomy & Astrophysics, 11F of Astronomy-Mathematics Building,
AS/NTU, No.1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.  Hsi-Wei Yen
Academia Sinica Institute of Astronomy & Astrophysics, 11F of Astronomy-
Mathematics Building, AS/NTU, No.1, Sec. 4, Roosevelt Rd, Taipei 10617,
Taiwan, R.O.C.  Yusuke Aso Korea Astronomy and Space Science Institute, 776
Daedeok-daero, Yuseong-gu, Daejeon 34055, Republic of Korea  Christian Flores
Academia Sinica Institute of Astronomy & Astrophysics, 11F of Astronomy-
Mathematics Building, AS/NTU, No.1, Sec. 4, Roosevelt Rd, Taipei 10617,
Taiwan, R.O.C.  Jeong-Eun Lee Department of Physics and Astronomy, Seoul
National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea  Shih-Ping Lai
Institute of Astronomy, National Tsing Hua University, No. 101, Section 2,
Kuang-Fu Road, Hsinchu 30013, Taiwan  Center for Informatics and Computation
in Astronomy, National Tsing Hua University, No. 101, Section 2, Kuang-Fu
Road, Hsinchu 30013, Taiwan  Department of Physics, National Tsing Hua
University, No. 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan  Academia
Sinica Institute of Astronomy & Astrophysics, 11F of Astronomy-Mathematics
Building, AS/NTU, No.1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.
Woojin Kwon Department of Earth Science Education, Seoul National University,
1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea  SNU Astronomy Research
Center, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826,
Republic of Korea  Itziar de Gregorio-Monsalvo European Southern Observatory,
Alonso de Cordova 3107, Casilla 19, Vitacura, Santiago, Chile  Rajeeb Sharma
Niels Bohr Institute, University of Copenhagen, Øster Voldgade 5-7, 1350,
Copenhagen K, Denmark  Chang Won Lee Korea Astronomy and Space Science
Institute, 776 Daedeok-daero, Yuseong-gu, Daejeon 34055, Republic of Korea
Division of Astronomy and Space Science, University of Science and Technology,
217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea

###### Abstract

We present the first results from the Early Planet Formation in Embedded Disks
(eDisk) ALMA Large Program toward Oph IRS43, a binary system of solar mass
protostars. The 1.3 mm dust continuum observations resolve a compact disk,
$\sim 6$ au radius, around the northern component and show that the disk
around the southern component is even smaller, $\lesssim 3$ au. CO, 13CO, and
C18O maps reveal a large cavity in a low mass envelope that shows kinematic
signatures of rotation and infall extending out to $\sim 2000$ au. An
expanding CO bubble centered on the extrapolated location of the source $\sim
130$ years ago suggests a recent outburst. Despite the small size of the
disks, the overall picture is of a remarkably large and dynamically active
region.

Protostars; Circumstellar disks; Planet formation; Millimeter astronomy;
stars: individual— IRS43

††software: CASA (McMullin et al. 2007), astropy (The Astropy Collaboration
2013, 2018).

,

##  1 Introduction

More than half of pre-main sequence stars are binaries (Raghavan et al.,
2010). Such systems are thought to form through the fragmentation of dense
cores and/or massive disks, and the interaction between the young stars may
significantly affect their surroundings and evolution (Offner et al., 2010).
However, disks can persist around each individual star and/or the system and
ultimately turn into stable planetary systems (Thebault & Haghighipour, 2015).
The study of young binary systems is therefore important both for a more
complete picture of star and planet formation in general and also to extend
the range of physical conditions for testing models.

The focus of this paper is on the protostellar binary system, Oph IRS43 (also
known as YLW 15 and GY 265), that was observed as part of the Atacama Large
Millimeter Array (ALMA) Large Program, Early Planet Formation in Embedded
Disks (hereafter eDisk). This program, described in Ohashi et al. (2023),
observed 12 Class 0 and 7 Class I protostars at high spatial resolution
(0$.\\!\\!^{{}^{\prime\prime}}$04) with the primary goal of studying the
properties of their accompanying disks.

Oph IRS43, hereafter IRS43, is a Class I embedded protostellar binary located
in the L1688 region of the Ophiuchus molecular cloud complex. We adopt a
distance to the source of 137.3 pc based on Very Long Baseline Array (VLBA)
parallax measurements of 12 other (single) young stellar objects in L1688
(Ortiz-León et al., 2017). The combined bolometric luminosity and temperature
of the system are $L_{\rm bol}=4.15\,L_{\odot},~{}T_{\rm bol}=193$ K (see
Ohashi et al., 2023).

