75 research outputs found
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
Discovery of a Wide Substellar Companion to a Nearby Low-Mass Star
We report the discovery of a wide (135+/-25 AU), unusually blue L5 companion
2MASS J17114559+4028578 to the nearby M4.5 dwarf G 203-50 as a result of a
targeted search for common proper motion pairs in the Sloan Digital Sky Survey
and the Two Micron All Sky Survey. Adaptive Optics imaging with Subaru
indicates that neither component is a nearly equal mass binary with separation
> 0.18", and places limits on the existence of additional faint companions. An
examination of TiO and CaH features in the primary's spectrum is consistent
with solar metallicity and provides no evidence that G 203-50 is metal poor. We
estimate an age for the primary of 1-5 Gyr based on activity. Assuming
coevality of the companion, its age, gravity and metallicity can be constrained
from properties of the primary, making it a suitable benchmark object for the
calibration of evolutionary models and for determining the atmospheric
properties of peculiar blue L dwarfs. The low total mass (M_tot=0.21+/-0.03
M_sun), intermediate mass ratio (q=0.45+/-0.14), and wide separation of this
system demonstrate that the star formation process is capable of forming wide,
weakly bound binary systems with low mass and BD components. Based on the
sensitivity of our search we find that no more than 2.2% of early-to-mid M
dwarfs (9.0 0.06 M_sun.Comment: 24 pages, 5 figures, accepted for publication in Ap
Interactions between brown-dwarf binaries and Sun-like stars
Several mechanisms have been proposed for the formation of brown dwarfs, but
there is as yet no consensus as to which -- if any -- are operative in nature.
Any theory of brown dwarf formation must explain the observed statistics of
brown dwarfs. These statistics are limited by selection effects, but they are
becoming increasingly discriminating. In particular, it appears (a) that brown
dwarfs that are secondaries to Sun-like stars tend to be on wide orbits, a\ga
100\,{\rm AU} (the Brown Dwarf Desert), and (b) that these brown dwarfs have a
significantly higher chance of being in a close (a\la 10\,{\rm AU}) binary
system with another brown dwarf than do brown dwarfs in the field. This then
raises the issue of whether these brown dwarfs have formed {\it in situ}, i.e.
by fragmentation of a circumstellar disc; or have formed elsewhere and
subsequently been captured. We present numerical simulations of the purely
gravitational interaction between a close brown-dwarf binary and a Sun-like
star. These simulations demonstrate that such interactions have a negligible
chance () of leading to the close brown-dwarf binary being captured by
the Sun-like star. Making the interactions dissipative by invoking the
hydrodynamic effects of attendant discs might alter this conclusion. However,
in order to explain the above statistics, this dissipation would have to favour
the capture of brown-dwarf binaries over single brown-dwarfs, and we present
arguments why this is unlikely. The simplest inference is that most brown-dwarf
binaries -- and therefore possibly also most single brown dwarfs -- form by
fragmentation of circumstellar discs around Sun-like protostars, with some of
them subsequently being ejected into the field.Comment: 10 pages, 8 figures, Accepted for publication in Astrophysics and
Space Scienc
A Hybrid Scenario for the Formation of Brown Dwarfs and Very Low Mass Stars
We present a calculation of protostellar disk formation and evolution in
which gaseous clumps (essentially, the first Larson cores formed via disk
fragmentation) are ejected from the disk during the early stage of evolution.
