495 research outputs found
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
The migration of gas giant planets in gravitationally unstable disks
Planets form in the disks of gas and dust that surround young stars. It is not known whether or not gas giant planets on wide orbits form the same way as Jupiter or form by the fragmentation of gravitationally unstable disks. Here we show that a giant planet that has formed in the outer regions of a protostellar disk initially migrates quickly toward the central star (migration timescale ~104 years) while accreting gas from the disk. However, in contrast with previous studies, we find that the planet eventually opens up a gap in the disk and the migration is essentially halted. At the same time, accretion-powered radiative feedback from the planet significantly limits its mass growth, keeping it within the planetary-mass regime, (i.e., below the deuterium burning limit) at least for the initial stages of disk evolution. Giant planets may therefore be able to survive on wide orbits despite their initial fast inward migration, consequently shaping the environment in which terrestrial planets that may harbor life can form
The dynamical evolution of low-mass hydrogen-burning stars, brown dwarfs, and planetary-mass objects formed through disk fragmentation
Theory and simulations suggest that it is possible to form low-mass hydrogen-burning stars, brown dwarfs (BDs), and planetary-mass objects (PMOs) via disk fragmentation. As disk fragmentation results in the formation of several bodies at comparable distances to the host star, their orbits are generally unstable. Here, we study the dynamical evolution of these objects. We set up the initial conditions based on the outcomes of the smoothed-particle hydrodynamics simulations of Stamatellos & Whitworth, and for comparison we also study the evolution of systems resulting from lower-mass fragmenting disks. We refer to these two sets of simulations as set 1 and set 2, respectively. At 10 Myr, approximately half of the host stars have one companion left, and approximately 22% (set 1) to 9.8% (set 2) of the host stars are single. Systems with multiple secondaries in relatively stable configurations are common (about 30% and 44%, respectively). The majority of the companions are ejected within1 Myr with velocities mostly below 5 km s−1, with some runaway escapers with velocities over 30 km s−1. Roughly 6% (set 1) and 2% (set 2) of the companions pair up into very low-mass binary systems, resulting in respective binary fractions of 3.2% and 1.2%. The majority of these pairs escape as very low-mass binaries, while others remain bound to the host star in hierarchical configurations (often with retrograde inner orbits). Physical collisions with the host star (0.43 and 0.18 events per host star for set 1 and set 2, respectively) and between companions (0.08 and 0.04 events per host star for set 1 and set 2, respectively) are relatively common and their frequency increases with increasing disk mass. Our study predicts observable properties of very low-mass binaries, low-mass hierarchical systems, the BD desert, and free-floating BDs and PMOs in and near young stellar groupings, which can be used to distinguish between different formation scenarios of very low-mass stars, BDs, and PMO
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
Monte Carlo Radiative Transfer in Embedded Prestellar Cores
We implement a Monte Carlo radiative transfer method, that uses a large
number of monochromatic luminosity packets to represent the radiation
transported through a system. These packets are injected into the system and
interact stochastically with it. We test our code against various benchmark
calculations and determine the number of packets required to obtain accurate
results under different circumstances. We then use this method to study cores
that are directly exposed to the interstellar radiation field (non-embedded
cores) and find similar results with previous studies. We also explore a large
number of models of cores that are embedded in the centre of a molecular cloud.
Our study indicates that the temperature profiles in embedded cores are less
steep than those in non-embedded cores. Deeply embedded cores (ambient cloud
with visual extinction larger than 15-25) are almost isothermal at around 7-8
K. The temperature inside cores surrounded by an ambient cloud of even moderate
thickness (Av~5) is less than 12 K, which is lower than previous studies have
assumed. Thus, previous mass calculations of embedded cores (for example in the
rho Ophiuchi protocluster), based on mm continuum observations, may
underestimate core masses by up to a factor of 2. Our study shows that the best
wavelength region to observe embedded cores is between 400 and 500 microns,
where the core is quite distinct from the background. We also predict that very
sensitive observations (~1-3 MJy/sr) at 170-200 microns can be used to estimate
how deeply a core is embedded in its parent molecular cloud. The upcoming
HERSCHEL mission (ESA, 2007) will, in principle, be able to detect these
features and test our models.Comment: 15 pages, 18 figures, accepted by A&
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
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
A natural formation scenario for misaligned and short-period eccentric extrasolar planets
Recent discoveries of strongly misaligned transiting exoplanets pose a
challenge to the established planet formation theory which assumes planetary
systems to form and evolve in isolation. However, the fact that the majority of
stars actually do form in star clusters raises the question how isolated
forming planetary systems really are. Besides radiative and tidal forces the
presence of dense gas aggregates in star-forming regions are potential sources
for perturbations to protoplanetary discs or systems. Here we show that
subsequent capture of gas from large extended accretion envelopes onto a
passing star with a typical circumstellar disc can tilt the disc plane to
retrograde orientation, naturally explaining the formation of strongly inclined
planetary systems. Furthermore, the inner disc regions may become denser, and
thus more prone to speedy coagulation and planet formation. Pre-existing
planetary systems are compressed by gas inflows leading to a natural occurrence
of close-in misaligned hot Jupiters and short-period eccentric planets. The
likelihood of such events mainly depends on the gas content of the cluster and
is thus expected to be highest in the youngest star clusters.Comment: 7 pages, 4 figures. Accepted for publication in MNRAS. Updated to
match published versio
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
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