Girart et al. (2000) first showed that IRS43 was a
$0\hbox{$.\\!\\!^{{}^{\prime\prime}}$}6$ binary from centimeter wavelength
observations with the Very Large Array (VLA) carried out in 1989. We follow
their naming convention and designate the northern source, VLA1, and the
southern source, VLA2. The latter is much brighter in the near-infrared
(Duchêne et al., 2007) and spectroscopy shows that it is a heavily extincted,
$A_{\rm V}=40$ mag, cool star with a KIV/V spectral type corresponding to an
effective temperature $\sim 4300$ K. It has a strong continuum excess (veiling
= 3.0 at $2.2\,\mu$m) that indicates a high accretion rate, $\sim
10^{-6}\,M_{\odot}$ yr-1, and is rapidly rotating at a rate, $v\sin i\sim 50$
${\rm km~{}s^{-1}}$, that is typical of embedded protostars (Greene & Lada,
2002).

Subsequent VLA imaging over 12 years revealed a common proper motion of 24
milli-arcsecond yr-1and relative orbital motion of the binary (Curiel et al.,
2003). ALMA observations of the source were presented by Brinch et al. (2016)
who used the extended time baseline and archival VLA data to extend the
astrometry and determine an orbital solution. They found that the motions are
consistent with a circular orbit in or near the plane of the sky with a period
of 450 years and a total mass of $2.01\pm 0.47\,M_{\odot}$ with an equal mass
ratio, i.e. two solar mass stars. These astrometric constraints on the proper
motion and total mass are essential to our interpretation of the eDisk data in
§3.

The first millimeter interferometric observations of IRS43 were made using the
Submillimeter Array (SMA) by Jørgensen et al. (2009) and Brinch & Jørgensen
(2013), revealing that IRS43 has strong lines on top of a weak continuum with
an extended, flattened structure in HCO+ but the $1.7^{\prime\prime}\times
1.4^{\prime\prime}$ resolution was too low to resolve the binary. The Brinch
et al. (2016) ALMA observations were the highest resolution measurements of
this system to date, $0\hbox{$.\\!\\!^{{}^{\prime\prime}}$}2$, and clearly
separated the disks around each protostar but they remained unresolved with an
upper limit to their radius of $\sim 20$ au. However, the HCO+ and HCN line
data suggested that the larger scale flattened structure was rotating around
the binary and that the misalignment between the stellar orbits and
circumbinary material testified to a turbulent origin.

IRS43 is also a well-known X-ray source that undergoes energetic flares every
$\sim 20$ hr quasi-periodically due most likely to a strong star-disk
interface (Montmerle et al., 2000). It stands out for having undergone the
brightest “superflare” ever witnessed in T Tauri stars, when in 1995 its X-ray
luminosity peaked at $L_{\rm X}\sim 10-100\,L_{\odot}$ and outshone the entire
system at all other wavelengths for a couple of hours (Grosso et al., 1997).
Although the positional accuracy of the data was unable to determine whether
the source was VLA1 or VLA2, such a superflare must have been powered by a
massive accretion event and would have been accompanied by full ionization of
the surroundings within a few tenths of an au. Our eDisk observations suggest
a different, though perhaps related, outburst event occurred $\sim 130$ yr
ago.

The rest of the paper is organized as follows: §2.1 describes the ALMA
observations, data reduction and imaging procedure. §3 presents the results
separated into subsections focused on the 1.3 mm dust continuum, the molecular
line data showing the kinematics of the system, and an expanding bubble that
signposts a significant recent outburst. §4 discusses the implications of our
findings and §5 summarizes the paper.

##  2 Observations and Data Reduction

###  2.1 Observations

The ALMA observations used in this work were taken in Cycle 7 as part of
program 2019.1.00261.L (PI: N. Ohashi). Detailed information on the
configurations, spectral setup and targeted lines, and the other observed
targets are in Ohashi et al. (2023). In brief, the IRS43 observations were
carried out in 5 execution blocks (EB) between May and October 2021 with
between 41 and 46 antennas in a compact and extended configuration (C43-5 and
C43-8, respectively) with baselines extending over 15-11500 m. The total on-
source integration time was 170 minutes in a single Band 6 spectral setting.
Here we present results from the 234 GHz continuum and the CO, 13CO, C18O, SO,
and H2CO lines at 219-230 GHz.

###  2.2 Calibration

The data were calibrated using the Common Astronomy Software Applications
(CASA) package (McMullin et al., 2007) version 6.2.1. To ensure uniformity in
the eDisk data products and comparison of structures and kinematics across the
sample, the project team created a specific eDisk data reduction routine111All
of the data reduction (i.e., self-calibration and imaging) can be found at
http://github.com/jjtobin/edisk., that builds on the calibration strategy
developed for the Disk Substructures at High Angular Resolution Project
(DSHARP) ALMA Large Program (Andrews et al., 2018). That general reduction
procedure is described in Ohashi et al. (2023) but an additional step (the
“two-pass” method) was required for 5 of the 19 sources, including IRS43,
where some of the delivered data had high phase decorrelation which we
describe below.