This is a universal process related to the phenomenon of ejection in multiple
systems of point masses. However, it occurs in our model entirely due to the
interaction of compact, gravitationally-bound gaseous clumps and is free from
the smoothing-length uncertainty that is characteristic of models using sink
particles. Clumps that survive ejection span a mass range of 0.08--0.35
, and have ejection velocities km s, which are
several times greater than the escape speed. We suggest that, upon contraction,
these clumps can form substellar or low-mass stellar objects with notable
disks, or even close-separation very-low-mass binaries. In this hybrid
scenario, allowing for ejection of clumps rather than finished
protostars/proto--brown-dwarfs, disk formation and the low velocity dispersion
of low-mass objects are naturally explained, while it is also consistent with
the observation of isolated low-mass clumps that are ejection products. We
conclude that clump ejection and the formation of isolated low mass stellar and
substellar objects is a common occurrence, with important implications for
understanding the initial mass function, the brown dwarf desert, and the
formation of stars in all environments and epochs.Comment: 20 pages, 6 figures, to appear in The Astrophysical Journa
Radiative equilibrium in Monte Carlo radiative transfer using frequency distribution adjustment
The Monte Carlo method is a powerful tool for performing radiative
equilibrium calculations, even in complex geometries. The main drawback of the
standard Monte Carlo radiative equilibrium methods is that they require
iteration, which makes them numerically very demanding. Bjorkman & Wood
recently proposed a frequency distribution adjustment scheme, which allows
radiative equilibrium Monte Carlo calculations to be performed without
iteration, by choosing the frequency of each re-emitted photon such that it
corrects for the incorrect spectrum of the previously re-emitted photons.
Although the method appears to yield correct results, we argue that its
theoretical basis is not completely transparent, and that it is not completely
clear whether this technique is an exact rigorous method, or whether it is just
a good and convenient approximation. We critically study the general problem of
how an already sampled distribution can be adjusted to a new distribution by
adding data points sampled from an adjustment distribution. We show that this
adjustment is not always possible, and that it depends on the shape of the
original and desired distributions, as well as on the relative number of data
points that can be added. Applying this theorem to radiative equilibrium Monte
Carlo calculations, we provide a firm theoretical basis for the frequency
distribution adjustment method of Bjorkman & Wood, and we demonstrate that this
method provides the correct frequency distribution through the additional
requirement of radiative equilibrium. We discuss the advantages and limitations
of this approach, and show that it can easily be combined with the presence of
additional heating sources and the concept of photon weighting. However, the
method may fail if small dust grains are included... (abridged)Comment: 17 pages, 2 figures, accepted for publication in New Astronom
Secular evolution of viscous and self-gravitating circumstellar discs
We add the effect of turbulent viscosity via the \alpha-prescription to
models of the self-consistent formation and evolution of protostellar discs.
Our models are non-axisymmetric and carried out using the thin-disc
approximation. Self-gravity plays an important role in the early evolution of a
disc, and the later evolution is determined by the relative importance of
gravitational and viscous torques. In the absence of viscous torques, a
protostellar disc evolves into a self-regulated state with disk-averaged Toomre
parameter Q \sim 1.5-2.0, non-axisymmetric structure diminishing with time, and
maximum disc-to-star mass ratio \xi = 0.14. We estimate an effective viscosity
parameter \alpha_eff associated with gravitational torques at the inner
boundary of our simulation to be in the range 10^{-4}-10^{-3} during the late
evolution. Addition of viscous torques with a low value \alpha = 10^{-4} has
little effect on the evolution, structure, and accretion properties of the
disc, and the self-regulated state is largely preserved. A sequence of
increasing values of \alpha results in the discs becoming more axisymmetric in
structure, being more gravitationally stable, having greater accretion rates,
