90 research outputs found
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 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
SPH simulations of star/planet formation triggered by cloud-cloud collisions
We present results of hydrodynamic simulations of star formation triggered by
cloud-cloud collisions. During the early stages of star formation, low-mass
objects form by gravitational instabilities in protostellar discs. A number of
these low-mass objects are in the sub-stellar mass range, including a few
objects of planetary mass. The disc instabilities that lead to the formation of
low-mass objects in our simulations are the product of disc-disc interactions
and/or interactions between the discs and their surrounding gas.Comment: 8 pages, 7 figures; accepted for publication in the proceedings of
IAU Symposium 249: Exoplanets: Detection, Formation and Dynamics, Y.-S. Sun,
S. Ferraz-Mello & J.-L. Zhou (eds.), Cambridge University Pres
On the evolution of the density pdf in strongly self-gravitating systems
The time evolution of the probability density function (PDF) of the mass
density is formulated and solved for systems in free-fall using a simple
appoximate function for the collapse of a sphere. We demonstrate that a
pressure-free collapse results in a power-law tail on the high-density side of
the PDF. The slope quickly asymptotes to the functional form
for the (volume-weighted) PDF and
for the corresponding mass-weighted
distribution. From the simple approximation of the PDF we derive analytic
descriptions for mass accretion, finding that dynamically quiet systems with
narrow density PDFs lead to retarded star formation and low star formation
rates. Conversely, strong turbulent motions that broaden the PDF accelerate the
collapse causing a bursting mode of star formation. Finally, we compare our
theoretical work with observations. The measured star formation rates are
consistent with our model during the early phases of the collapse. Comparison
of observed column density PDFs with those derived from our model suggests that
observed star-forming cores are roughly in free-fall.Comment: accepted for publication, 13 page
How do brown dwarves form?
We review and evaluate four mechanisms for forming brown dwarves: (i)
dynamical ejection of a stellar embryo from its placental prestellar core; (ii)
opacity-limited fragmentation of a shock-compressed layer; (iii) gravitational
instabilities in discs, triggered by impulsive interactions with other discs or
naked stars; and (iv) photo-erosion of pre-existing cores. All these mechanisms
can produce free-floating brown dwarves, but only (ii) and (iii) are likely to
produce brown dwarves in multiple systems, and (i) has difficulty delivering
brown dwarves with discs.Comment: To appear in the proceedings of "Low Mass Stars and Brown Dwarfs:
IMF, Accretion and Activity" (Volterra, 2004). 6 pages, 1 figur
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