####  2.2.1 Combining multiple datasets with high phase noise

Each execution block was first passed through the standard ALMA data reduction
pipeline (version 2021.2.0.128) to remove atmospheric and instrumental effects
using the quasar calibrators followed by self-calibration on the source itself
to increase the signal-to-noise ratio. The top panel of Figure 1 shows the
real part of the source visibility amplitudes for each dataset across a range
of overlapping baseline lengths. There is some scatter due to time variability
in the calibrators and/or source. Notably, there is a significant loss of flux
at long baselines in the first short-baseline track (SB1) due to decorrelation
caused by high phase noise. The middle panel of Figure 1 shows what the
visibility amplitudes would be had we followed the standard eDisk reduction
procedure where the fluxes are normalized to a common scale first and then
self-calibrated (see Ohashi et al. (2023)). In the “two-pass” method, however,
we first run the self-calibration to completion without scaling the fluxes. We
then determined which EB had the the highest quality data based on the
smoothness of the visibility-amplitude profiles and quality of the images made
on a per-EB basis, and selected this to be the reference EB. The reference EB
showed the least amount of decorrelation and lowest level of imaging artifacts
due to phase noise. For IRS43, this was the third long-baseline (LB3). We
subsequently rescaled the other EBs to the same average amplitude for
$uv-$distances out to $800\,{\rm k}\lambda$, and re-ran the data reduction
script from the start with the pre-determined scaling factors. The result of
this second pass through the calibration process, shown in the bottom panel of
Figure 1, has a much more uniform flux profile and reduced scatter across all
datasets and baselines (consistent with a $\sim 10$% absolute flux calibraiton
uncertainty) which enables a higher quality image to be produced from their
combination.

Figure 1: Top: Visibility amplitudes as a function of uv-distance after the
standard pipeline calibration. Note that short-baseline 1 (SB1) decreases
anomalously due to loss of coherence on long baselines. Middle: Same as top
panel but after the standard self-calibration procedure (for detailed
description see Ohashi et al., 2023). Bottom: Same as top panel but after
joint self-calibration using the two-pass method. The procedure is described
in Section 2.2.1. For these data, the long-baseline 3 (LB3) was chosen as the
reference, or best, dataset to which all the other datasets were scaled.

###  2.3 Imaging

Due to the range of spatial scales and surface brightness of the various
features, we imaged the data using different baseline ranges and visibility
weightings. To provide an overall view and show the faint, extended continuum
emission, we used the short-baseline data only (hereafter SB) and natural
weighting (Briggs ${\tt robust}=2$) with a correspondingly relatively large
beam size $0\hbox{$.\\!\\!^{{}^{\prime\prime}}$}31\times
0\hbox{$.\\!\\!^{{}^{\prime\prime}}$}24$ and low continuum RMS noise levels,
0.015 mJy beam-1. For the highest resolution view of the disks, we combined
the short- and long-baseline data (hereafter SB+LB) and weighted more toward
the longer baselines (Briggs weighting ${\tt robust}=-1$ or $-0.5$) to achieve
much smaller beam sizes $\sim 0\hbox{$.\\!\\!^{{}^{\prime\prime}}$}04$ but at
the expense of higher continuum RMS $\sim 0.05$ mJy beam-1. The line maps were
all produced from the SB+LB data using a $2000\,{\rm k}\lambda$ taper and
intermediate weighting (Briggs ${\tt robust}=0.5$) with typical beam sizes
$\sim 0\hbox{$.\\!\\!^{{}^{\prime\prime}}$}15$ and RMS levels $\sim 1.5$ mJy
beam-1 ${\rm km~{}s^{-1}}$. For the weaker lines, we smoothed the data to a
beam size of $0\hbox{$.\\!\\!^{{}^{\prime\prime}}$}25$.

##  3 Results

###  3.1 Continuum data

Figure 2: 1.3 mm continuum image using only the short baseline data with a log
stretch to emphasize the faint, extended emission around the IRS43 binary. The
coordinates are arcsecond offsets from the position of VLA1 and the
synthesized beam on the bottom left corresponds to
0$.\\!\\!^{{}^{\prime\prime}}$31 $\times$ 0$.\\!\\!^{{}^{\prime\prime}}$24.

####  3.1.1 Overall view

A map of the large scale 1.3 mm continuum emission toward the region,
optimized to highlight low surface brightness features more than high
resolution, is shown in Figure 2. There are 3 compact sources corresponding to
disks around each member of the IRS43 binary and also the infrared source, GY
263. We present higher resolution images of these in the following subsection.
There is also an elongated, slightly curved structure that extends $\sim
7^{\prime\prime}\approx 10^{3}$ au on either side of the binary. The line
data, presented in §3.2, reveal this to be a rotating, infalling envelope. The
flux density of this extended continuum emission (without the disks) is 36
mJy, which corresponds to a total dust mass of only $10\,M_{\oplus}$ at a
temperature of 30 K derived from H2CO line ratios. Assuming an ISM gas-to-dust
ratio of 100, the implied total mass is $3\times 10^{-3}\,M_{\odot}$. Note,
however, that the envelope size is greater than the maximum recoverable size
of the observations, $\sim 3^{\prime\prime}$, so this envelope mass
measurement should be considered a lower limit.

