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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 We present a study connecting the physical properties of protostellar envelopes to the morphology of the envelope-scale magnetic field. We used the Atacama Large Millimeter/submillimeter Array (ALMA) polarization observations of 61 young protostars at 0.87 mm on ~400–3000 au scales from theB-field Orion Protostellar Survey to infer the envelope-scale magnetic field, and we used the dust emission to measure the envelope properties on comparable scales. We find that protostars showing standard hourglass magnetic field morphology tend to have larger masses, and the nonthermal velocity dispersion is positively correlated with the bolometric luminosity and dust temperature of the envelope. Combining with the disk properties taken from the Orion VLA/ALMA Nascent Disk and Multiplicity survey, we connect envelope properties to fragmentation. Our results show a positive correlation between the fragmentation level and the angle dispersion of the magnetic field, suggesting that the envelope fragmentation tends to be suppressed by the magnetic field. We also find that protostars exhibiting standard hourglass magnetic field structure tend to have a smaller disk and smaller angle dispersion of the magnetic field than other field configurations, especially the rotated hourglass, but also the spiral and others, suggesting a more effective magnetic braking in the standard hourglass morphology of magnetic fields. Nevertheless, significant misalignment between the magnetic field and outflow axes tends to reduce magnetic braking, leading to the formation of larger disks. 

Abstract The polarization spectrum, or wavelength dependence of the polarization fraction, of interstellar dust emission provides important insights into the grain alignment mechanism of interstellar dust grains. We investigate the far-infrared polarization spectrum of a realistic simulated high-mass star-forming cloud under various models of grain alignment and emission. We find that neither a homogeneous grain alignment model nor a grain alignment model that includes collisional dealignment is able to produce the falling spectrum seen in observations. On the other hand, we find that a grain alignment model with grain alignment efficiency dependent on local temperature is capable of producing a falling spectrum that is in qualitative agreement with observations of OMC-1. For the model most in agreement with OMC-1, we find no correlation between the temperature and the slope of the polarization spectrum. However, we do find a positive correlation between the column density and the slope of the polarization spectrum. We suggest this latter correlation to be the result of wavelength-dependent polarization by absorption. 

Abstract Magnetic fields likely play an important role in the formation of young protostars. Multiscale and multiwavelength dust polarization observations can reveal the inferred magnetic field from scales of the cloud to core to protostar. We present continuum polarization observations of the young protostellar triple system IRAS 16293-2422 at 89μm using HAWC+ on SOFIA. The inferred magnetic field is very uniform with an average field angle of 89° ± 23° (E of N), which is different from the ∼170° field morphology seen at 850μm at larger scales (≳2000 au) with JCMT POL-2 and at 1.3 mm on smaller scales (≲300 au) with Atacama Large Millimeter/submillimeter Array. The HAWC+ magnetic field direction is aligned with the known E-W outflow. This alignment difference suggests that the shorter wavelength HAWC+ data is tracing the magnetic field associated with warmer dust likely from the outflow cavity, whereas the longer wavelength data are tracing the bulk magnetic field from cooler dust. Also, we show in this source the dust emission peak is strongly affected by the observing wavelength. The dust continuum peaks closer to source B (northern source) at shorter wavelengths and progressively moves toward the southern A source with increasing wavelength (from 22 to 850μm). 

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. 

Abstract VLA 1623 West is an ambiguous source that has been described as a shocked cloudlet as well as a protostellar disk. We use deep ALMA 1.3 and 0.87 mm observations to constrain its shape and structure to determine its origins better. We use a series of geometric models to fit the uv visibilities at both wavelengths with GALARIO . Although the real visibilities show structures similar to what has been identified as gaps and rings in protoplanetary disks, we find that a modified flat-topped Gaussian model at high inclination provides the best fit to the observations. This fit agrees well with expectations for an optically thick, highly inclined disk. Nevertheless, we find that the geometric models consistently yield positive residuals at the four corners of the disk at both wavelengths. We interpret these residuals as evidence that the disk is flared in the millimeter dust. We use a simple toy model for an edge-on flared disk and find that the residuals best match a disk with flaring that is mainly restricted to the outer disk at R ≳ 30 au. Thus, VLA 1623W may represent a young protostellar disk where the large dust grains have not yet had enough time to settle into the midplane. This result may have implications for how disk evolution and vertical dust settling impact the initial conditions leading to planet formation. 

