986 research outputs found
The role of accretion disks in the formation of massive stars
We present radiation hydrodynamics simulations of the collapse of massive
pre-stellar cores. We treat frequency dependent radiative feedback from stellar
evolution and accretion luminosity at a numerical resolution down to 1.27 AU.
In the 2D approximation of axially symmetric simulations, it is possible for
the first time to simulate the whole accretion phase of several 10^5 yr for the
forming massive star and to perform a comprehensive scan of the parameter
space. Our simulation series show evidently the necessity to incorporate the
dust sublimation front to preserve the high shielding property of massive
accretion disks. Our disk accretion models show a persistent high anisotropy of
the corresponding thermal radiation field, yielding to the growth of the
highest-mass stars ever formed in multi-dimensional radiation hydrodynamics
simulations. Non-axially symmetric effects are not necessary to sustain
accretion. The radiation pressure launches a stable bipolar outflow, which
grows in angle with time as presumed from observations. For an initial mass of
the pre-stellar host core of 60, 120, 240, and 480 Msol the masses of the final
stars formed in our simulations add up to 28.2, 56.5, 92.6, and at least 137.2
Msol respectively.Comment: 4 pages, 2 figures, Computational Star Formation Proceedings IAU
Symposium No. 270, 2010, Ed.: J. Alves, B. Elmegreen, J. Girart & V. Trimbl
Turbulent Mixing in the Outer Solar Nebula
The effects of turbulence on the mixing of gases and dust in the outer Solar
nebula are examined using 3-D MHD calculations in the shearing-box
approximation with vertical stratification. The turbulence is driven by the
magneto-rotational instability. The magnetic and hydrodynamic stresses in the
turbulence correspond to an accretion time at the midplane about equal to the
lifetimes of T Tauri disks, while accretion in the surface layers is thirty
times faster. The mixing resulting from the turbulence is also fastest in the
surface layers. The mixing rate is similar to the rate of radial exchange of
orbital angular momentum, so that the Schmidt number is near unity. The
vertical spreading of a trace species is well-matched by solutions of a damped
wave equation when the flow is horizontally-averaged. The damped wave
description can be used to inexpensively treat mixing in 1-D chemical models.
However, even in calculations reaching a statistical steady state, the
concentration at any given time varies substantially over horizontal planes,
due to fluctuations in the rate and direction of the transport. In addition to
mixing species that are formed under widely varying conditions, the turbulence
intermittently forces the nebula away from local chemical equilibrium. The
different transport rates in the surface layers and interior may affect
estimates of the grain evolution and molecular abundances during the formation
of the Solar system.Comment: To appear in the Astrophysical Journal; 20 pages, 9 figure
An implicit method for radiative transfer with the diffusion approximation in SPH
An implicit method for radiative transfer in SPH is described. The diffusion
approximation is used, and the hydrodynamic calculations are performed by a
fully three--dimensional SPH code. Instead of the energy equation of state for
an ideal gas, various energy states and the dissociation of hydrogen molecules
are considered in the energy calculation for a more realistic temperature and
pressure determination. In order to test the implicit code, we have performed
non--isothermal collapse simulations of a centrally condensed cloud, and have
compared our results with those of finite difference calculations performed by
MB93. The results produced by the two completely different numerical methods
agree well with each other.Comment: 25 pages, 9 figure
Certainty Closure: Reliable Constraint Reasoning with Incomplete or Erroneous Data
Constraint Programming (CP) has proved an effective paradigm to model and
solve difficult combinatorial satisfaction and optimisation problems from
disparate domains. Many such problems arising from the commercial world are
permeated by data uncertainty. Existing CP approaches that accommodate
uncertainty are less suited to uncertainty arising due to incomplete and
erroneous data, because they do not build reliable models and solutions
guaranteed to address the user's genuine problem as she perceives it. Other
fields such as reliable computation offer combinations of models and associated
methods to handle these types of uncertain data, but lack an expressive
framework characterising the resolution methodology independently of the model.