####  3.1.2 Circumstellar disks

Including the long-baseline data and using different weighting mechanisms, we
can zoom into each of the three disks. Figure 3 shows the two disks in the
IRS43 binary from the SB+LB data weighted heavily to long baselines (Briggs
${\tt robust}=-1$). The disks are extremely compact. Their positions and sizes
were determined from Gaussian fits in the image plane using the CASA imfit
routine and listed in Table 3. VLA1 is resolved in both major and minor axes,
with a deconvolved FWHM size of 12 au, and, assuming a flat disk, has an
inclination inferred from the arc-cosine of their ratio equal to $78\arcdeg$.
VLA2 is just barely resolved along the beam minor axis with a remarkably small
deconvolved FWHM size of 2.3 au. The source is faint so we checked different
weighting schemes and found that it was detected at slightly higher signal-to-
noise but also resolved in a slightly larger beam with a similar deconvolved
size (2.7 au) with Briggs ${\tt robust}=-0.5$. To our knowledge, this is the
smallest resolved protostellar disk in the ALMA literature to date.

The flux densities are 10.6 mJy and 1.1 mJy for VLA1 and VLA2, respectively.
Using the simplest assumptions of optically thin, isothermal emission, these
convert to dust masses $3.6\,M_{\oplus}$ and $0.38\,M_{\oplus}$ at a
temperature $T_{\rm dust}=30$ K appropriate for Class I sources (Tobin et al.,
2015) and a dust opacity $\kappa_{\nu}=2.3\,{\rm cm^{2}\,g^{-1}}$ (Beckwith et
al., 1990). These are likely lower limits, especially for VLA1, since the high
brightness temperature indicates that the emission is in fact optically thick.

Figure 3: High resolution 1.3 mm continuum image of the compact dusty disks around each member of the IRS43 binary. This was produced by strongly weighting the long baseline visibilities and is shown on an Asinh intensity scale. The beam size is $0\hbox{$.\\!\\!^{{}^{\prime\prime}}$}046\times 0\hbox{$.\\!\\!^{{}^{\prime\prime}}$}030$ ($\sim 6\,{\rm au}\times 4$ au). Table 3: Gaussian fits to the continuum sources Source | ICRS R.A. | ICRS Dec. | Deconvolved FWHM, PA | Peak $I_{\nu},T_{\rm b}$ | $F_{\nu}$ | incl | Dust Mass
---|---|---|---|---|---|---|---
| [h m s] | [d m s] | [mas,°] | [mJy beam-1, K] | [mJy] | [°] | [$M_{\oplus}$]
VLA1 | 16:27:26.906 |  $-$24:40:50.81 | 89$\times$18, 133.5 $\pm$ 1.1 | 3.30, 62 | 10.62 | 78 | 3.4
VLA2 | 16:27:26.911 |  $-$24:40:51.40 | 19$\times$16, 62.5 $\pm$ 6.3 | 0.88, 20 | 1.08 | 32 | 0.34

The third disk lies around the known Class II source, GY 263 (Allen et al.,
2002), 6$.\\!\\!^{{}^{\prime\prime}}$6 (900 au) to the North-West of IRS43.
Figure 4 shows a high resolution image from the SB+LB data with Briggs ${\tt
robust}=-0.5$. Although this disk was not the target of the eDisk program, it
is nonetheless interesting in its own right as we serendipitously find a
central hole. The disk flux of 13.1 mJy implies a dust mass,
$4.5\,M_{\oplus}$, and the ring has a radius of
$0\hbox{$.\\!\\!^{{}^{\prime\prime}}$}17=23$ au with an inclination of
$24^{\circ}$ at a position angle of $130^{\circ}$. This tells us that GY 263
is a small transition disk (e.g., van der Marel et al., 2022) while also
demonstrating both the ability of the eDisk data to image small scale disk
features and the clear visual differences with the two embedded disks.
Nevertheless, given the large projected distance between GY 263 and IRS43, and
because we do not see any signs of interaction in the line data or in archival
datasets, we consider the sources to be essentially physically independent and
do not discuss GY 263 further here.

Figure 4: Zooming in on the continuum image toward the infrared source GY 263
with a beam size of $0\hbox{$.\\!\\!^{{}^{\prime\prime}}$}056\times
0\hbox{$.\\!\\!^{{}^{\prime\prime}}$}039$ and an Asinh intensity scale. There
is a well resolved central cavity showing that this is a small transition
disk.

