319 research outputs found
Brown dwarfs forming in discs: where to look for them?
A large fraction of the observed brown dwarfs may form by gravitational
fragmentation of unstable discs. This model reproduces the brown dwarf desert,
and provides an explanation the existence of planetary-mass objects and for the
binary properties of low-mass objects. We have performed an ensemble of
radiative hydrodynamic simulations and determined the statistical properties of
the low-mass objects produced by gravitational fragmentation of discs. We
suggest that there is a population of brown dwarfs loosely bound on wide orbits
(100-5000 AU) around Sun-like stars that surveys of brown dwarf companions
should target. Our simulations also indicate that planetary-mass companions to
Sun-like stars are unlikely to form by disc fragmentation.Comment: To appear in the proceedings of the conference "New technologies for
probing the diversity of brown dwarfs and exoplanets", Shanghai 200
Can giant planets form by gravitational fragmentation of discs?
Gravitational fragmentation has been proposed as a mechanism for the
formation of giant planets in close orbits around solar-type stars. However, it
is debatable whether this mechanism can function in the inner regions (R<40 AU)
of real discs. We use a newly developed method for treating the energy equation
and the equation of state, which accounts for radiative transfer effects in SPH
simulations of circumstellar discs. The different chemical and internal states
of hydrogen and the properties of dust at different densities and temperatures
(ice coated dust grains at low temperatures, ice melting, dust sublimation) are
all taken into account by the new method.We present radiative hydrodynamic
simulations of the inner regions of massive circumstellar discs and examine two
cases: (i) a disc irradiated by a cool background radiation field
(T_bgr=10K)and (ii) a disc heated by radiation from its central star
(T_bgr~1/R). In neither case does the disc fragment: in the former because it
cannot cool fast enough and in the latter because it is not gravitationally
unstable. Our results (a) corroborate previous numerical results using
different treatments for the hydrodynamics and the radiative transfer, and (b)
confirm our own earlier analytic predictions. We conclude that disc
fragmentation is unlikely to be able to produce giant planets around solar-type
stars at radii <40 AU.Comment: Accepted by A&A, 10 pages, high-resolution available at
http://www.astro.cf.ac.uk/pub/Dimitrios.Stamatellos/publications
The statistical Analysis of Star Clusters
We review a range of stastistical methods for analyzing the structures of
star clusters, and derive a new measure which both quantifies, and
distinguishes between, a (relatively smooth) large-scale radial density
gradient and multi-scale (fractal) sub-clustering. Q is derived from the
normalised correlation length and the normalised edge length of the minimal
spanning tree for each cluster
The formation of brown dwarfs in discs: Physics, numerics, and observations
A large fraction of brown dwarfs and low-mass stars may form by gravitational
fragmentation of relatively massive (a few 0.1 Msun), extended (a few hundred
AU) discs around Sun-like stars. We present an ensemble of radiative
hydrodynamic simulations that examine the conditions for disc fragmentation. We
demonstrate that this model can explain the low-mass IMF, the brown dwarf
desert, and the binary properties of low-mass stars and brown dwarfs. Observing
discs that are undergoing fragmentation is possible but very improbable, as the
process of disc fragmentation is short lived (discs fragment within a few
thousand years).Comment: 4 pages, for the proceedings of IAU Symposium 270: Computational Star
Formation, Barcelona, 201
Brown dwarf formation by gravitational fragmentation of massive, extended protostellar discs
We suggest that low-mass hydrogen-burning stars like the Sun should sometimes
form with massive extended discs; and we show, by means of radiation
hydrodynamic simulations, that the outer parts of such discs (R>100 AU) are
likely to fragment on a dynamical timescale (10^3 to $10^4 yr), forming
low-mass companions: principally brown dwarfs (BDs), but also very low-mass
hydrogen-burning stars and planetary-mass objects. A few of the BDs formed in
this way remain attached to the primary star, orbiting at large radii. The
majority are released into the field, by interactions amongst themselves; in so
doing they acquire only a low velocity dispersion (<2 km/s), and therefore they
usually retain small discs, capable of registering an infrared excess and
sustaining accretion. Some BDs form close BD/BD binaries, and these binaries
can survive ejection into the field. This BD formation mechanism appears to
avoid some of the problems associated with the `embryo ejection' scenario, and
to answer some of the questions not yet answered by the `turbulent
fragmentation' scenario.Comment: 5 pages, accepted for publication in MNRAS Letter
Episodic accretion, radiative feedback, and their role in low-mass star formation
It is speculated that the accretion of material onto young protostars is
episodic. We present a computational method to include the effects of episodic
accretion in radiation hydrodynamic simulations of star formation. We find that
during accretion events protostars are "switched on", heating and stabilising
the discs around them. However, these events typically last only a few hundred
years, whereas the intervals in between them may last for a few thousand years.
During these intervals the protostars are effectively "switched off", allowing
gravitational instabilities to develop in their discs and induce fragmentation.
Thus, episodic accretion promotes disc frag- mentation, enabling the formation
of low-mass stars, brown dwarfs and planetary-mass objects. The frequency and
the duration of episodic accretion events may be responsible for the low-mass
end of the IMF, i.e. for more than 60% of all stars.Comment: To appear in the proceedings of the 9th Pacific Rim Conference of
Stellar Astrophysics, Lijiang, China, 201
The formation of brown dwarfs and low-mass stars by disc fragmentation
We suggest that a high proportion of brown dwarfs are formed by gravitational
fragmentation of massive, extended discs around Sun-like stars. We argue that
such discs should arise frequently, but should be observed infrequently,
precisely because they fragment rapidly. By performing an ensemble of
radiation-hydrodynamic simulations, we show that such discs typically fragment
within a few thousand years to produce mainly brown dwarfs (including
planetary-mass brown dwarfs) and low-mass hydrogen-burning stars. Subsequently
most of the brown dwarfs are ejected by mutual interactions. We analyse the
properties of these objects that form by disc fragmentation, and compare them
with observations.Comment: 4 pages, 2 figures, to appear in the proceedings of the Cool Stars 15
conferenc
The intrinsic shapes of starless cores in Ophiuchus
Using observations of cores to infer their intrinsic properties requires the
solution of several poorly constrained inverse problems. Here we address one of
these problems, namely to deduce from the projected aspect ratios of the cores
in Ophiuchus their intrinsic three-dimensional shapes. Four models are
proposed, all based on the standard assumption that cores are randomly
orientated ellipsoids, and on the further assumption that a core's shape is not
correlated with its absolute size. The first and simplest model, M1, has a
single free parameter, and assumes that the relative axes of a core are drawn
randomly from a log-normal distribution with zero mean and standard deviation
\sigma o. The second model, M2a, has two free parameters, and assumes that the
log-normal distribution (with standard deviation \sigma o) has a finite mean,
\mu o, defined so that \mu o<0 means elongated (prolate) cores are favoured,
whereas \mu o>0 means flattened (oblate) cores are favoured. Details of the
third model (M2b, two free parameters) and the fourth model (M4, four free
parameters) are given in the text. Markov chain Monte Carlo sampling and
Bayesian analysis are used to map out the posterior probability density
functions of the model parameters, and the relative merits of the models are
compared using Bayes factors. We show that M1 provides an acceptable fit to the
Ophiuchus data with \sigma o ~ 0.57+/-0.06; and that, although the other models
sometimes provide an improved fit, there is no strong justification for the
introduction of their additional parameters.Comment: 10 pages, 8 figures. Accepted by MNRA
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