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ABSTRACT Recent high angular resolution ALMA observations have revealed numerous gaps in protoplanetary discs. A popular interpretation has been that planets open them. Most previous investigations of planet gap-opening have concentrated on viscous discs. Here, we carry out 2D (axisymmetric) global simulations of gap opening by a planet in a wind-launching non-ideal MHD disc with consistent thermochemistry. We find a strong concentration of poloidal magnetic flux in the planet-opened gap, where the gas dynamics are magnetically dominated. The magnetic field also drives a fast (nearly sonic) meridional gas circulation in the denser disc regions near the inner and outer edges of the gap, which may be observable through high-resolution molecular line observations. The gap is more ionized than its denser surrounding regions, with a better magnetic field–matter coupling. In particular, it has a much higher abundance of molecular ion HCO+, consistent with ALMA observations of the well-studied AS 209 protoplanetary disc that has prominent gaps and fast meridional motions reaching the local sound speed. Finally, we provide fitting formulae for the ambipolar and Ohmic diffusivities as a function of the disc local density, which can be used for future 3D simulations of planet gap-opening in non-ideal MHD discs where thermochemistry is too computationally expensive to evolve self-consistently with the magneto-hydrodynamics. 

Abstract We use molecular line data from the Atacama Large Millimeter/submillimeter Array, Submillimeter Array, James Clerk Maxwell Telescope, and NANTEN2 to study the multiscale (∼15–0.005 pc) velocity statistics in the massive star formation region NGC 6334. We find that the nonthermal motions revealed by the velocity dispersion function (VDF) stay supersonic over scales of several orders of magnitude. The multiscale nonthermal motions revealed by different instruments do not follow the same continuous power law, which is because the massive star formation activities near central young stellar objects have increased the nonthermal motions in small-scale and high-density regions. The magnitudes of VDFs vary in different gas materials at the same scale, where the infrared dark clump N6334S in an early evolutionary stage shows a lower level of nonthermal motions than other more evolved clumps due to its more quiescent star formation activity. We find possible signs of small-scale-driven (e.g., by gravitational accretion or outflows) supersonic turbulence in clump N6334IV with a three-point VDF analysis. Our results clearly show that the scaling relation of velocity fields in NGC 6334 deviates from a continuous and universal turbulence cascade due to massive star formation activities. 

ABSTRACT We compare the structure of synthetic dust polarization with synthetic molecular line emission from radiative transfer calculations using a three-dimensional, turbulent collapsing-cloud magnetohydrodynamics simulation. The histogram of relative orientation (HRO) technique and the projected Rayleigh statistic (PRS) are considered. In our trans-Alfvénic (more strongly magnetized) simulation, there is a transition to perpendicular alignment at densities above ∼4 × 103 cm−3. This transition is recovered in most of our synthetic observations of optically thin molecular tracers; however, for 12CO it does not occur and the PRS remains in parallel alignment across the whole observer space. We calculate the physical depth of the optical depth τ = 1 surface and find that for 12CO it is largely located in front of the cloud mid-plane, suggesting that 12CO is too optically thick and instead mainly probes low-volume density gas. In our super-Alfvénic simulation, the magnetic field becomes significantly more tangled, and all observed tracers tend towards no preference for perpendicular or parallel alignment. An observable difference in alignment between optically thin and optically thick tracers may indicate the presence of a dynamically important magnetic field, though there is some degeneracy with viewing angle. We convolve our data with a Gaussian beam and compare it with HRO results of the Vela C molecular cloud. We find good agreement between these results and our sub-Alfvénic simulations when viewed with the magnetic field in the plane of the sky (especially when sensitivity limitations are considered), though the observations are also consistent with an intermediately inclined magnetic field. 

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. 

Abstract Magnetic fields play essential roles in protoplanetary disks. Magnetic fields in the disk atmosphere are of particular interest, as they are connected to the wind-launching mechanism. In this work, we study the polarization of the light scattered off of magnetically aligned grains in the disk atmosphere, focusing on the deviation of the polarization orientation from the canonical azimuthal direction, which may be detectable in near-IR polarimetry with instruments such as VLT/SPHERE. We show with a simple disk model that the polarization can even be oriented along the radial (rather than azimuthal) direction, especially in highly inclined disks with toroidally dominated magnetic fields. This polarization reversal is caused by the anisotropy in the polarizability of aligned grains and is thus a telltale sign of such grains. We show that the near-IR light is scattered mostly byμm-sized grains or smaller at theτ= 1 surface and such grains can be magnetically aligned if they contain superparamagnetic inclusions. For comparison with observations, we generate synthetic maps of the ratios ofU_ϕ/IandQ_ϕ/I, which can be used to infer the existence of (magnetically) aligned grains through a negativeQ_ϕ(polarization reversal) and/or a significant level ofU_ϕ/I. We show that two features observed in the existing data, an asymmetric distribution ofU_ϕwith respect to the disk minor axis and a spatial distribution ofU_ϕthat is predominantly positive or negative, are incompatible with scattering by spherical grains in an axisymmetric disk. They provide indirect evidence for scattering by aligned nonspherical grains. 

Abstract 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, water 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. 

