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Abstract We analyze the dense gas kinematics in two class 0/I protostellar cores, Per 30 and NGC 1333 IRAS 7, in the Perseus Molecular Cloud to determine whether their velocity structures are indicative of rotation. We examine the hyperfine structure of the N₂H⁺J= 1–0 transition by combining 3″ (900 au) Atacama Large Millimeter/submillimeter Array measurements with 9″ (2700 au) measurements from the Green Bank Telescope. We use theCASA Feathermethod to combine these data in order to maximize our sensitivity across spatial scales. We fit the N₂H⁺spectra to constrain the centroid velocity of the gas at each pixel and use these values to calculate the linear velocity gradient and specific angular momentum within apertures centered on each protostar with radii ranging from 5″ to 60″. Our results indicate that the velocity structure probed by the N₂H⁺emission is likely not a result of core rotation. These findings are consistent with other studies in the literature that indicate rotation is often not evident on scales ≲1000 au. We instead suggest that the velocity structure we see is a result of torques caused by irregular density distributions in these protostellar systems. 

ABSTRACT We investigate the dynamics of dust concentration in actively accreting, substructured, non-ideal magnetohydrodynamic wind-launching discs using two-dimensional and three-dimensional (3D) simulations incorporating pressureless dust fluids of various grain sizes and their aerodynamic feedback on gas dynamics. Our results reveal that mm/cm-sized grains are preferentially concentrated within the inner 5–10 au of the disc, where the dust-to-gas surface density ratio (local metallicity Z) significantly exceeds the canonical 0.01, reaching values up to 0.25. This enhancement arises from the interplay of dust settling and complex gas flows in the meridional plane, including mid-plane accretion streams at early times, mid-plane expansion driven by magnetically braked surface accretion at later times, and vigorous meridional circulation in spontaneously formed gas rings. The resulting size-dependent dust distribution has a strong spatial variation, with large grains preferentially accumulating in dense rings, particularly in the inner disc, while being depleted in low-density gas gaps. In 3D, these rings and gaps are unstable to Rossby wave instability, generating arc-shaped vortices that stand out more prominently than their gas counterparts in the inner disc because of preferential dust concentration at small radii. The substantial local enhancement of the dust relative to the gas could promote planetesimal formation via streaming instability, potentially aided by the ‘azimuthal drift’ streaming instability that operates efficiently in accreting discs and a lower Toomre Q expected in younger discs. Our findings suggest that actively accreting young discs may provide favourable conditions for early planetesimal formation, which warrants further investigation. 

Abstract Star formation is a fundamental, yet poorly understood, process of the Universe. It is important to study how star formation occurs in different galactic environments. Thus, here, in the first of a series of papers, we introduce the Low-metallicity Star Formation (LZ-STAR) survey of the Sh2-284 (hereafter S284) region, which, atZ ∼  0.3–0.5Z_⊙, is one of the lowest-metallicity star-forming regions of our Galaxy. LZ-STAR is a multifacility survey, including observations with JWST, the Atacama Large Millimeter/submillimeter Array (ALMA), Hubble Space Telescope, Chandra, and Gemini. As a starting point, we report JWST and ALMA observations of one of the most massive protostars in the region, S284p1. The observations of shock-excited molecular hydrogen reveal a symmetric, bipolar outflow originating from the protostar, spanning several parsecs, and fully covered by the JWST field of view and ALMA observations of CO(2–1) emission. These allow us to infer that the protostar has maintained a relatively stable orientation of disk accretion over its formation history. The JWST near-infrared continuum observations detect a centrally illuminated bipolar outflow cavity around the protostar, as well as a surrounding cluster of low-mass young stars. We develop new radiative transfer models of massive protostars designed for the low metallicity of S284. Fitting these models to the protostar’s spectral energy distribution implies a current protostellar mass of ∼10M_⊙has formed from an initial ∼100M_⊙core over the last ∼3 × 10⁵yr. Overall, these results indicate that massive stars can form in an ordered manner in low-metallicity, protocluster environments. 