We present a unifying framework that extends the CP formalism in both model
and solutions, to tackle ill-defined combinatorial problems with incomplete or
erroneous data. The certainty closure framework brings together modelling and
solving methodologies from different fields into the CP paradigm to provide
reliable and efficient approches for uncertain constraint problems. We
demonstrate the applicability of the framework on a case study in network
diagnosis. We define resolution forms that give generic templates, and their
associated operational semantics, to derive practical solution methods for
reliable solutions.Comment: Revised versio
Photon Bubbles in the Circumstellar Envelopes of Young Massive Stars
We show that the optically-thick dusty envelopes surrounding young high-mass
stars are subject to the photon bubble instability. The infrared radiation
passing through the envelope amplifies magnetosonic disturbances, with growth
rates in our local numerical radiation MHD calculations that are consistent
with a linear analysis. Modes with wavelengths comparable to the gas pressure
scale height grow by more than two orders of magnitude in a thousand years,
reaching non-linear amplitudes within the envelope lifetime. If the magnetic
pressure in the envelope exceeds the gas pressure, the instability develops
into trains of propagating shocks. Radiation escapes readily through the
low-density material between the shocks, enabling accretion to continue despite
the Eddington limit imposed by the dust opacity. The supersonic motions arising
from the photon bubble instability can help explain the large velocity
dispersions of hot molecular cores, while conditions in the shocked gas are
suitable for maser emission. We conclude that the photon bubble instability may
play a key role in the formation of massive stars.Comment: Accepted by the Astrophysical Journal; 18 pages, 4 figure
Resolving the Formation of Protogalaxies. III. Feedback from the First Stars
The first stars form in dark matter halos of masses ~10^6 M_sun as suggested
by an increasing number of numerical simulations. Radiation feedback from these
stars expels most of the gas from their shallow potential well of their
surrounding dark matter halos. We use cosmological adaptive mesh refinement
simulations that include self-consistent Population III star formation and
feedback to examine the properties of assembling early dwarf galaxies. Accurate
radiative transport is modeled with adaptive ray tracing. We include supernova
explosions and follow the metal enrichment of the intergalactic medium. The
calculations focus on the formation of several dwarf galaxies and their
progenitors. In these halos, baryon fractions in 10^8 solar mass halos decrease
by a factor of 2 with stellar feedback and by a factor of 3 with supernova
explosions. We find that radiation feedback and supernova explosions increase
gaseous spin parameters up to a factor of 4 and vary with time. Stellar
feedback, supernova explosions, and H_2 cooling create a complex, multi-phase
interstellar medium whose densities and temperatures can span up to 6 orders of
magnitude at a given radius. The pair-instability supernovae of Population III
stars alone enrich the halos with virial temperatures of 10^4 K to
approximately 10^{-3} of solar metallicity. We find that 40% of the heavy
elements resides in the intergalactic medium (IGM) at the end of our
calculations. The highest metallicity gas exists in supernova remnants and very
dilute regions of the IGM.Comment: 15 pages, 16 figures, accepted to ApJ. Many changes, including
estimates of metal line cooling. High resolution images and movies available
at http://www.slac.stanford.edu/~jwise/research/PGalaxies3
A Keck High Resolution Spectroscopic Study of the Orion Nebula Proplyds
We present the results of spectroscopy of four bright proplyds in the Orion
Nebula obtained at a velocity resolution of 6 km/s. After careful isolation of
the proplyd spectra from the confusing nebular radiation, the emission line
profiles are compared with those predicted by realistic dynamic/photoionization
models of the objects. The spectral line widths show a clear correlation with
ionization potential, which is consistent with the free expansion of a
transonic, ionization-stratified, photoevaporating flow. Fitting models of such
a flow simultaneously to our spectra and HST emission line imaging provides
direct measurements of the proplyd size, ionized density and outflow velocity.
These measurements confirm that the ionization front in the proplyds is
approximately D-critical and provide the most accurate and robust estimate to
date of the proplyd mass loss rate. Values of 0.7E-6 to 1.5E-6 Msun/year are
found for our spectroscopic sample, although extrapolating our results to a
larger sample of proplyds implies that 0.4E-6 Msun/year is more typical of the
proplyds as a whole. In view of the reported limits on the masses of the
circumstellar disks within the proplyds, the length of time that they can have
been exposed to ionizing radiation should not greatly exceed 10,000 years - a
factor of 30 less than the mean age of the proplyd stars. We review the various
mechanisms that have been proposed to explain this situation, and conclude that
none can plausibly work unless the disk masses are revised upwards by a
substantial amount.Comment: 23 pages, 8 figures, uses emulateapj.sty, accepted for publication in
The Astronomical Journal (scheduled November 1999
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