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1. Grain growth during protostellar disc formation

   https://doi.org/10.1093/mnras/stac2030  

   Tu, Yisheng; Li, Zhi-Yun; Lam, Ka Ho (July 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Recent observations indicate that mm/cm-sized grains may exist in the embedded protostellar discs. How such large grains grow from the micron size (or less) in the earliest phase of star formation remains relatively unexplored. In this study, we take a first step to model the grain growth in the protostellar environment, using 2D (axisymmetric) radiation hydrodynamic and grain growth simulations. We show that the grain growth calculations can be greatly simplified by the ‘terminal velocity approximation’, where the dust drift velocity relative to the gas is proportional to its stopping time, which is proportional to the grain size. We find that the grain–grain collision from size-dependent terminal velocity alone is too slow to convert a significant fraction of the initially micron-sized grains into mm/cm sizes during the deeply embedded Class 0 phase. Substantial grain growth is achieved when the grain–grain collision speed is enhanced by a factor of 4. The dust growth above and below the disc midplane enables the grains to settle faster towards the midplane, which increases the local dust-to-gas ratio, which, in turn, speeds up further growth there. How this needed enhancement can be achieved is unclear, although turbulence is a strong possibility that deserves further exploration. 

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2. Finding Substructures in Protostellar Disks in Ophiuchus

   https://doi.org/10.3847/1538-3881/acf653  

   Michel, Arnaud; Sadavoy, Sarah I; Sheehan, Patrick D; Looney, Leslie W; Cox, Erin G; Tobin, John J; van_der_Marel, Nienke; Segura-Cox, Dominique M (October 2023, The Astronomical Journal)

   Abstract High-resolution, millimeter observations of disks at the protoplanetary stage reveal substructures such as gaps, rings, arcs, spirals, and cavities. While many protoplanetary disks host such substructures, only a few at the younger protostellar stage have shown similar features. We present a detailed search for early disk substructures in Atacama Large Millimeter/submillimeter Array 1.3 and 0.87 mm observations of ten protostellar disks in the Ophiuchus star-forming region. Of this sample, four disks have identified substructure, two appear to be smooth disks, and four are considered ambiguous. The structured disks have wide Gaussian-like rings (σ_R/R_disk∼ 0.26) with low contrasts (C< 0.2) above a smooth disk profile, in comparison to protoplanetary disks where rings tend to be narrow and have a wide variety of contrasts (σ_R/R_disk∼ 0.08 andCranges from 0 to 1). The four protostellar disks with the identified substructures are among the brightest sources in the Ophiuchus sample, in agreement with trends observed for protoplanetary disks. These observations indicate that substructures in protostellar disks may be common in brighter disks. The presence of substructures at the earliest stages suggests an early start for dust grain growth and, subsequently, planet formation. The evolution of these protostellar substructures is hypothesized in two potential pathways: (1) the rings are the sites of early planet formation, and the later observed protoplanetary disk ring–gap pairs are secondary features, or (2) the rings evolve over the disk lifetime to become those observed at the protoplanetary disk stage. 

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3. Hall effect in protostellar disc formation and evolution

   https://doi.org/10.1093/mnras/staa041  

   Zhao, Bo; Caselli, Paola; Li, Zhi-Yun; Krasnopolsky, Ruben; Shang, Hsien; Lam, Ka Ho (March 2020, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The Hall effect is recently shown to be efficient in magnetized dense molecular cores and could lead to a bimodal formation of rotationally supported discs (RSDs) in the first core phase. However, how such Hall dominated systems evolve in the protostellar accretion phase remains unclear. We carry out 2D axisymmetric simulations including Hall effect and ohmic dissipation, with realistic magnetic diffusivities computed from our equilibrium chemical network. We find that Hall effect only becomes efficient when the large population of very small grains (VSGs: ≲100 Å) is removed from the standard Mathis–Rumpl–Nordsieck size distribution. With such an enhanced Hall effect, however, the bimodality of disc formation does not continue into the main accretion phase. The outer part of the initial ∼40 au disc formed in the anti-aligned configuration ($$\boldsymbol {\Omega \cdot B}\lt 0$$) flattens into a thin rotationally supported Hall current sheet as Hall effect moves the poloidal magnetic field radially inward relative to matter, leaving only the inner ≲10–20 au RSD. In the aligned configuration ($$\boldsymbol {\Omega \cdot B}\gt 0$$), disc formation is suppressed initially but a counter-rotating disc forms subsequently due to efficient azimuthal Hall drift. The counter-rotating disc first grows to ∼30 au as Hall effect moves the magnetic field radially outward, but only the inner ≲10 au RSD is long lived like in the anti-aligned case. Besides removing VSGs, cosmic ray ionization rate should be below a few 10−16 s−1 for Hall effect to be efficient in disc formation. We conclude that Hall effect produces small ≲10–20 au discs regardless of the polarity of the magnetic field, and that radially outward diffusion of magnetic fields remains crucial for disc formation and growth. 

