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1. Imaging the Water Snowline around Protostars with Water and HCO ⁺ Isotopologues

   https://doi.org/10.3847/1538-4357/ac3080  

   van ’t Hoff, Merel L. ; Harsono, Daniel ; van Gelder, Martijn L. ; Hsieh, Tien-Hao ; Tobin, John J. ; Jensen, Sigurd S. ; Hirano, Naomi ; Jørgensen, Jes K. ; Bergin, Edwin A. ; van Dishoeck, Ewine F. ( January 2022 , The Astrophysical Journal)

   Abstract The water snowline location in protostellar envelopes provides crucial information about the thermal structure and the mass accretion process as it can inform about the occurrence of recent (≲1000 yr) accretion bursts. In addition, the ability to image water emission makes these sources excellent laboratories to test indirect snowline tracers such as H 13 CO + . We study the water snowline in five protostellar envelopes in Perseus using a suite of molecular-line observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) at ∼0.″2−0.″7 (60–210 au) resolution. B1-c provides a textbook example of compact H 2 18 O (3 1,3 −2 2,0 ) and HDO (3 1,2 −2 2,1 ) emission surrounded by a ring of H 13 CO + ( J = 2−1) and HC 18 O + ( J = 3−2). Compact HDO surrounded by H 13 CO + is also detected toward B1-bS. The optically thick main isotopologue HCO + is not suited to trace the snowline, and HC 18 O + is a better tracer than H 13 CO + due to a lower contribution from the outer envelope. However, because a detailed analysis is needed to derive a snowline location from H 13 COmore »+ or HC 18 O + emission, their true value as a snowline tracer will lie in the application in sources where water cannot be readily detected. For protostellar envelopes, the most straightforward way to locate the water snowline is through observations of H 2 18 O or HDO. Including all subarcsecond-resolution water observations from the literature, we derive an average burst interval of ∼10,000 yr, but high-resolution water observations of a larger number of protostars are required to better constrain the burst frequency.« less
2. Resolved molecular line observations reveal an inherited molecular layer in the young disk around TMC1A

   https://doi.org/10.1051/0004-6361/202038697  

   Harsono, D. ; van der Wiel, M. H. ; Bjerkeli, P. ; Ramsey, J. P. ; Calcutt, H. ; Kristensen, L. E. ; Jørgensen, J. K. ( February 2021 , Astronomy & Astrophysics)

   Context. Physical processes that govern the star and planet formation sequence influence the chemical composition and evolution of protoplanetary disks. Recent studies allude to an early start to planet formation already during the formation of a disk. To understand the chemical composition of protoplanets, we need to constrain the composition and structure of the disks from whence they are formed. Aims. We aim to determine the molecular abundance structure of the young disk around the TMC1A protostar on au scales in order to understand its chemical structure and any possible implications for disk formation. Methods. We present spatially resolved Atacama Large Millimeter/submillimeter Array observations of CO, HCO + , HCN, DCN, and SO line emission, as well as dust continuum emission, in the vicinity of TMC1A. Molecular column densities are estimated both under the assumption of optically thin emission from molecules in local thermodynamical equilibrium (LTE) as well as through more detailed non-LTE radiative transfer calculations. Results. Resolved dust continuum emission from the disk is detected between 220 and 260 GHz. Rotational transitions from HCO + , HCN, and SO are also detected from the inner 100 au region. We further report on upper limits to vibrational HCN υ 2more »= 1, DCN, and N 2 D + lines. The HCO + emission appears to trace both the Keplerian disk and the surrounding infalling rotating envelope. HCN emission peaks toward the outflow cavity region connected with the CO disk wind and toward the red-shifted part of the Keplerian disk. From the derived HCO + abundance, we estimate the ionization fraction of the disk surface, and find values that imply that the accretion process is not driven by the magneto-rotational instability. The molecular abundances averaged over the TMC1A disk are similar to its protostellar envelope and other, older Class II disks. We meanwhile find a discrepancy between the young disk’s molecular abundances relative to Solar System objects. Conclusions. Abundance comparisons between the disk and its surrounding envelope for several molecular species reveal that the bulk of planet-forming material enters the disk unaltered. Differences in HCN and H 2 O molecular abundances between the disk around TMC1A, Class II disks, and Solar System objects trace the chemical evolution during disk and planet formation.« less
3. New Constraints on Protoplanetary Disk Gas Masses in Lupus

   https://doi.org/10.3847/1538-4357/ac517e  

   Anderson, Dana E. ; Cleeves, L. Ilsedore ; Blake, Geoffrey A. ; Bergin, Edwin A. ; Zhang, Ke ; Carpenter, John M. ; Schwarz, Kamber R. ( March 2022 , The Astrophysical Journal)