Abstract We present 870μm polarimetric observations toward 61 protostars in the Orion molecular clouds with ∼400 au (1″) resolution using the Atacama Large Millimeter/submillimeter Array. We successfully detect dust polarization and outflow emission in 56 protostars; in 16 of them the polarization is likely produced by self-scattering. Self-scattering signatures are seen in several Class 0 sources, suggesting that grain growth appears to be significant in disks at earlier protostellar phases. For the rest of the protostars, the dust polarization traces the magnetic field, whose morphology can be approximately classified into three categories: standard-hourglass, rotated-hourglass (with its axis perpendicular to outflow), and spiral-like morphology. A total of 40.0% (±3.0%) of the protostars exhibit a mean magnetic field direction approximately perpendicular to the outflow on several × 10²–10³au scales. However, in the remaining sample, this relative orientation appears to be random, probably due to the complex set of morphologies observed. Furthermore, we classify the protostars into three types based on the C¹⁷O (3–2) velocity envelope’s gradient: perpendicular to outflow, nonperpendicular to outflow, and unresolved gradient (≲1.0 km s⁻¹arcsec⁻¹). In protostars with a velocity gradient perpendicular to outflow, the magnetic field lines are preferentially perpendicular to outflow, with most of them exhibiting a rotated hourglass morphology, suggesting that the magnetic field has been overwhelmed by gravity and angular momentum. Spiral-like magnetic fields are associated with envelopes having large velocity gradients, indicating that the rotation motions are strong enough to twist the field lines. All of the protostars with a standard-hourglass field morphology show no significant velocity gradient due to the strong magnetic braking. 

Abstract We present Very Large Array observations toward the Class 0 protostar L1157 MMS at 6.8 and 9 mm with a resolution of ∼0.″04 (14 au). We detect two sources within L1157 MMS and interpret these sources as a binary protostar with a separation of ∼16 au. The material directly surrounding the binary system within the inner 50 au radius of the system has an estimated mass of 0.11M_☉, calculated from the observed dust emission. We interpret the observed binary system in the context of previous observations of its flattened envelope structure, low rates of envelope rotation from 5000 to 200 au scales, and an ordered, poloidal magnetic field aligned with the outflow. Thus, L1157 MMS is a prototype system for magnetically regulated collapse, and the presence of a compact binary within L1157 MMS demonstrates that multiple star formation can still occur within envelopes that likely have dynamically important magnetic fields. 

ABSTRACT Polarized dust continuum emission has been observed with Atacama Large Millimeter/submillimeter Array in an increasing number of deeply embedded protostellar systems. It generally shows a sharp transition going from the protostellar envelope to the disc scale, with the polarization fraction typically dropping from $${\sim } 5{{\ \rm per\ cent}}$$ to $${\sim } 1{{\ \rm per\ cent}}$$ and the inferred magnetic field orientations becoming more aligned with the major axis of the system. We quantitatively investigate these observational trends using a sample of protostars in the Perseus molecular cloud and compare these features with a non-ideal magnetohydrodynamic disc formation simulation. We find that the gas density increases faster than the magnetic field strength in the transition from the envelope to the disc scale, which makes it more difficult to magnetically align the grains on the disc scale. Specifically, to produce the observed $${\sim } 1{{\ \rm per\ cent}}$$ polarization at $${\sim } 100\, \mathrm{au}$$ scale via grains aligned with the B-field, even relatively small grains of $$1\, \mathrm{\mu m}$$ in size need to have their magnetic susceptibilities significantly enhanced (by a factor of ∼20) over the standard value, potentially through superparamagnetic inclusions. This requirement is more stringent for larger grains, with the enhancement factor increasing linearly with the grain size, reaching ∼2 × 104 for millimetre-sized grains. Even if the required enhancement can be achieved, the resulting inferred magnetic field orientation in the simulation does not show a preference for the major axis, which is inconsistent with the observed pattern. We thus conclude that the observed trends are best described by the model where the polarization on the envelope scale is dominated by magnetically aligned grains and that on the disc scale by scattering. 

Abstract We present H -band (1.65 μ m) and SOFIA HAWC+ 154 μ m polarization observations of the low-mass core L483. Our H -band observations reveal a magnetic field that is overwhelmingly in the E–W direction, which is approximately parallel to the bipolar outflow that is observed in scattered IR light and in single-dish 12 CO observations. From our 154 μ m data, we infer a ∼45° twist in the magnetic field within the inner 5″ (1000 au) of L483. We compare these new observations with published single-dish 350 μ m polarimetry and find that the 10,000 au scale H -band data match the smaller-scale 350 μ m data, indicating that the collapse of L483 is magnetically regulated on these larger scales. We also present high-resolution 1.3 mm Atacama Large Millimeter/submillimeter Array data of L483 that reveals it is a close binary star with a separation of 34 au. The plane of the binary of L483 is observed to be approximately parallel to the twisted field in the inner 1000 au. Comparing this result to the ∼1000 au protostellar envelope, we find that the envelope is roughly perpendicular to the 1000 au HAWC+ field. Using the data presented, we speculate that L483 initially formed as a wide binary and the companion star migrated to its current position, causing an extreme shift in angular momentum thereby producing the twisted magnetic field morphology observed. More observations are needed to further test this scenario.