1,145 research outputs found
Mass accretion rates in self-regulated disks of T Tauri stars
We have studied numerically the evolution of protostellar disks around
intermediate and upper mass T Tauri stars (0.25 M_sun < M_st < 3.0 M_sun) that
have formed self-consistently from the collapse of molecular cloud cores. In
the T Tauri phase, disks settle into a self-regulated state, with low-amplitude
nonaxisymmetric density perturbations persisting for at least several million
years. Our main finding is that the global effect of gravitational torques due
to these perturbations is to produce disk accretion rates that are of the
correct magnitude to explain observed accretion onto T Tauri stars. Our models
yield a correlation between accretion rate M_dot and stellar mass M_st that has
a best fit M_dot \propto M_st^{1.7}, in good agreement with recent
observations. We also predict a near-linear correlation between the disk
accretion rate and the disk mass.Comment: Accepted for publication in ApJ Letter
Self-regulated gravitational accretion in protostellar discs
We present a numerical model for the evolution of a protostellar disc that
has formed self-consistently from the collapse of a molecular cloud core. The
global evolution of the disc is followed for several million years after its
formation. The capture of a wide range of spatial and temporal scales is made
possible by use of the thin-disc approximation. We focus on the role of
gravitational torques in transporting mass inward and angular momentum outward
during different evolutionary phases of a protostellar disc with disc-to-star
mass ratio of order 0.1. In the early phase, when the infall of matter from the
surrounding envelope is substantial, mass is transported inward by the
gravitational torques from spiral arms that are a manifestation of the
envelope-induced gravitational instability in the disc. In the late phase, when
the gas reservoir of the envelope is depleted, the distinct spiral structure is
replaced by ongoing irregular nonaxisymmetric density perturbations. The
amplitude of these density perturbations decreases with time, though this
process is moderated by swing amplification aided by the existence of the
disc's sharp outer edge. Our global modelling of the protostellar disc reveals
that there is typically a residual nonzero gravitational torque from these
density perturbations, i.e. their effects do not exactly cancel out in each
region. In particular, the net gravitational torque in the inner disc tends to
be negative during first several million years of the evolution, while the
outer disc has a net positive gravitational torque. Our global model of a
self-consistently formed disc shows that it is also self-regulated in the late
phase, so that it is near the Toomre stability limit, with a near-uniform
Toomre parameter Q\approx 1.5-2.0. (Abstract abridged).Comment: 9 pages, 9 figures, accepted for publication in MNRA
The Burst Mode of Accretion in Primordial Star Formation
We present simulation results for the formation and long-term evolution of a
primordial protostellar disk harbored by a first star. Using a 2+1D
nonaxisymmetric thin disk numerical simulation, together with a barotropic
relation for the gas, we are able to probe ~20 kyr of the disk's evolution.
During this time period we observe fragmentation leading to loosely bound
gaseous clumps within the disk. These are then torqued inward and accreted onto
the growing protostar, giving rise to a burst phenomenon. The luminous feedback
produced by this mechanism may have important consequences for the subsequent
growth of the protostar.Comment: 3 pages, 2 figures, to appear in proceedings of First Stars IV
meeting (Kyoto, Japan; 2012
Optical Color Gradients in Star-Forming Ring Galaxies
We compute radial color gradients produced by an outwardly propagating
circular wave of star formation and compare our results with color gradients
observed in the classical ring galaxy, the ``Cartwheel''. We invoke two
independent models of star formation in the ring galaxies. The first one is the
conventional density wave scenario, in which an intruder galaxy creates a
radially propagating density wave accompanied by an enhanced star formation
following the Schmidt law. The second scenario is a pure self-propagating star
formation model, in which the intruder only sets off the first burst of stars
at the point of impact. Both models give essentially the same results.
Systematic reddening of B-V, V-K colors towards the center, such as that
observed in the Cartwheel, can be obtained only if the abundance of heavy
elements in the star-forming gas is a few times below solar. The B-V and V-K
color gradients observed in the Cartwheel can be explained as a result of
mixing of stellar populations born in a star-forming wave propagating through a
low-metallicity gaseous disk, and a pre-existing stellar disk of the size of
the gaseous disk with color properties typical to those observed in nearby disk
galaxies.Comment: 16 pages, 12 figures; accepted for publication in the Astrophysical
Journa
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