###  3.2 Line data

Spectral line emission is detected predominantly from $v_{\rm LSR}=-7$ to 2
${\rm km~{}s^{-1}}$ and 6 to 15 ${\rm km~{}s^{-1}}$ in CO and its
isotopologues, H2CO, and SO. There is very little signal around the central
velocities of the system, 2 to 6 ${\rm km~{}s^{-1}}$, in CO and 13CO. The most
likely cause is spatial filtering of large scale emission, although there may
also be some absorption by an intervening cold molecular layer. The actual
central velocity of each source cannot be precisely determined as we do not
detect any line emission that can be clearly associated with either of the two
disks. However, from the more red- and blue-shifted parts of the spectra, we
can study the kinematics of the envelope seen in the extended continuum.

####  3.2.1 Overall view

Figure 5: Velocity integrated maps of the CO, 13CO, and C18O emission (from -3
to +11 km/s) in red, green, and blue, respectively. The maps are centered on
VLA1 and each autoscaled from their minimum to maximum and shown on a linear
scaling. The CO map has been smoothed to 0$.\\!\\!^{{}^{\prime\prime}}$25 and
the isotopologues to 0$.\\!\\!^{{}^{\prime\prime}}$5 to enhance the extended
structures. The positions of the three disks are shown by the white crosses
and other prominent features are labeled.

The IRS43 protostellar binary lies at the center of a molecular filament or
envelope that we detect in CO 2–1 and its isotopologues 13CO and C18O, as well
as H2CO $3_{0,3}-2_{0,2}$ and SO $6_{5}-5_{4}$. A 3-color CO, 13CO, and C18O
moment 0 map is shown in Figure 5. The overlay of the 3 lines gives a better
representation of the molecular gas structure than any single line due to the
range of column densities that they trace and the high level of absorption and
interferometric filtering of large scale structure.

The faint extended dust emission in Figure 2 is best seen in the CO
isotopologues because the emission from the optically thick CO line is
relatively uniform resulting in a weak interferometric response. The total
integrated 13CO emission over the entirety (760 square arcseconds) of the
mapped structure is 39 Jy ${\rm km~{}s^{-1}}$ which corresponds to a mean
column density of $1.1\times 10^{15}$ cm-2 at 30 K and a total gas mass $\sim
4\times 10^{-4}\,M_{\odot}$ for an abundance $[^{13}{\rm CO}]/[{\rm
H}_{2}]=2\times 10^{-6}$ (Ripple et al., 2013), which in good agreement with
the dust-derived mass. The CO emission is more prominent along the bright rims
of a cavity that extends north (most prominently) and south of the
circumbinary envelope. There is a hole in the northern structure that is more
apparent in the channel maps which we discuss further in §3.3.

There is a slight enhancement of CO toward VLA1 and VLA2 in the moment map
but, partially due to the strong obscuration or filtering around the central
velocities, we were unable to identify line emission in any molecule or
transition that could be clearly identified withither disk. Williams & Best
(2014) calculated the 13CO and C18O emission for a grid of disk models with
varying masses and abundances, marginalizing over size, surface density, and
temperature profiles. From the non-detections here, we estimate a $3\sigma$
upper limit to the gas mass of $0.3\,M_{\rm Jup}$ for each disk. This is about
30 and 300 times the dust mass for VLA1 and VLA2, respectively.

The observing passband contains two additional H2CO lines, $3_{2,1}-2_{2,0}$
and $3_{2,2}-2_{2,1}$, that were weakly detected in the envelope. The relative
strength of these higher excitation transitions constrain the gas temperature.
The spatially and velocity integrated ratio of line strengths for the
${3_{0,3}-2_{0,2}}/{3_{2,1}-2_{2,0}}$ transitions is $7\pm 1.5$, from which we
derive an average rotational temperature of $28\pm 4$ K assuming LTE and using
the radiative transfer code RADEX (van der Tak et al., 2007).

####  3.2.2 Kinematics

Figure 6: Structure and kinematics of the IRS43 envelope as seen in the five
brightest lines detected in our observations: the 2–1 transitions of CO, 13CO,
and C18O, H2CO $3_{0,3}-2_{0,2}$, and SO $6_{5}-5_{4}$. The resolution of the
data is indicated in the lower left corner of each panel. The left column
shows the peak emission map on an Asinh scale over the following ranges in mJy
beam-1; CO: 10–220; 13CO: 15–90; C18O: 10–30; H2CO: 5–15; SO: 10–30. The large
and small crosses indicate the positions of VLA1 and VLA2, respectively. The
middle column is the first moment for each line on the same velocity scale
from 0 (red) to 6 ${\rm km~{}s^{-1}}$ (blue) and shows the East-West velocity
gradient in the envelope. The rectangular box shows the cut along the RA axis
with a width of 0$.\\!\\!^{{}^{\prime\prime}}$5 in Dec used for the position-
velocity diagrams displayed in the right column. All 5 tracers show emission
at velocities greater than expected for pure Keplerian velocity for an edge-on
geometry and a central mass of $2.5\,M_{\odot}$ (the maximum mass allowed by
the astrometry, shown by the solid cyan line). The dotted white line brackets
the range of projected velocities for a combination of rotation and free-fall
collapse with the same central mass and matches the data much better. The
prominent SO emission centered on VLA1 has a high velocity gradient and may be
due to an infall shock onto the disk.