ABSTRACT Recent high angular resolution ALMA observations have revealed rich information about protoplanetary discs, including ubiquitous substructures and three-dimensional gas kinematics at different emission layers. One interpretation of these observations is embedded planets. Previous 3D planet–disc interaction studies are either based on viscous simulations or non-ideal magnetohydrodynamics (MHD) simulations with simple prescribed magnetic diffusivities. This study investigates the dynamics of gap formation in 3D non-ideal MHD discs using non-ideal MHD coefficients from the look-up table that is self-consistently calculated based on the thermochemical code. We find a concentration of the poloidal magnetic flux in the planet-opened gap (in agreement with previous work) and enhanced field-matter coupling due to gas depletion, which together enable efficient magnetic braking of the gap material, driving a fast accretion layer significantly displaced from the disc mid-plane. The fast accretion helps deplete the gap further and is expected to negatively impact the planet growth. It also affects the corotation torque by shrinking the region of horseshoe orbits on the trailing side of the planet. Together with the magnetically driven disc wind, the fast accretion layer generates a large, persistent meridional vortex in the gap, which breaks the mirror symmetry of gas kinematics between the top and bottom disc surfaces. Finally, by studying the kinematics at the emission surfaces, we discuss the implications of planets in realistic non-ideal MHD discs on kinematics observations. 

Abstract The vertical settling of dust grains in a circumstellar disk, characterized by their scale height, is a pivotal process in the formation of planets. This study offers in-depth analysis and modeling of the radial scale height profile of dust grains in the HL Tau system, leveraging high-resolution polarization observations. We resolve the inner disk’s polarization, revealing a significant nearside–farside asymmetry, with the nearside being markedly brighter than the farside in polarized intensity. This asymmetry is attributed to a geometrically thick inner dust disk, suggesting a large aspect ratio ofH/R≥ 0.15, whereHis the dust scale height andRis the radius. The first ring at 20 au exhibits an azimuthal contrast, with polarization enhanced along the minor axis, indicating a moderately thick dust ring withH/R ≈ 0.1. The absence of the nearside–farside asymmetry at larger scales implies a thin dust layer, withH/R < 0.05. Taken together, these findings depict a disk with a turbulent inner region and a settled outer disk, requiring a variable turbulence model withαincreasing from 10⁻⁵at 100 au to 10^−2.5at 20 au. This research sheds light on dust settling and turbulence levels within protoplanetary disks, providing valuable insights into the mechanisms of planet formation. 

Abstract We present new Atacama Large Millimeter/submillimeter Array observations that, for the first time, detect hydrogen and helium radio recombination lines from a protoplanetary disk. We imaged the Orion Nebula Cluster at 3.1 mm with a spectral setup that covered then= 42 → 41 transitions of hydrogen (H41α) and helium (He41α). The unprecedented sensitivity of these observations enables us to search for radio recombination lines toward the positions of ∼200 protoplanetary disks. We detect H41αfrom 17 disks, all of which are HST-identified “proplyds.” The detected H41αemission is spatially coincident with the locations of proplyd ionization fronts, indicating that proplyd H41αemission is produced by gas that has been photoevaporated off the disk and ionized by UV radiation from massive stars. We measure the fluxes and widths of the detected H41αlines and find line fluxes of ∼30–800 mJy km s⁻¹and line widths of ∼30–90 km s⁻¹. The derived line widths indicate that the broadening of proplyd H41αemission is dominated by outflowing gas motions associated with external photoevaporation. The derived line fluxes, when compared with measurements of 3.1 mm free–free flux, imply that the ionization fronts of H41α-detected proplyds have electron temperatures of ∼6000–11,000 K and electron densities of ∼10⁶–10⁷cm⁻³. Finally, we detect He41αtoward one H41α-detected source and find evidence that this system is helium-rich. Our study demonstrates that radio recombination lines are readily detectable in ionized photoevaporating disks, providing a new way to measure disk properties in clustered star-forming regions. 

Abstract The dust emission polarization spectrum—how the polarization percentage changes with wavelength—serves as a probe of dust grain properties in star-forming regions. In this paper, we present 89–214μm polarization spectrum measurements obtained from SOFIA/HAWC+ for three star-forming clouds: OMC1, M17, and W3. We find that all three clouds have an overall decreasing polarization percentage with increasing wavelength (i.e., a “falling polarization spectrum”). We use SOFIA and Herschel data to create column density and temperature maps for each cloud. We fit for the slope of the polarization spectrum at each sky position in each cloud, and using the Pearsonrcoefficient, we probe each cloud for possible correlations of slope with column density and slope with temperature. We also create plots of slope versus column density and slope versus temperature for each cloud. For the case of OMC1, our results are consistent with those presented by J. Michail et al., who carried out a similar analysis for that cloud. Our plots of polarization spectrum slope versus column density reveal that for each cloud there exists a critical column density below which a falling polarization spectrum is not observed. For these more diffuse sight lines, the polarization spectrum is instead flat or slightly rising. This finding is consistent with a hypothesis presented 25 yr ago in a paper led by R. Hildebrand based on Kuiper Airborne Observatory data. This hypothesis is that regions shielded from near-IR radiation are required to produce a sharply falling polarization spectrum. 