   Gas mass is a fundamental quantity of protoplanetary disks that directly relates to their ability to form planets. Because we are unable to observe the bulk H₂content of disks directly, we rely on indirect tracers to provide quantitative mass estimates. Current estimates for the gas masses of the observed disk population in the Lupus star-forming region are based on measurements of isotopologues of CO. However, without additional constraints, the degeneracy between H₂mass and the elemental composition of the gas leads to large uncertainties in such estimates. Here, we explore the gas compositions of seven disks from the Lupus sample representing a range of CO-to-dust ratios. With Band 6 and 7 ALMA observations, we measure line emission for HCO⁺, HCN, and N₂H⁺. We find a tentative correlation among the line fluxes for these three molecular species across the sample, but no correlation with¹³CO or submillimeter continuum fluxes. For the three disks where N₂H⁺is detected, we find that a combination of high disk gas masses and subinterstellar C/H and O/H are needed to reproduce the observed values. We find increases of ∼10–100× previous mass estimates are required to match the observed line fluxes. This work highlights how multimolecular studies are essentialmore »for constraining the physical and chemical properties of the gas in populations of protoplanetary disks, and that CO isotopologues alone are not sufficient for determining the mass of many observed disks.

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4. Disks and Outflows in the Intermediate-mass Star-forming Region NGC 2071 IR

   https://doi.org/10.3847/1538-4357/ac7464  

   Cheng, Yu ; Tobin, John J. ; Yang, Yao-Lun ; van ’t Hoff, Merel L. R. ; Sadavoy, Sarah I. ; Osorio, Mayra ; Díaz-Rodríguez, Ana Karla ; Anglada, Guillem ; Karnath, Nicole ; Sheehan, Patrick D. ; et al ( July 2022 , The Astrophysical Journal)

   We present Atacama Large Millimeter Array band 6/7 (1.3 mm/0.87 mm) and Very Large Array Ka-band (9 mm) observations toward NGC 2071 IR, an intermediate-mass star-forming region. We characterize the continuum and associated molecular line emission toward the most luminous protostars, i.e., IRS1 and IRS3, on ∼100 au (0.″2) scales. IRS1 is partly resolved in the millimeter and centimeter continuum, which shows a potential disk. IRS3 has a well-resolved disk appearance in the millimeter continuum and is further resolved into a close binary system separated by ∼40 au at 9 mm. Both sources exhibit clear velocity gradients across their disk major axes in multiple spectral lines including C¹⁸O, H₂CO, SO, SO₂, and complex organic molecules like CH₃OH,¹³CH₃OH, and CH₃OCHO. We use an analytic method to fit the Keplerian rotation of the disks and give constraints on physical parameters with a Markov Chain Monte Carlo routine. The IRS3 binary system is estimated to have a total mass of 1.4–1.5M_⊙. IRS1 has a central mass of 3–5M_⊙based on both kinematic modeling and its spectral energy distribution, assuming that it is dominated by a single protostar. For both IRS1 and IRS3, the inferred ejection directions from different tracers, including radio jet, watermore »maser, molecular outflow, and H₂emission, are not always consistent, and for IRS1 these can be misaligned by ∼50°. IRS3 is better explained by a single precessing jet. A similar mechanism may be present in IRS1 as well but an unresolved multiple system in IRS1 is also possible.

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5. Kinematics around the B335 protostar down to au scales

   https://doi.org/10.1051/0004-6361/201935948  

   Bjerkeli, Per ; Ramsey, Jon P. ; Harsono, Daniel ; Calcutt, Hannah ; Kristensen, Lars E. ; van der Wiel, Matthijs H. ; Jørgensen, Jes K. ; Muller, Sébastien ; Persson, Magnus V. ( November 2019 , Astronomy & Astrophysics)

   Context. The relationship between outflow launching and the formation of accretion disks around young stellar objects is still not entirely understood, which is why spectrally and spatially resolved observations are needed. Recently, the Atacama Large Millimetre/sub-millimetre Array (ALMA) carried out long-baseline observations towards a handful of young sources, revealing connections between outflows and the inner regions of disks. Aims. Here we aim to determine the small-scale kinematical and morphological properties of the outflow from the isolated protostar B335 for which no Keplerian disk has, so far, been observed on scales down to 10 au. Methods. We used ALMA in its longest-baseline configuration to observe emission from CO isotopologues, SiO, SO 2 , and CH 3 OH. The proximity of B335 provides a resolution of ~3 au (0.03′′). We also combined our long-baseline data with archival observations to produce a high-fidelity image covering scales up to 700 au (7′′). Results. 12 CO has an X-shaped morphology with arms ~50 au in width that we associate with the walls of an outflow cavity, similar to what is observed on larger scales. Long-baseline continuum emission is confined to <7 au from the protostar, while short-baseline continuum emission follows the 12 CO outflow andmore »cavity walls. Methanol is detected within ~30 au of the protostar. SiO is also detected in the vicinity of the protostar, but extended along the outflow. Conclusions. The 12 CO outflow does not show any clear signs of rotation at distances ≳30 au from the protostar. SiO traces the protostellar jet on small scales, but without obvious rotation. CH 3 OH and SO 2 trace a region <16 au in diameter, centred on the continuum peak, which is clearly rotating. Using episodic, high-velocity, 12 CO features, we estimate the launching radius of the outflow to be <0.1 au and dynamical timescales of the order of a few years.« less