5,793 research outputs found
On the shape of the mass-function of dense clumps in the Hi-GAL fields. II. Using Bayesian inference to study the clump mass function
Context. Stars form in dense, dusty clumps of molecular clouds, but little is
known about their origin, their evolution and their detailed physical
properties. In particular, the relationship between the mass distribution of
these clumps (also known as the "clump mass function", or CMF) and the stellar
initial mass function (IMF), is still poorly understood. Aims. In order to
better understand how the CMF evolve toward the IMF, and to discern the "true"
shape of the CMF, large samples of bona-fide pre- and proto-stellar clumps are
required. Two such datasets obtained from the Herschel infrared GALactic Plane
Survey (Hi-GAL) have been described in paper I. Robust statistical methods are
needed in order to infer the parameters describing the models used to fit the
CMF, and to compare the competing models themselves. Methods. In this paper we
apply Bayesian inference to the analysis of the CMF of the two regions
discussed in Paper I. First, we determine the Bayesian posterior probability
distribution for each of the fitted parameters. Then, we carry out a
quantitative comparison of the models used to fit the CMF. Results. We have
compared the results from several methods implementing Bayesian inference, and
we have also analyzed the impact of the choice of priors and the influence of
various constraints on the statistical conclusions for the preferred values of
the parameters. We find that both parameter estimation and model comparison
depend on the choice of parameter priors. Conclusions. Our results confirm our
earlier conclusion that the CMFs of the two Hi-GAL regions studied here have
very similar shapes but different mass scales. Furthermore, the lognormal model
appears to better describe the CMF measured in the two Hi-GAL regions studied
here. However, this preliminary conclusion is dependent on the choice of
parameters priors.Comment: Submitted for publication to A&A on November 12, 2013. This paper
contains 11 pages and 7 figure
Theory of Cluster Formation: Effects of Magnetic Fields
Stars form predominantly in clusters inside dense clumps of molecular clouds
that are both turbulent and magnetized. The typical size and mass of the
cluster-forming clumps are pc and 10 M,
respectively. Here, we discuss some recent progress on numerical simulations of
clustered star formation in such parsec-scale dense clumps with emphasis on the
role of magnetic fields. The simulations have shown that magnetic fields tend
to slow down global gravitational collapse and thus star formation, especially
in the presence of protostellar outflow feedback. Even a relatively weak can
retard star formation significantly, because the field is amplified by
supersonic turbulence to an equipartition strength. However, in such a case,
the distorted field component dominates the uniform one. In contrast, if the
field is moderately strong, the uniform component remains dominant. Such a
difference in the magnetic structure is observed in simulated polarization maps
of dust thermal emission. Recent polarization measurements show that the field
lines in nearby cluster-forming clumps are spatially well-ordered, indicative
of a rather strong field. In such strongly-magnetized clumps, star formation
should proceed relatively slowly; it continues for at least several global
free-fall times of the parent dense clump ( a few yr).Comment: 8 pages, proceedings of Computational Star Formation (IAU 270
Clustered Star Formation in Magnetic Clouds: Properties of Dense Cores Formed in Outflow-Driven Turbulence
We investigate the physical properties of dense cores formed in turbulent,
magnetized, parsec-scale clumps of molecular clouds, using three-dimensional
numerical simulations that include protostellar outflow feedback. The dense
cores are identified in the simulated density data cube through a clumpfind
algorithm. We find that the core velocity dispersion does not show any clear
dependence on the core size, in contrast to Larson's linewidth-size relation,
but consistent with recent observations. In the absence of a magnetic field,
the majority of the cores have supersonic velocity dispersions. A
moderately-strong magnetic field reduces the dispersion to a subsonic or at
most transonic value typically. Most of the cores are out of virial
equilibrium, with the external pressure dominating the self-gravity. The
implication is that the core evolution is largely controlled by the
outflow-driven turbulence. Even an initially-weak magnetic field can retard
star formation significantly, because the field is amplified by the
outflow-driven turbulence to an equipartition strength, with the distorted
field component dominating the uniform one. In contrast, for a
moderately-strong field, the uniform component remains dominant. Such a
difference in the magnetic structure is evident in our simulated polarization
maps of dust thermal emission; it provides a handle on the field strength.