Abstract We present 870μm Atacama Large Millimeter/submillimeter Array polarization observations of thermal dust emission from the iconic, edge-on debris diskβPic. While the spatially resolved map does not exhibit detectable polarized dust emission, we detect polarization at the ∼3σlevel when averaging the emission across the entire disk. The corresponding polarization fraction isP_frac= 0.51% ± 0.19%. The polarization position angleχis aligned with the minor axis of the disk, as expected from models of dust grains aligned via radiative alignment torques (RAT) with respect to a toroidal magnetic field (B-RAT) or with respect to the anisotropy in the radiation field (k-RAT). When averaging the polarized emission across the outer versus inner thirds of the disk, we find that the polarization arises primarily from the SW third. We perform synthetic observations assuming grain alignment via bothk-RAT andB-RAT. Both models produce polarization fractions close to our observed value when the emission is averaged across the entire disk. When we average the models in the inner versus outer thirds of the disk, we find thatk-RAT is the likely mechanism producing the polarized emission inβPic. A comparison of timescales relevant to grain alignment also yields the same conclusion. For dust grains with realistic aspect ratios (i.e.,s> 1.1), our models imply low grain-alignment efficiencies. 

Abstract We present ALMA dust polarization and molecular line observations toward four clumps (I(N), I, IV, and V) in the massive star-forming region NGC 6334. In conjunction with large-scale dust polarization and molecular line data from JCMT, Planck, and NANTEN2, we make a synergistic analysis of relative orientations between magnetic fields (θ_B), column density gradients (θ_NG), local gravity (θ_LG), and velocity gradients (θ_VG) to investigate the multi-scale (from ∼30 to 0.003 pc) physical properties in NGC 6334. We find that the relative orientation betweenθ_Bandθ_NGchanges from statistically more perpendicular to parallel as column density ( $N_{{\mathrm{H}}_2}$) increases, which is a signature of trans-to-sub-Alfvénic turbulence at complex/cloud scales as revealed by previous numerical studies. Becauseθ_NGandθ_LGare preferentially aligned within the NGC 6334 cloud, we suggest that the more parallel alignment betweenθ_Bandθ_NGat higher $N_{{\mathrm{H}}_2}$is because the magnetic field line is dragged by gravity. At even higher $N_{{\mathrm{H}}_2}$, the angle betweenθ_Bandθ_NGorθ_LGtransits back to having no preferred orientation, or statistically slightly more perpendicular, suggesting that the magnetic field structure is impacted by star formation activities. A statistically more perpendicular alignment is found betweenθ_Bandθ_VGthroughout our studied $N_{{\mathrm{H}}_2}$range, which indicates a trans-to-sub-Alfvénic state at small scales as well, and this signifies that magnetic field has an important role in the star formation process in NGC 6334. The normalized mass-to-flux ratio derived from the polarization-intensity gradient (KTH) method increases with $N_{{\mathrm{H}}_2}$, but the KTH method may fail at high $N_{{\mathrm{H}}_2}$due to the impact of star formation feedback. 

Abstract We characterize protostellar multiplicity in²⁰Current address: Niels Bohr Institute, University of Copenhagen, Øster Voldgade 5–7, DK-1350, Copenhagen K, Denmark.the Orion molecular clouds using Atacama Large Millimeter/submillimeter Array 0.87 mm and Very Large Array 9 mm continuum surveys toward 328 protostars. These observations are sensitive to projected spatial separations as small as ∼20 au, and we consider source separations up to 10⁴au as potential companions. The overall multiplicity fraction (MF) and companion fraction (CF) for the Orion protostars are 0.30 ± 0.03 and 0.44 ± 0.03, respectively, considering separations from 20 to 10⁴au. The MFs and CFs are corrected for potential contamination by unassociated young stars using a probabilistic scheme based on the surface density of young stars around each protostar. The companion separation distribution as a whole is double peaked and inconsistent with the separation distribution of solar-type field stars, while the separation distribution of Flat Spectrum protostars is consistent solar-type field stars. The multiplicity statistics and companion separation distributions of the Perseus star-forming region are consistent with those of Orion. Based on the observed peaks in the Class 0 separations at ∼100 au and ∼10³au, we argue that multiples with separations <500 au are likely produced by both disk fragmentation and turbulent fragmentation with migration, and those at ≳10³au result primarily from turbulent fragmentation. We also find that MFs/CFs may rise from Class 0 to Flat Spectrum protostars between 100 and 10³au in regions of high young stellar object density. This finding may be evidence for the migration of companions from >10³au to <10³au, and that some companions between 10³and 10⁴au must be (or become) unbound. 

ABSTRACT We present the first results of high-spectral resolution (0.023 km s−1) N2H+ observations of dense gas dynamics at core scales (∼0.01 pc) using the recently commissioned Argus instrument on the Green Bank Telescope (GBT). While the fitted linear velocity gradients across the cores measured in our targets nicely agree with the well-known power-law correlation between the specific angular momentum and core size, it is unclear if the observed gradients represent core-scale rotation. In addition, our Argus data reveal detailed and intriguing gas structures in position–velocity (PV) space for all five targets studied in this project, which could suggest that the velocity gradients previously observed in many dense cores actually originate from large-scale turbulence or convergent flow compression instead of rigid-body rotation. We also note that there are targets in this study with their star-forming discs nearly perpendicular to the local velocity gradients, which, assuming the velocity gradient represents the direction of rotation, is opposite to what is described by the classical theory of star formation. This provides important insight on the transport of angular momentum within star-forming cores, which is a critical topic on studying protostellar disc formation.