The early evolution of viscous and self-gravitating circumstellar disks with a dust component

The long-term evolution of a circumstellar disk starting from its formation and ending in the T Tauri phase was simulated numerically with the purpose of studying the evolution of dust in the disk with distinct values of viscous \alpha-parameter and dust fragmentation velocity v_frag. We solved numerical hydrodynamics equations in the thin-disk limit, which are modified to include a dust component consisting of two parts: sub-micron-sized dust and grown dust with a maximum radius a_r. The former is strictly coupled to the gas, while the latter interacts with the gas via friction. The conversion of small to grown dust, dust growth, and dust self-gravity are also considered. We found that the process of dust growth known for the older protoplanetary phase also holds for the embedded phase of disk evolution. The dust growth efficiency depends on the radial distance from the star - a_r is largest in the inner disk and gradually declines with radial distance. In the inner disk, a_r is limited by the dust fragmentation barrier. The process of small-to-grown dust conversion is very fast once the disk is formed. The total mass of grown dust in the disk (beyond 1 AU) reaches tens or even hundreds of Earth masses already in the embedded phase of star formation and even a greater amount of grown dust drifts in the inner, unresolved 1 AU of the disk. Dust does not usually grow to radii greater than a few cm. A notable exception are models with \alpha <= 10^{-3}, in which case a zone with reduced mass transport develops in the inner disk and dust can grow to meter-sized boulders in the inner 10 AU. Grown dust drifts inward and accumulates in the inner disk regions. This effect is most pronounced in the \alpha <= 10^{-3} models where several hundreds of Earth masses can be accumulated in a narrow region of several AU from the star by the end of embedded phase. (abridged).Comment: accepted by Astronomy & Astrophysic

An Overall Picture of the Gas Flow In Massive Cluster Forming Region: The Case of G10.6-0.4

The massive clump G10.6-0.4 is an OB cluster forming region, in which multiple UC HII regions have been identified. In the present study, we report arcsecond resolution observations of the CS (1-0) transition, the NH$_{3}$ (3,3) main hyperfine inversion transition, the CH$_{3}$OH J=5 transitions, and the centimeter free-free continuum emissions in this region. The comparisons of the molecular line emissions with the free--free continuum emissions reveal a 0.5 pc scale massive molecular envelope which is being partially dispersed by the dynamically-expanding bipolar ionized cavity. The massive envelope is rotationally flattened and has an enhanced molecular density in the mid-plane. In the center of this massive clump lies a compact ($<$0.1 pc) hot ($\gtrsim$100 K) toroid, in which a cluster of O--type stars has formed. This overall geometry is analogous to the standard core collapse picture in the low-mass star forming region, with a central (proto-)stellar object, a disk, an envelope, and a bipolar outflow and outflow cavity. However, G10.6-0.4 has a much larger physical size scale ($\le$0.1 pc for typical low--mass star forming core). Based on the observations, we propose a schematic picture of the OB cluster forming region, which incorporates the various physical mechanisms. This model will be tested with the observations of other embedded OB clusters, and with numerical simulations.Comment: 34 pages, 12 figures, accepted by Ap

Gravitational instability, spiral substructure, and modest grain growth in a typical protostellar disk: Modeling multi-wavelength dust continuum observation of TMC1A

Embedded, Class 0/I protostellar disks represent the initial condition for planet formation. This calls for better understandings of their bulk properties and the dust grains within them. We model multi-wavelength dust continuum observations of the disk surrounding the Class I protostar TMC1A to provide insight on these properties. The observations can be well fit by a gravitationally self-regulated (i.e., marginally gravitationally unstable and internally heated) disk model, with surface density $\Sigma \sim 1720 (R/10au)^{-1.96} g/cm^2$ and midplane temperature $T_{mid} \sim 185 (R/10au)^{-1.27} K$. The observed disk contains a $m=1$ spiral substructure; we use our model to predict the spiral's pitch angle and the prediction is consistent with the observations. This agreement serves as both a test of our model and strong evidence of the gravitational nature of the spiral. Our model estimates a maximum grain size $a_{max}\sim 196(R/10au)^{-2.45} \mu m$, which is consistent with grain growth being capped by a fragmentation barrier with threshold velocity $\sim 1 m/s$. We further demonstrate that observational properties of TMC1A are typical among the observed population of Class 0/I disks, which hints that traditional methods of disk data analyses based on Gaussian fitting and the assumption of the optically thin dust emission could have systematically underestimated disk size and mass and overestimated grain size.Comment: 16 pages, 8 figures, accepted for publication in Ap