The envelope shows a clear velocity gradient, redshifted to blueshifted from
east to west centered on $\sim 4$ ${\rm km~{}s^{-1}}$. A collage of peak
emission, first moment, and position-velocity diagrams for the five brightest
lines is shown in Figure 6. The optically thick CO emission is the most
spatially distributed and shows the temperature structure of the gas,
highlighting the rims of the cavity as noted above. The average brightness
temperature over the envelope is 25 K but the cavity rims are about a factor
of two higher. The 13CO emission is more optically thin and highlights the
envelope, with the peak emission here showing a flared appearance. The less
abundant and optically thinner C18O line is detected where the CO and 13CO
lines are filtered out which provides an important view of the structure and
kinematics of the envelope near the source velocity. The peak emission map is
more uniform and less tightly pinched than the 13CO but also centered around
VLA1, and the velocity gradient in the position-velocity map extends linearly
through the center. The H2CO transition shown here was observed at relatively
low velocity resolution, 1.34 ${\rm km~{}s^{-1}}$, but is also less affected
by spatial filtering than CO and 13CO near the central velocities and has a
similar spatial and kinematic appearance as C18O. Finally, the SO line was
also weakly detected in the envelope but, unlike the other tracers, peaked
strongly on VLA1.

The position-velocity diagrams shown in the third column of Figure 6 show that
the velocity gradient across the envelope is not simply inherited shear or
rotation of the molecular cloud or filament from which the stars formed, but
increases around the stars due to their gravity. The binary orbit has been
accurately determined from the two decades of high resolution VLA and ALMA
astrometry and the total mass of the system is constrained to be
$M_{\ast}=2.01\pm 0.47\,M_{\odot}$ (Brinch et al., 2016). However, the gas
motions are significantly faster than Keplerian even for the maximum projected
velocity case of the rotation axis being in the plane of the sky (solid cyan
lines in Figure 6) and a central mass equal to the upper limit of
$2.5\,M_{\odot}$. This therefore indicates that the envelope is not purely
rotating.

Following Cesaroni et al. (2011) who considered the motions in an envelope
around a young massive star, we include a free-fall component perpendicular to
the rotation, which results in a radial velocity profile,

$v(r)=\left(\frac{GM_{\ast}}{r}\right)^{1/2}\biggl{(}\frac{x}{r}+\sqrt{2}\frac{z}{r}\biggr{)},$
(1)

where $x$ is the physical distance in the East-West direction in the plane of
the sky, $z$ is the physical distance along the line-of-sight, such that the
radius from the center of mass of the binary is $r=(x^{2}+z^{2})^{1/2}$. The
dotted white lines in Figure 6 show the allowed range (minimum to maximum) of
projected velocities that we would expect from this rotating and infalling
model for the highest stellar mass consistent with the astrometry,
$M_{\ast}=2.5\,M_{\odot}$. This matches the outer extent of the emission for
each line in Figure 6 where the pure-Keplerian fit is too low. In addition,
there is some flow toward the observer that lie within the expected bounds of
the model for the less optically thin lines, C18O, H2CO, and SO, where we can
see through to the back side. Together, this demonstrates that the envelope is
not only rotating but continuing to fall onto the system and the gravitational
influence of the protostars is felt to beyond an outer radius $R>2500$ au,
limited by the map size and sensitivity of the data.

Only the SO line actually peaks on the source, and even then only on VLA1, not
VLA2. The position-velocity diagram shows a high velocity gradient across
VLA1, though it does not exceed the velocity range of the rotating-infalling
envelope model. Inspection of channel maps shows that the gradient follows the
same East-West direction as the larger scale envelope. The weak, distributed
SO emission in the envelope is similar to that seen in other embedded
protostars (Tychoniec et al., 2021), but the strong enhancement on the source
may signpost infall from the envelope onto the disk similar to observations by
Sakai et al. (2014) and modeled by van Gelder et al. (2021). The velocity
gradient of this feature is approximately perpendicular to the major axis of
the dust disk so it is also possible that it may be a small outflow or jet.

###  3.3 Signature of a recent outburst

Understanding protostellar feedback and how newborn stars clear their
surroundings is a key question in star formation and learning about the origin
of the stellar mass function. Neither the CO, SO or any other line have high
velocity line wings indicative of an unbound outflow. Moreover, we do not
detect SiO 5–4 in the passband, although it is detected in the powerful jets
from several other eDisk sources. It is therefore unclear what is the cause of
the $\sim 10^{3}$ au cavity blown out of the envelope. However, inspection of
the CO channel maps reveal a possible clue to the recent accretion and
outburst history of IRS43.

