135 research outputs found
Analytical theory for the initial mass function: III time dependence and star formation rate
The present paper extends our previous theory of the stellar initial mass
function (IMF) by including the time-dependence, and by including the impact of
magnetic field. The predicted mass spectra are similar to the time independent
ones with slightly shallower slopes at large masses and peak locations shifted
toward smaller masses by a factor of a few. Assuming that star-forming clumps
follow Larson type relations, we obtain core mass functions in good agreement
with the observationally derived IMF, in particular when taking into account
the thermodynamics of the gas. The time-dependent theory directly yields an
analytical expression for the star formation rate (SFR) at cloud scales. The
SFR values agree well with the observational determinations of various Galactic
molecular clouds. Furthermore, we show that the SFR does not simply depend
linearly on density, as sometimes claimed in the literature, but depends also
strongly on the clump mass/size, which yields the observed scatter. We stress,
however, that {\it any} SFR theory depends, explicitly or implicitly, on very
uncertain assumptions like clump boundaries or the mass of the most massive
stars that can form in a given clump, making the final determinations uncertain
by a factor of a few. Finally, we derive a fully time-dependent model for the
IMF by considering a clump, or a distribution of clumps accreting at a constant
rate and thus whose physical properties evolve with time. In spite of its
simplicity, this model reproduces reasonably well various features observed in
numerical simulations of converging flows. Based on this general theory, we
present a paradigm for star formation and the IMF.Comment: accepted for publication in Ap
Ambipolar diffusion in low-mass star formation. I. General comparison with the ideal MHD case
In this paper, we provide a more accurate description of the evolution of the
magnetic flux redistribution during prestellar core collapse by including
resistive terms in the magnetohydrodynamics (MHD) equations. We focus more
particularly on the impact of ambipolar diffusion. We use the adaptive mesh
refinement code RAMSES to carry out such calculations. The resistivities
required to calculate the ambipolar diffusion terms were computed using a
reduced chemical network of charged, neutral and grain species. The inclusion
of ambipolar diffusion leads to the formation of a magnetic diffusion barrier
in the vicinity of the core, preventing accumulation of magnetic flux in and
around the core and amplification of the field above 0.1G. The mass and radius
of the first Larson core remain similar between ideal and non-ideal MHD models.
This diffusion plateau has crucial consequences on magnetic braking processes,
allowing the formation of disk structures. Magnetically supported outflows
launched in ideal MHD models are weakened when using non-ideal MHD. Contrary to
ideal MHD misalignment between the initial rotation axis and the magnetic field
direction does not significantly affect the results for a given mu, showing
that the physical dissipation truly dominate over numerical diffusion. We
demonstrate severe limits of the ideal MHD formalism, which yield unphysical
behaviours in the long-term evolution of the system. This includes counter
rotation inside the outflow, interchange instabilities, and flux redistribution
triggered by numerical diffusion, none observed in non-ideal MHD. Disks with
Keplerian velocity profiles form in all our non-ideal MHD simulations, with
final mass and size which depend on the initial magnetisation. This ranges from
a few 0.01 solar masses and 20-30 au for the most magnetised case (mu=2) to 0.2
solar masses and 40-80 au for a lower magnetisation (mu=5).Comment: Accepted in A&A section 7 (on Wednesday, september the 16th, year
2015
Radiative, magnetic and numerical feedbacks on small-scale fragmentation
Radiative feedback and magnetic field are understood to have a strong impact
on the protostellar collapse. We present high resolution numerical calculations
of the collapse of a 1 solar mass dense core in solid body rotation, including
both radiative transfer and magnetic field. Using typical parameters for
low-mass cores, we study thoroughly the effect of radiative transfer and
magnetic field on the first core formation and fragmentation. We show that
including the two aforementioned physical processes does not correspond to the
simple picture of adding them separately. The interplay between the two is
extremely strong, via the magnetic braking and the radiation from the accretion
shock.Comment: 4 pages, 2 figures ; to appear in "IAU Symposium 270: Computational
Star formation", Eds. J. Alves, B. Elmegreen, J. Girart, V. Trimbl
ofw: An R package to select continuous variables for multiclass classification with a stochastic wrapper method
When dealing with high dimensional and low sample size data, feature selection is often needed to help reduce the dimension of the variable space while optimizing the classification task. Few tools exist for selecting variables in such data sets, especially when classes are numerous ( > 2). We have developed ofw, an R package that implements, in the context of classification, the meta algorithm "optimal feature weighting". We focus on microarray data, although the method can be applied to any p >> n problems with continuous variables. The aim is to select relevant variables and to numerically evaluate the resulting variable selection. Two versions are proposed with the application of supervised multiclass classifiers such as classification and regression trees and support vector machines. Furthermore, a weighted approach can be chosen to deal with unbalanced multiclasses, a common characteristic in microarray data sets
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