Recent polarization measurements show that the field lines in cluster-forming
clumps are spatially well-ordered. It is indicative of a moderately-strong,
dynamically important, field which, in combination with outflow feedback, can
keep the rate of star formation in embedded clusters at the
observationally-inferred, relatively-slow rate of several percent per free-fall
time.Comment: 49 pages, 16 figures accepted by The Astrophysical Journa
Incremental Learning of Nonparametric Bayesian Mixture Models
Clustering is a fundamental task in many vision applications.
To date, most clustering algorithms work in a
batch setting and training examples must be gathered in a
large group before learning can begin. Here we explore
incremental clustering, in which data can arrive continuously.
We present a novel incremental model-based clustering
algorithm based on nonparametric Bayesian methods,
which we call Memory Bounded Variational Dirichlet
Process (MB-VDP). The number of clusters are determined
flexibly by the data and the approach can be used to automatically
discover object categories. The computational requirements
required to produce model updates are bounded
and do not grow with the amount of data processed. The
technique is well suited to very large datasets, and we show
that our approach outperforms existing online alternatives
for learning nonparametric Bayesian mixture models
A Case Study of Small Scale Structure Formation in 3D Supernova Simulations
It is suggested in observations of supernova remnants that a number of large-
and small-scale structures form at various points in the explosion.
Multidimensional modeling of core-collapse supernovae has been undertaken since
SN1987A, and both simulations and observations suggest/show that
Rayleigh-Taylor instabilities during the explosion is a main driver for the
formation of structure in the remnants.
We present a case study of structure formation in 3D in a \msol{15} supernova
for different parameters. We investigate the effect of moderate asymmetries and
different resolutions of the formation and morphology of the RT unstable
region, and take first steps at determining typical physical quantities (size,
composition) of arising clumps. We find that in this progenitor the major RT
unstable region develops at the He/OC interface for all cases considered. The
RT instabilities result in clumps that are overdense by 1-2 orders of magnitude
with respect to the ambient gas, have size scales on the level of a few % of
the remnant diameter, and are not diffused after the first yrs of the
remnant evolution, in the absence of a surrounding medium.Comment: 59 pages, 34 figure
Circumplanetary disks around young giant planets: a comparison between core-accretion and disk instability
Circumplanetary disks can be found around forming giant planets, regardless
of whether core accretion or gravitational instability built the planet. We
carried out state-of-the-art hydrodynamical simulations of the circumplanetary
disks for both formation scenarios, using as similar initial conditions as
possible to unveil possible intrinsic differences in the circumplanetary disk
mass and temperature between the two formation mechanisms. We found that the
circumplanetary disks mass linearly scales with the circumstellar disk mass.
Therefore, in an equally massive protoplanetary disk, the circumplanetary disks
formed in the disk instability model can be only a factor of eight more massive
than their core-accretion counterparts. On the other hand, the bulk
circumplanetary disk temperature differs by more than an order of magnitude
between the two cases. The subdisks around planets formed by gravitational
instability have a characteristic temperature below 100 K, while the core
accretion circumplanetary disks are hot, with temperatures even greater than
1000 K when embedded in massive, optically thick protoplanetary disks. We
explain how this difference can be understood as the natural result of the
different formation mechanisms. We argue that the different temperatures should
persist up to the point when a full-fledged gas giant forms via disk
instability, hence our result provides a convenient criteria for observations
to distinguish between the two main formation scenarios by measuring the bulk
temperature in the planet vicinity.Comment: 12 pages, 9 figures, 1 table, accepted for publication at MNRA
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