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4. Protostellar discs fed by dense collapsing gravomagneto sheetlets

   https://doi.org/10.1093/mnras/stad3843  

   Tu, Yisheng; Li, Zhi-Yun; Lam, Ka Ho; Tomida, Kengo; Hsu, Chun-Yen (December 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Stars form from the gravitational collapse of turbulent, magnetized molecular cloud cores. Our non-ideal MHD simulations reveal that the intrinsically anisotropic magnetic resistance to gravity during the core collapse naturally generates dense gravomagneto sheetlets within inner protostellar envelopes – disrupted versions of classical sheet-like pseudo-discs. They are embedded in a magnetically dominant background, where less dense materials flow along the local magnetic field lines and accumulate in the dense sheetlets. The sheetlets, which feed the disc predominantly through its upper and lower surfaces, are the primary channels for mass and angular momentum transfer from the envelope to the disc. The protostellar disc inherits a small fraction (up to 10 per cent) of the magnetic flux from the envelope, resulting in a disc-averaged net vertical field strength of 1–10 mG and a somewhat stronger toroidal field, potentially detectable through ALMA Zeeman observations. The inherited magnetic field from the envelope plays a dominant role in disc angular momentum evolution, enabling the formation of gravitationally stable discs in cases where the disc field is relatively well-coupled to the gas. Its influence remains significant even in marginally gravitationally unstable discs formed in the more magnetically diffusive cases, removing angular momentum at a rate comparable to or greater than that caused by spiral arms. The magnetically driven disc evolution is consistent with the apparent scarcity of prominent spirals capable of driving rapid accretion in deeply embedded protostellar discs. The dense gravomagneto sheetlets observed in our simulations may correspond to the ‘accretion streamers’ increasingly detected around protostars. 

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5. Protostellar Cores in Sagittarius B2 N and M

   https://doi.org/10.3847/1538-4357/ad0383  

   Budaiev, Nazar; Ginsburg, Adam; Jeff, Desmond; Goddi, Ciriaco; Meng, Fanyi; Sánchez-Monge, Álvaro; Schilke, Peter; Schmiedeke, Anika; Yoo, Taehwa (January 2024, The Astrophysical Journal)

   Abstract We present 500 and 700 au resolution 1 and 3 mm Atacama Large Millimeter/submillimeter Array observations, respectively, of protostellar cores in protoclusters Sagittarius B2 (Sgr B2) North (N) and Main (M), parts of the most actively star-forming cloud in our Galaxy. Previous lower-resolution (5000 au) 3 mm observations of this region detected ∼150 sources inferred to be young stellar objects (YSOs) withM> 8M_⊙. With a 10-fold increase in resolution, we detect 371 sources at 3 mm and 218 sources in the smaller field of view at 1 mm. The sources seen at low resolution are observed to fragment into an average of two objects. About one-third of the observed sources fragment. Most of the sources we report are marginally resolved and are at least partially optically thick. We determine that the observed sources are most consistent with Stage 0/I YSOs, i.e., rotationally supported disks with an active protostar and an envelope, that are warmer than those observed in the solar neighborhood. We report source-counting-based inferred stellar mass and the star formation rate of the cloud: 2800M_⊙and 0.0038M_⊙yr⁻¹for Sgr B2 N and 6900M_⊙and 0.0093M_⊙yr⁻¹for Sgr B2 M, respectively. 

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