Abstract Polarization observations of the Milky Way and many other spiral galaxies have found a close correspondence between the orientation of spiral arms and magnetic field lines on scales of hundreds of parsecs. This paper presents polarization measurements at 214μm toward 10 filamentary candidate “bones” in the Milky Way using the High-resolution Airborne Wide-band Camera on the Stratospheric Observatory for Infrared Astronomy. These data were taken as part of the Filaments Extremely Long and Dark: A Magnetic Polarization Survey and represent the first study to resolve the magnetic field in spiral arms at parsec scales. We describe the complex yet well-defined polarization structure of all 10 candidate bones, and we find a mean difference and standard deviation of −74° ± 32° between their filament axis and the plane-of-sky magnetic field, closer to a field perpendicular to their length rather than parallel. By contrast, the 850μm polarization data from Planck on scales greater than 10 pc show a nearly parallel mean difference of 3° ± 21°. These findings provide further evidence that magnetic fields can change orientation at the scale of dense molecular clouds, even along spiral arms. Finally, we use a power law to fit the dust polarization fraction as a function of total intensity on a cloud-by-cloud basis and find indices between −0.6 and −0.9, with a mean and standard deviation of −0.7 ± 0.1. The polarization, dust temperature, and column density data presented in this work are publicly available online. 

Abstract Circumstellar disk dust polarization in the (sub)millimeter is, for the most part, not from dust grain alignment with magnetic fields but rather indicative of a combination of dust self-scattering with a yet unknown alignment mechanism that is consistent with mechanical alignment. While the observational evidence for scattering has been well established, that for mechanical alignment is less so. Circum-multiple dust structures in protostellar systems provide a unique environment to probe different polarization alignment mechanisms. We present ALMA Band 4 and Band 7 polarization observations toward the multiple young system L1448 IRS3B. The polarization in the two bands are consistent with each other, presenting multiple polarization morphologies. On the size scale of the inner envelope surrounding the circum-multiple disk, the polarization is consistent with magnetic field dust grain alignment. On the very small scale of compact circumstellar regions, we see polarization that is consistent with scattering around sourceaandc, which are likely the most optically thick components. Finally, we see polarization that is consistent with mechanical alignment of dust grains along the spiral dust structures, which would suggest that the dust is tracing the relative gas flow along the spiral arms. If the gas-flow dust grain alignment mechanism is dominant in these cases, disk dust polarization may provide a direct probe of the small-scale kinematics of the gas flow relative to the dust grains. 

ABSTRACT The majority of stars are in binary/multiple systems. How such systems form in turbulent, magnetized cores of molecular clouds in the presence of non-ideal magnetohydrodynamic (MHD) effects remains relatively underexplored. Through athena++-based non-ideal MHD adaptive mesh refinement simulations with ambipolar diffusion, we show that the collapsing protostellar envelope is dominated by dense gravo-magneto-sheetlets, a turbulence-warped version of the classic pseudodisc produced by anisotropic magnetic resistance to the gravitational collapse, in agreement with previous simulations of turbulent, magnetized single-star formation. The sheetlets feed mass, magnetic fields, and angular momentum to a Dense ROtation-Dominated (DROD) structure, which fragments into binary/multiple systems. This DROD fragmentation scenario is a more dynamic variant of the traditional disc fragmentation scenario for binary/multiple formation, with dense spiral filaments created by inhomogeneous feeding from the highly structured larger-scale sheetlets rather than the need for angular momentum transport, which is dominated by magnetic braking. Provided that the local material is sufficiently demagnetized, with a plasma-$$\beta$$ of 10 or more, collisions between the dense spiralling filaments play a key role in facilitating gravitational collapse and stellar companion formation by pushing the local magnetic Toomre parameter $$Q_\mathrm{m}$$ below unity. This mechanism can naturally produce in situ misaligned systems on the 100-au scale, often detected with high-resolution Atacama Large Millimeter Array (ALMA) observations. Our simulations also highlight the importance of non-ideal MHD effects, which affect whether fragmentation occurs and, if so, the masses and orbital parameters of the stellar companions formed.