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Register a new userMulti-scale Dust Polarization and Spiral-like Stokes-I Residual in the Class I Protostellar System TMC-1A
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We have observed the Class I protostar TMC-1A in the Taurus molecular cloud using the Submillimeter Array (SMA) and the Atacama Large Millimeter/submillimeter Array (ALMA) in the linearly polarized 1.3 mm continuum emission at angular resolutions of ~3 and ~0.3, respectively. The ALMA observations also include CO, 13CO, and C18O J=2-1 spectral lines. The SMA observations trace magnetic fields on the 1000-au scale, the directions of which are neither parallel nor perpendicular to the outflow direction. Applying the Davis-Chandrasekhar-Fermi method to the SMA polarization angle dispersion, we estimate a field strength in the TMC-1A envelope of 1-5 mG. It is consistent with the field strength needed to reduce the radial infall velocity to the observed value, which is substantially less than the local} free-fall velocity. The ALMA polarization observations consist of two distinct components -- a central component and a north/south component. The central component shows polarization directions in the disk minor axis to be azimuthal, suggesting dust self-scattering in the TMC-1A disk. The north/south component is located along the outflow axis and the polarization directions are aligned with the outflow direction. We discuss possible origins of this polarization structure, including grain alignment by a toroidal magnetic field and mechanical alignment by the gaseous outflow. In addition, we discover a spiral-like residual in the total intensity (Stokes I) for the first time. The C18O emission suggests that material in the spiral-like structure is infalling at a speed that is 20% of the local Keplerian speed.
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Recent observational progress has challenged the dust grain-alignment theories used to explain the polarized dust emission routinely observed in star-forming cores. In an effort to improve our understanding of the dust grain alignment mechanism(s), we have gathered a dozen ALMA maps of (sub)millimeter-wavelength polarized dust emission from Class 0 protostars, and carried out a comprehensive statistical analysis of dust polarization quantities. We analyze the statistical properties of the polarization fraction P_frac and dispersion of polarization position angles S. More specifically, we investigate the relationship between S and P_frac as well as the evolution of the product S*P_frac as a function of the column density of the gas in the protostellar envelopes. We find a significant correlation in the polarized dust emission from protostellar envelopes seen with ALMA; the power-law index differs significantly from the one observed by Planck in star-forming clouds. The product S*P_frac, which is sensitive to the dust grain alignment efficiency, is approximately constant across three orders of magnitude in envelope column density. This suggests that the grain alignment mechanism producing the bulk of the polarized dust emission in star-forming cores may not depend systematically on the local conditions such as local gas density. Ultimately, our results suggest dust alignment mechanism(s) are efficient at producing dust polarized emission in the various local conditions typical of Class 0 protostars. The grain alignment efficiency found in these objects seems to be higher than the efficiency produced by the standard RAT alignment of paramagnetic grains. Further study will be needed to understand how more efficient grain alignment via, e.g., different irradiation conditions, dust grain characteristics, or additional grain alignment mechanisms can reproduce the observations.
Water is a key volatile that provides insights into the initial stages of planet formation. The low water abundances inferred from water observations toward low-mass protostellar objects may point to a rapid locking of water as ice by large dust grains during star and planet formation. However, little is known about the water vapor abundance in newly formed planet-forming disks. We aim to determine the water abundance in embedded Keplerian disks through spatially-resolved observations of H$_2^{18}$O lines to understand the evolution of water during star and planet formation. We present H$_2^{18}$O line observations with ALMA and NOEMA millimeter interferometers toward five young stellar objects. NOEMA observed the 3$_{1,3}$ - $2_{2,0}$ line (E$_{rm up}$ = 203.7 K) while ALMA targeted the $4_{1,4}$ - $3_{2,1}$ line (E$_{rm up}$ = 322.0 K). Water column densities are derived considering optically thin and thermalized emission. Our observations are sensitive to the emission from the known Keplerian disks around three out of the five Class I objects in the sample. No H$_2^{18}$O emission is detected toward any of our five Class I disks. We report upper limits to the integrated line intensities. The inferred water column densities in Class I disks are N < 10$^{15}$ cm$^{-2}$ on 100 au scales which include both disk and envelope. The upper limits imply a disk-averaged water abundance of $lesssim 10^{-6}$ with respect to H$_2$ for Class I objects. After taking into account the physical structure of the disk, the upper limit to the water abundance averaged over the inner warm disk with $T>$ 100 K is between 10$^{-7}$ up to 10$^{-5}$. Water vapor is not abundant in warm protostellar envelopes around Class I protostars. Upper limits to the water vapor column densities in Class I disks are at least two orders magnitude lower than values found in Class 0 disk-like structures.
The dynamics of dust and gas can be quite different from each other when the dust is poorly coupled to the gas. In protoplanetary discs, it is well known that this decoupling of the dust and gas can lead to diverse spatial structures and dust-to-gas ratios. In this paper, we study the dynamics of dust and gas during the earlier phase of protostellar collapse, before a protoplanetary disc is formed. We find that for dust grains with sizes < 10 micron, the dust is well coupled during the collapse of a rotating, pre-stellar core and there is little variation of the dust-to-gas ratio during the collapse. However, if larger grains are present, they may have trajectories that are very different from the gas during the collapse, leading to mid-plane settling and/or oscillations of the dust grains through the mid-plane. This may produce variations in the dust-to-gas ratio and very different distributions of large and small dust grains at the very earliest stages of star formation, if large grains are present in pre-stellar cores.
Asymmetrical features in disks provide indirect evidences of embedded objects, such as planets. Observed with the Atacama Large Millimeter/submillimeter Array (ALMA), the circumstellar disk in MWC 758 traced with thermal dust continuum emission at wavelengths of 0.9 mm with an angular resolution up to 0.1 (15 au) exhibits an asymmetrical dust ring with additional features. In order to analyze the structures azimuthally and radially, we split the dust ring into small segments in azimuth. For each segment, we fit two-Gaussian functions to the radial intensity profile. The obtained best-fit parameters as a function of azimuth are analyzed. Three spiral-like arm structures are identified. When fitting the 0.9 mm features with the spiral density wave theory using the WKB approximation, two sets of disk aspect ratios are found: one solution gives relatively low values (~0.03) while the other solution is at the upper bound of the free parameter (~0.2). The planet locations suggested by the upper-bound result are similar to the ones determined by Benisty et al. (2015) for the NIR polarized intensity image. Comparing the reported spiral-like structures with the higher angular-resolution (0.04) ALMA image in Dong et al. (2018), we identify different structures in the West of the disk due to differences in the adopted analysis methods and the respective resolutions of the images.
We estimate the levels of turbulence in the envelopes of Class 0 and I protostars using a model based on measurements of the peak separation of double-peaked asymmetric line profiles. We use observations of 20 protostars of both Class 0 and I taken in the HCO+ (J=3-2) line that show the classic double-peaked profile. We find that some Class 0 sources show high levels of turbulence whilst others demonstrate much lower levels. In Class I protostars we find predominantly low levels of turbulence. The observations are consistent with a scenario in which Class 0 protostars form in a variety of environments and subsequently evolve into Class I protostars. The data do not appear to be consistent with a recently proposed scenario in which Class 0 protostars can only form in extreme environments.
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