Figure 7 presents channel maps for the redshifted side of the envelope
emission. We find a ring like structure with a common spatial center but
varying radius at different velocities (linearly proportional to the relative
velocity difference) outlined by white dashed ellipses in each subplot.
Remarkably, this ring center lies at the extrapolated position of the binary
130 years ago, as determined from its proper motion, $\mu_{\alpha}=-7.6$,
$\mu_{\delta}=-25.3$ mas year-1 (Brinch et al., 2016), and illustrated by the
solid blue line and open circle. We expect the molecular envelope is co-moving
with the stars but, if something disrupted the surrounding gas, there could be
features that decouple from this common motion. Consequently, we interpret
this CO feature as an expanding molecular ring that is the remnant of a
protostellar outburst event that occurred at the end of the 19th century.

Figure 7: Channel maps of CO for the red-shifted side of its emission. The
velocity of each channel is indicated at the top right of each panel in units
of ${\rm km~{}s^{-1}}$. The maps are centered on VLA1 (red cross) and the cyan
line shows the proper motion of the source projected back in time. The white
dashed ellipses, which have the same inclination and center (cyan open circle)
and radii that scale linearly with velocity, outline a ring-like feature in
each channel. We interpret this as an expanding bubble centered on the
location of the binary 130 years ago.

We can make a rough estimate of the energy of the bubble from its size, $\sim
500$ au in radius, and expansion speed, $\sim 500\,{\rm au}/130\,{\rm yr}=20$
${\rm km~{}s^{-1}}$. If the density of the pre-burst gas was $n_{\rm
H_{2}}=10^{3}$ ${\rm cm^{-3}}$, then the mass of gas in the bubble is about
$1\,M_{\oplus}$ and its total kinetic energy is $\sim 2\times 10^{40}$ erg.
This is about an order of magnitude greater than the total energy of the X-ray
superflare observed by Grosso et al. (1997), $L_{\rm X}\sim
10^{1.5}\,L_{\odot}$ over a couple of hours. However, for a typical mechanical
efficiency of a few percent for converting from stellar outburst scales to the
ISM, the total energy requirement is perhaps similar to an EXor event but less
than a FUor (Hartmann et al., 2016).

##  4 Discussion

The two disks around VLA1 and VLA2 are remarkably compact, just a few au in
radius. They are the smallest in the eDisk sample (Ohashi et al., 2023) and
may be the smallest yet measured with ALMA. It is well known that binary
systems can severely truncate disks (Artymowicz & Lubow, 1994) but the 74 au
separation of the two sources appears to be too large for this to be the
explanation. For the $\sim 3$ au disk around VLA2 to be dynamically affected,
the semi-minor axis of the binary orbit would have to be no greater than 9 au
which implies a very high orbital eccentricity greater than 0.88.
Interestingly, Manara et al. (2019) came to a similar conclusion for disks in
multiple systems in Taurus. In this case, however, the positions of two
sources have been accurately measured over 25 yr (5% of the orbital period)
and is consistent with a circular orbit (Brinch et al., 2016).

ALMA surveys of Class II objects demonstrate that many dust disks are quite
small, $\lower 2.15277pt\hbox{$\;\buildrel<\over{\sim}\;$}20$ au in radius
(Manara et al., 2022). A possible explanation is the radial drift of
millimeter and larger sized grains with high Stokes numbers and a lack of
pressure bumps to resist it (Facchini et al., 2019). Drift timescales are very
short so this could indeed be important for protostellar disks (Tobin et al.,
2020).

The high brightness temperature in VLA1 shows that the continuum emission is
likely optically thick (Figure 3) and its mass may well be significantly
underestimated. VLA2 is about 10 times fainter so the optical depth correction
may be small and it may well have low mass. However, this is hard to reconcile
with its high accretion rate (Greene & Lada, 2002). Infall from the envelope
and rapid transport through the disk may explain this. Alternatively, if most
of the mass were extremely centrally concentrated, into a $\sim 1$ au radial
region, the emission would be both optically thick and beam diluted to a low
brightness temperature in the $\sim 0\hbox{$.\\!\\!^{{}^{\prime\prime}}$}05$
beam.

Infrared observations show that the high visual extinction to VLA2, $A_{\rm
V}=40$ mag, is much greater than the inferred value from the mean column
density of the large scale circumbinary envelope, $N_{\rm H_{2}}\sim 10^{20}$
cm-2 ($A_{\rm V}\simeq 0.2$ mag), which is additional evidence for localized
dust around the protostar. Moreover, strong CO ice absorption at $4.67\,\mu$m
reveals the presence of dense, cold molecular gas enveloping both stars
(Herczeg et al., 2011).

