375 research outputs found
Resolution Requirements and Resolution Problems in Simulations of Radiative Feedback in Dusty Gas
In recent years a number of authors have introduced methods to model the
effects of radiation pressure feedback on flows of interstellar and
intergalactic gas, and have posited that the forces exerted by stars' radiation
output represents an important feedback mechanism capable of halting accretion
and thereby regulating star formation. However, numerical simulations have
reached widely varying conclusions about the effectiveness of this feedback. In
this paper I show that much of the divergence in the literature is a result of
failure to obey an important resolution criterion: whether radiation feedback
is able to reverse an accretion flow is determined on scales comparable to the
dust destruction radius, which is AU even for the most luminous
stellar sources. Simulations that fail to resolve this scale can produce
unphysical results, in many cases leading to a dramatic overestimate of the
effectiveness of radiation feedback. Most published simulations of radiation
feedback on molecular cloud and galactic scales fail to satisfy this condition.
I show how the problem can be circumvented by introducing a new subgrid model
that explicitly accounts for momentum balance on unresolved scales, making it
possible to simulate dusty accretion flows safely even at low resolution.Comment: 15 pages, 5 figures, MNRAS in press; this version has some added
discussion, but no changes to figures or conclusion
The Star Formation Law in Molecule-Poor Galaxies
In this paper, I investigate the processes that regulate the rate of star
formation in regions of galaxies where the neutral interstellar medium is
predominantly composed of non-star-forming HI. In such regions, found today
predominantly in low-metallicity dwarf galaxies and in the outer parts of large
spirals, the star formation rate per unit area and per unit mass is much
smaller than in more molecule-rich regions. While in molecule-rich regions the
ultraviolet radiation field produced by efficient star formation forces the
density of the cold neutral medium to a value set by two-phase equilibrium, I
show that the low rates of star formation found in molecule-poor regions
preclude this condition. Instead, the density of the cold neutral gas is set by
the requirements of hydrostatic balance. Using this result, I extend the
Krumholz, McKee, & Tumlinson model for star formation and the atomic to
molecular transition to the molecule-poor regime. This "KMT+" model matches a
wide range of observations of the star formation rate and the balance between
the atomic and molecular phases in dwarfs and in the outer parts of spirals,
and is well-suited to implementation as a subgrid recipe for star formation in
cosmological simulations and semi-analytic models. I discuss the implications
of this model for star formation over cosmological times.Comment: 18 pages, 9 figures, accepted for publication in MNRA
VADER: A Flexible, Robust, Open-Source Code for Simulating Viscous Thin Accretion Disks
The evolution of thin axisymmetric viscous accretion disks is a classic
problem in astrophysics. While models based on this simplified geometry provide
only approximations to the true processes of instability-driven mass and
angular momentum transport, their simplicity makes them invaluable tools for
both semi-analytic modeling and simulations of long-term evolution where two-
or three-dimensional calculations are too computationally costly. Despite the
utility of these models, the only publicly-available frameworks for simulating
them are rather specialized and non-general. Here we describe a highly
flexible, general numerical method for simulating viscous thin disks with
arbitrary rotation curves, viscosities, boundary conditions, grid spacings,
equations of state, and rates of gain or loss of mass (e.g., through winds) and
energy (e.g., through radiation). Our method is based on a conservative,
finite-volume, second-order accurate discretization of the equations, which we
solve using an unconditionally-stable implicit scheme. We implement Anderson
acceleration to speed convergence of the scheme, and show that this leads to
factor of speed gains over non-accelerated methods in realistic
problems, though the amount of speedup is highly problem-dependent. We have
implemented our method in the new code Viscous Accretion Disk Evolution
Resource (VADER), which is freely available for download from
https://bitbucket.org/krumholz/vader/ under the terms of the GNU General Public
License.Comment: 58 pages, 13 figures, accepted to Astronomy & Computing; this version
includes more discussion, but no other changes; code is available for
download from https://bitbucket.org/krumholz/vader
Numerical Simulations of Radiatively-Driven Dusty Winds
[abridged] Radiation pressure on dust grains may be an important mechanism in
driving winds in a wide variety of astrophysical systems. However, the
efficiency of the coupling between the radiation field and the dusty gas is
poorly understood in environments characterized by high optical depths. We
present a series of idealized numerical experiments, performed with the
radiation-hydrodynamic code ORION, in which we study the dynamics of such winds
and quantify their properties. We find that, after wind acceleration begins,
radiation Rayleigh-Taylor instability forces the gas into a configuration that
reduces the rate of momentum transfer from the radiation field to the gas by a
factor ~ 10 - 100 compared to an estimate based on the optical depth at the
base of the atmosphere; instead, the rate of momentum transfer from a driving
radiation field of luminosity L to the gas is roughly L/c multiplied by one
plus half the optical depth evaluated using the photospheric temperature, which
is far smaller than the optical depth one would obtain using the interior
temperature. When we apply our results to conditions appropriate to ULIRGs and
star clusters, we find that the asymptotic wind momentum flux from such objects
should not significantly exceed that carried by the direct radiation field,
L/c. This result constrains the expected mass loss rates from systems that
exceed the Eddington limit to be of order the so-called "single-scattering"
limit, and not significantly higher. We present an approximate fitting formula
for the rate of momentum transfer from radiation to dusty gas through which it
passes, which is suitable for implementation in sub-grid models of galaxy
formation. Finally, we provide a first map of the column density distribution
of gas in a radiatively-driven wind as a function of velocity, and velocity
dispersion.Comment: 19 pages, 17 figures, MNRAS in press; some additional discussion
compared to previous version, no changes in conclusion
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