Grain Growth in the Dust Ring with Crescent around Very Low Mass Star ZZ Tau IRS with JVLA

The azimuthal asymmetries of dust rings in protoplanetary disks such as a crescent around young stars are often interpreted as dust traps, and thus as ideal locations for planetesimal and planet formations. Whether such dust traps effectively promote planetesimal formation in disks around very-low-mass stars (VLM; a mass of $\lesssim$0.2~$M_\odot$) is debatable, as the dynamical and grain growth timescales in such systems are long. To investigate grain growth in such systems, we studied the dust ring with crescent around the VLM star ZZ~Tau~IRS using the Karl G. Jansky Very Large Array (JVLA) at centimeter wavelengths. Significant signals were detected around ZZ~Tau~IRS. To estimate the maximum grain size ($a_{\rm max}$) in the crescent, we compared the observed spectral energy distribution (SED) with SEDs for various $a_{\rm max}$ values predicted by radiative transfer calculations. We found $a_{\rm max} \gtrsim$~1~mm and $\lesssim$~60~$\mu$m in the crescent and ring, respectively, though our modeling efforts rely on uncertain dust properties. Our results suggest that grain growth occurred in the ZZ~Tau~IRS disk, relative to sub-micron-sized interstellar medium. Planet formation in crescent with mm-sized pebbles might proceed more efficiently than in other regions with sub-millimeter-sized pebbles via pebble accretion scenarios.Comment: 11 pages, 3 figures, accepted in Ap

Detection of 40-48 GHz dust continuum linear polarization towards the Class 0 young stellar object IRAS 16293-2422

We performed the new JVLA full polarization observations at 40-48 GHz (6.3-7.5 mm) towards the nearby ($d$ $=$147$\pm$3.4 pc) Class 0 YSO IRAS 16293-2422, and compare with the previous SMA observations reported by Rao et al. (2009; 2014). We observed the quasar J1407+2827 which is weakly polarized and can be used as a leakage term calibrator for $<$9 GHz observations, to gauge the potential residual polarization leakage after calibration. We did not detect Stokes Q, U, and V intensities from the observations of J1407+2827, and constrain (3-$\sigma$) the residual polarization leakage after calibration to be $\lesssim$0.3\%. We detect linear polarization from one of the two binary components of our target source, IRAS\,16293-2422\,B. The derived polarization position angles from our observations are in excellent agreement with those detected from the previous observations of the SMA, implying that on the spatial scale we are probing ($\sim$50-1000 au), the physical mechanisms for polarizing the continuum emission do not vary significantly over the wavelength range of $\sim$0.88-7.5 mm. We hypothesize that the observed polarization position angles trace the magnetic field which converges from large scale to an approximately face-on rotating accretion flow. In this scenario, magnetic field is predominantly poloidal on $>$100 au scales, and becomes toroidal on smaller scales. However, this interpretation remains uncertain due to the high dust optical depths at the central region of IRAS\,16293-2422\,B and the uncertain temperature profile. We suggest that dust polarization at wavelengths comparable or longer than 7\,mm may still trace interstellar magnetic field. Future sensitive observations of dust polarization in the fully optically thin regime will have paramount importance for unambiguously resolving the magnetic field configuration.Comment: 14 pages, 7 figures, accepted to A&A. Comments are welcom