595 research outputs found
A grid of 1D low-mass star formation collapse models
The current study was developed to provide a database of relatively simple
numerical simulations of protostellar collapse, as a template library for
observations of cores and very young protostars, and for researchers who wish
to test their chemical modeling under dynamic astrophysical conditions. It was
also designed to identify statistical trends that may appear when running many
models of the formation of low-mass stars by varying the initial conditions. A
large set of 143 calculations of the gravitational collapse of an isolated
sphere of gas with uniform temperature and a Bonnor-Ebert like density profile
was undertaken using a 1D fully implicit Lagrangian radiation hydrodynamics
code. The parameter space covered initial masses from 0.2 to 8 Msun,
temperatures of 5-30 K and radii between 3000 and 30,000 AU. A spread in the
thermal evolutionary tracks of the runs was found, due to differing initial
conditions and optical depths. Within less than an order of magnitude, all
first and second Larson cores had masses and radii independent of the initial
conditions. The time elapsed between the formation of the first and second
cores was found to strongly depend on the first core mass accretion rate, and
no first core in our grid of models lived for longer than 2000 years, before
the onset of the second collapse. The end product of a protostellar cloud
collapse, the second Larson core, is, at birth, a canonical object with a mass
and radius of about 3 Mjup and 8 Rjup, independent of its initial conditions.
The evolution sequence which brings the gas to stellar densities can however
proceed in a variety of scenarios, on different timescales, along different
isentropes, but each story line can largely be predicted by the initial
conditions. All the data from the simulations are publicly available at this
address: http://starformation.hpc.ku.dk/grid-of-protostars.Comment: 24 pages, 14 figures, accepted for publication in A&
Infall-Driven Protostellar Accretion and the Solution to the Luminosity Problem
We investigate the role of mass infall in the formation and evolution of
protostars. To avoid ad hoc initial and boundary conditions, we consider the
infall resulting self-consistently from modeling the formation of stellar
clusters in turbulent molecular clouds. We show that infall rates in turbulent
clouds are comparable to accretion rates inferred from protostellar
luminosities or measured in pre-main-sequence stars. They should not be
neglected in modeling the luminosity of protostars and the evolution of disks,
even after the embedded protostellar phase. We find large variations of infall
rates from protostar to protostar, and large fluctuations during the evolution
of individuals protostars. In most cases, the infall rate is initially of order
10\msun\ yr, and may either decay rapidly in the formation of
low-mass stars, or remain relatively large when more massive stars are formed.
The simulation reproduces well the observed characteristic values and scatter
of protostellar luminosities and matches the observed protostellar luminosity
function. The luminosity problem is therefore solved once realistic
protostellar infall histories are accounted for, with no need for extreme
accretion episodes. These results are based on a simulation of randomly-driven
magneto-hydrodynamic turbulence on a scale of 4pc, including self-gravity,
adaptive-mesh refinement to a resolution of 50AU, and accreting sink particles.
The simulation yields a low star formation rate, consistent with the
observations, and a mass distribution of sink particles consistent with the
observed stellar initial mass function during the whole duration of the
simulation, forming nearly 1,300 sink particles over 3.2 Myr.Comment: 21 pages, 16 figures, accepted for publication in Ap
The stellar IMF from Isothermal MHD Turbulence
We address the turbulent fragmentation scenario for the origin of the stellar
initial mass function (IMF), using a large set of numerical simulations of
randomly driven supersonic MHD turbulence. The turbulent fragmentation model
successfully predicts the main features of the observed stellar IMF assuming an
isothermal equation of state without any stellar feedback. As a test of the
model, we focus on the case of a magnetized isothermal gas, neglecting stellar
feedback, while pursuing a large dynamic range in both space and timescales
covering the full spectrum of stellar masses from brown dwarfs to massive
stars. Our simulations represent a generic 4 pc region within a typical
Galactic molecular cloud, with a mass of 3000 Msun and an rms velocity 10 times
the isothermal sound speed and 5 times the average Alfven velocity, in
agreement with observations. We achieve a maximum resolution of 50 au and a
maximum duration of star formation of 4.0 Myr, forming up to a thousand sink
particles whose mass distribution closely matches the observed stellar IMF. A
large set of medium-size simulations is used to test the sink particle
algorithm, while larger simulations are used to test the numerical convergence
of the IMF and the dependence of the IMF turnover on physical parameters
predicted by the turbulent fragmentation model. We find a clear trend toward
numerical convergence and strong support for the model predictions, including
the initial time evolution of the IMF. We conclude that the physics of
isothermal MHD turbulence is sufficient to explain the origin of the IMF.Comment: 25 pages, 21 figures, Accepted by Ap
Bassem Chit and Revolutionary Socialism in Lebanon
This article discusses the Lebanese activist and writer Bassem Chit as an example of the intellectual rebel in Lebanon and the Arab world. It analyses the ideological tradition of revolutionary socialism and the Arab left. Through an analysis of interviews and articles, Haugbolle attempts to locate the place and nature of intellectual production in the organisation of revolutionary activity, and the particular role rebel intellectuals play in bringing about social change. It draws on the sociology of intellectuals, in particular Gramsci, in the analysis of Bassem Chitâs work and his post mortem veneration
- âŚ