146 research outputs found
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) main hyperfine inversion transition, the CHOH 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
(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 (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 and midplane temperature . The observed disk contains a 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 , which
is consistent with grain growth being capped by a fragmentation barrier with
threshold velocity . 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 0.2~) 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 () in the crescent, we compared the
observed spectral energy distribution (SED) with SEDs for various
values predicted by radiative transfer calculations. We found ~1~mm and ~60~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 ( 1473.4 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-) the residual polarization leakage after calibration to
be 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 (50-1000 au), the physical mechanisms for
polarizing the continuum emission do not vary significantly over the wavelength
range of 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
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