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1. 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)

   null (Ed.)

   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 2 = 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. 

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2. Digging into the Interior of Hot Cores with ALMA (DIHCA). II. Exploring the Inner Binary (Multiple) System Embedded in G335 MM1 ALMA1

   https://doi.org/10.3847/1538-4357/ac5bd8  

   Olguin, Fernando A. ; Sanhueza, Patricio ; Ginsburg, Adam ; Chen, Huei-Ru Vivien ; Zhang, Qizhou ; Li, Shanghuo ; Lu, Xing ; Sakai, Takeshi ( April 2022 , The Astrophysical Journal)

   Abstract We observed the high-mass protostellar core G335.579–0.272 ALMA1 at ∼200 au (0.″05) resolution with the Atacama Large Millimeter/submillimeter Array (ALMA) at 226 GHz (with a mass sensitivity of 5 σ = 0.2 M ⊙ at 10 K). We discovered that at least a binary system is forming inside this region, with an additional nearby bow-like structure (≲1000 au) that could add an additional member to the stellar system. These three sources are located at the center of the gravitational potential well of the ALMA1 region and the larger MM1 cluster. The emission from CH 3 OH (and many other tracers) is extended (>1000 au), revealing a common envelope toward the binary system. We use CH 2 CHCN line emission to estimate an inclination angle of the rotation axis of 26° with respect to the line of sight based on geometric assumptions and derive a kinematic mass of the primary source (protostar+disk) of 3.0 M ⊙ within a radius of 230 au. Using SiO emission, we find that the primary source drives the large-scale outflow revealed by previous observations. Precession of the binary system likely produces a change in orientation between the outflow at small scales observed here and large scales observed in previous works. The bow structure may have originated from the entrainment of matter into the envelope due to the widening or precession of the outflow, or, alternatively, an accretion streamer dominated by the gravity of the central sources. An additional third source, forming due to instabilities in the streamer, cannot be ruled out as a temperature gradient is needed to produce the observed absorption spectra. 

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3. 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 and 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. 

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4. Physical conditions in the warped accretion disk of a massive star: 349 GHz ALMA observations of G023.01−00.41

   https://doi.org/10.1051/0004-6361/202040000  

   Sanna, A. ; Giannetti, A. ; Bonfand, M. ; Moscadelli, L. ; Kuiper, R. ; Brand, J. ; Cesaroni, R. ; Caratti o Garatti, A. ; Pillai, T. ; Menten, K. M. ( November 2021 , Astronomy & Astrophysics)

   Young massive stars warm up the large amount of gas and dust that condenses in their vicinity, exciting a forest of lines from different molecular species. Their line brightness is a diagnostic tool of the gas’s physical conditions locally, which we use to set constraints on the environment where massive stars form. We made use of the Atacama Large Millimeter/submillimeter Array at frequencies near 349 GHz, with an angular resolution of 0′′.1, to observe the methyl cyanide (CH 3 CN) emission which arises from the accretion disk of a young massive star. We sample the disk midplane with twelve distinct beams, where we get an independent measure of the gas’s (and dust’s) physical conditions. The accretion disk extends above the midplane, showing a double-armed spiral morphology projected onto the plane of the sky, which we sample with ten additional beams: Along these apparent spiral features, gas undergoes velocity gradients of about 1 km s −1 per 2000 au. The gas temperature ( T ) rises symmetrically along each side of the disk, from about 98 K at 3000 au to 289 K at 250 au, following a power law with radius R −0.43 . The CH 3 CN column density ( N ) increases from 9.2 × 10 15 cm −2 to 8.7 × 10 17 cm −2 at the same radii, following a power law with radius R −1.8 . In the framework of a circular gaseous disk observed approximately edge-on, we infer an H 2 volume density in excess of 4.8 ×10 9 cm −3 at a distance of 250 au from the star. We study the disk stability against fragmentation following the methodology by Kratter et al. (2010, ApJ, 708, 1585), which is appropriate under rapid accretion, and we show that the disk is marginally prone to fragmentation along its whole extent. 

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5. Viscous heating in the disk of the outbursting star FU Orionis

   https://doi.org/10.1051/0004-6361/202039370  

   Labdon, Aaron ; Kraus, Stefan ; Davies, Claire L. ; Kreplin, Alexander ; Monnier, John D. ; Le Bouquin, Jean-Baptiste ; Anugu, Narsireddy ; ten Brummelaar, Theo ; Setterholm, Benjamin ; Gardner, Tyler ; et al ( February 2021 , Astronomy & Astrophysics)

   null (Ed.)

   Context. FU Orionis is the archetypal FUor star, a subclass of young stellar objects (YSOs) that undergo rapid brightening events, often gaining between four and six magnitudes on timescales of days. This brightening is often associated with a massive increase in accretion, which is one of the most ubiquitous processes in astrophysics for bodies ranging from planets and stars to super-massive black holes. We present multi-band interferometric observations of the FU Ori circumstellar environment, including the first J -band interferometric observations of a YSO. Aims. We investigate the morphology and temperature gradient of the innermost regions of the accretion disk around FU Orionis. We aim to characterise the heating mechanisms of the disk and comment on potential outburst-triggering processes. Methods. Recent upgrades to the MIRC-X instrument at the CHARA array have allowed for the first dual-band J and H observations of YSOs. Using baselines up to 331 m, we present high-angular-resolution data of a YSO covering the near-infrared bands J , H , and K . The unprecedented spectral range of the data allowed us to apply temperature gradient models to the innermost regions of FU Ori. Results. We spatially resolved the innermost astronomical unit of the disk and determine the exponent of the temperature gradient of the inner disk to T ∝ r −0.74 ± 0.02 . This agrees with theoretical works that predict T ∝ r −0.75 for actively accreting, steady-state disks, which is a value only obtainable through viscous heating within the disk. We found a disk that extends down to the stellar surface at 0.015 ± 0.007 au, where the temperature is found to be 5800 ± 700 K. We found a disk inclined at 32 ± 4° with a minor-axis position angle of 34 ± 11°. Conclusions. We demonstrate that J -band interferometric observations of YSOs are feasible with the MIRC-X instrument at CHARA. The temperature gradient power-law derived for the inner disk is consistent with theoretical predictions for steady-state, optically thick, viciously heated accretion disks. 

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