A long standing problem in star formation is the low average luminosity of
protostars despite the need to gain of order a solar mass in a million years
(Kenyon & Hartmann, 1995). Punctuated bursts of accretion are a potential
solution and a handful of protostars have been observed to dramatically
brighten due to such an event (Hartmann et al., 2016). Although stars may
indeed grow “when we are not looking”, here we have found that associated
outburst events may leave detectable signatures in the structure and
kinematics of their surroundings. This was only possible due to the long-term
astrometric monitoring of this binary system and consequent accurate
measurements of its proper motion. Now, a decade into the ALMA era, there is
potential to measure protostellar proper motions for other sources and search
for fossil interactions with their surroundings.

It is unclear what caused the outburst 130 years ago. It does not appear to be
a close passage by the other member of the binary since the orbital period is
much longer, $\sim 450$ yr, although it is worth considering that the system
is part of a moderately clustered region where star formation is influenced by
interactions and much more dynamic than the classical picture of isolated core
collapse (Bate, 2012). Rather, it is likely related to the highly active
nature of one of the individual stars, as witnessed by the recent X-ray
superflare. The analysis here was limited by the small disk sizes and lack of
detectable lines in the ALMA data, but the system has strong CO ro-vibrational
line emission and absorption in the M-band at 4 $\mu$m (Herczeg et al., 2011)
which may provide an alternative means to study the disk properties.

The eDisk survey of 12 Class 0 and 7 Class I objects revealed a diverse set of
disk properties (Ohashi et al., 2023). Ranked by bolometric temperature, IRS43
is one of the more evolved sources but has the smallest disks in the sample.
It is one of four binary systems and has the smallest projected separation
though not by a significant margin. The circumstellar envelope surrounding the
source is punctured by a large cavity but there are no signs of molecular
outflows from either of the two protostars, in contrast to the rest of the
sources in the survey. IRS43 mostly stands out due to its compact disks and
high X-ray activity, properties that might possibly be related.

##  5 Summary

These observations of Oph IRS43, a Class I protobinary system, are part of a
homogeneous dataset of 19 embedded protostellar disks in the eDisk ALMA Large
Program. Our main results are as follows:

  * •

We detect disks around each member of the IRS43 binary in millimeter continuum
emission. The northern source, VLA1, is about 10 times brighter than the
southern source, VLA2, opposite to how they appear in the infrared. Both disks
are extremely small, just a few au in radius, and masses are low but,
especially for VLA1, likely underestimated due to high optical depth. A third
disk is detected around the Class II source, GY 263, that lies within in the
ALMA field-of-view and is found to be a transition disk with a $\sim 20$ au
radius cleared central cavity.

  * •

We map a flattened circumbinary envelope over $\sim 2000$ au in the East-West
direction in the dust continuum and multiple molecular species, 13CO, C18O,
H2CO, and SO. The CO emission extends in the North-South direction and
delineates the rim of a wide cavity. The envelope is moving faster than
expected for Keplerian rotation and we deduce that the motions must include a
component of infall onto the system. The shock as it hits the disk may explain
the enhanced SO emission toward VLA1.

  * •

We discovered an expanding ring of CO emission in the channel maps with a
center that sits at the projected position of the system 130 years ago. We
interpret this as the signature of a fossil outburst with an energy estimated
to be greater than the X-ray superflare observed in 1995 but lower than FU Ori
type events.

The eDisk survey has revealed a wide range of disk properties in embedded
protostellar systems. Here, we have presented the results from a first look at
the data for one individual system. Future work will include looking for
trends across the sample. However, it is already clear that disks form and
evolve in quite heterogeneous ways. This has ramifications for studies of
subsequent stages from the T Tauri Class II phase all the way to exoplanets.
Understanding the nature of survey outliers such as the compact disks in IRS43
will be an important part of a complete picture of star and planet formation.

This paper makes use of the following ALMA data: ADS/JAO.ALMA#2019.1.00261.L.
ALMA is a partnership of ESO (representing its member states), NSF (USA) and
NINS (Japan), together with NRC (Canada), NSTC 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. The National Radio
Astronomy Observatory is a facility of the National Science Foundation
operated under cooperative agreement by Associated Universities, Inc. Funding
support is acknowledged from NSF GRFP grant No. 2236415 and NSF AST-2107841
(SN), NSF AST-2107841 (JPW), NASA RP 80NSSC22K1159 (JJT), NSF AST-2108794
(LWL), NSTC 109-2112-M-001-051, 110- 2112-M-001-031 (NO, CF), NASA
80NSSC20K0533 and NSF AST-1910106 (ZYL), Independent Research Fund Denmark
grant No. 0135-00123B (JKJ, RS), PID2020-114461GB-I00 funded by
MCIN/AEI/10.13039/501100011033 (IdG), NSTC 110-2628-M-001-003-MY3 from the
Taiwan National Science and Technology Council and AS-CDA-111-M03 from the
Academia Sinica Career Development Award (H-WY).

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