larger sizes, shorter lifetimes, and lower disc-to-star mass ratios. For
\alpha=10^{-2}, the model is viscous-dominated and the self-regulated state
largely disappears by late times. (Abridged)Comment: 13 pages, 11 figures, accepted for publication in MNRA
Recent advances on IMF research
Here I discuss recent work on brown dwarfs, massive stars and the IMF in
general. The stellar IMF can be well described by an invariant two-part power
law in present-day star-formation events within the Local Group of galaxies. It
is nearly identical in shape to the pre-stellar core mass function. The
majority of brown dwarfs follow a separate IMF. Evidence from globular clusters
and ultra-compact dwarf galaxies has emerged that IMFs may have been top heavy
depending on the star-formation rate density. The IGIMF then ranges from bottom
heavy at low galaxy-wide star formation rates to being top-heavy in
galaxy-scale star bursts.Comment: 6 pages, LaTeX, to appear in The Labyrinth of Star Formation, 18-22
June 2012, Crete, (eds.) D. Stamatellos, S. Goodwin, and D. Ward-Thompson,
Springer, in press; replaced version: very minor corrections plus the
addition of reference Smith & Lucey (2013) on the bottom-heavy IMF in
elliptical galaxie
A search for pre-substellar cores and proto-brown dwarf candidates in Taurus: multiwavelength analysis in the B213-L1495 clouds
In an attempt to study whether the formation of brown dwarfs (BDs) takes
place as a scaled-down version of low-mass stars, we conducted IRAM30m/MAMBO-II
observations at 1.2 mm in a sample of 12 proto-BD candidates selected from
Spitzer/IRAC data in the B213-L1495 clouds in Taurus. Subsequent observations
with the CSO at 350 micron, VLA at 3.6 and 6 cm, and IRAM30m/EMIR in the
12CO(1-0), 13CO(1-0), and N2H+(1-0) transitions were carried out toward the two
most promising Spitzer/IRAC source(s), J042118 and J041757. J042118 is
associated with a compact (<10 arcsec or <1400 AU) and faint source at 350
micron, while J041757 is associated with a partially resolved (~16 arcsec or
~2000 AU) and stronger source emitting at centimetre wavelengths with a flat
spectral index. The corresponding masses of the dust condensations are ~1 and
~5 Mjup for J042118 and J041757, respectively. In addition, about 40 arcsec to
the northeast of J041757 we detect a strong and extended submillimetre source,
J041757-NE, which is not associated with NIR/FIR emission down to our detection
limits, but is clearly detected in 13CO and N2H+ at ~7 km/s, and for which we
estimated a total mass of ~100 Mjup, close to the mass required to be
gravitationally bound. In summary, our observational strategy has allowed us to
find in B213-L1495 two proto-BD candidates and one pre-substellar core
candidate, whose properties seem to be consistent with a scaled-down version of
low-mass stars.Comment: MNRAS, 424, 2778; corrected typos, mass estimate refined in Section
3.2.1 and Section 5.3; conclusions unchange
Disk masses in the embedded and T Tauri phases of stellar evolution
(Abridged). Motivated by a growing concern that masses of circumstellar disks
may have been systematically underestimated by conventional observational
methods, we present a numerical hydrodynamics study of time-averaged disk
masses () around low-mass Class 0, Class I, and Class II objects. Mean
disk masses (\overline{M}_d}) are then calculated by weighting the
time-averaged disk masses according to the corresponding stellar masses using a
power-law weight function with a slope typical for the Kroupa initial mass
function of stars. Two distinct types of disks are considered: self-gravitating
disks, in which mass and angular momentum are redistributed exclusively by
gravitational torques, and viscous disks, in which both the gravitational and
viscous torques are at work. We find that self-gravitating disks have mean
masses that are slowly increasing along the sequence of stellar evolution
phases. More specifically, Class 0/I/II self-gravitating disks have mean masses
\overline{M}_d=0.09, 0.10, and 0.12 M_sun, respectively. Viscous disks have
similar mean masses (\overline{M}_d=0.10-0.11 M_sun) in the Class 0/I phases
but almost a factor of 2 lower mean mass in the Class II phase
(\overline{M}_d,CII=0.06 M_sun). In each evolution phase, time-averaged disk
masses show a large scatter around the mean value. Our obtained mean disk
masses are larger than those recently derived by Andrews & Williams and Brown
et al., regardless of the physical mechanisms of mass transport in the disk.Comment: 13 pages, 7 figures, accepted for publication in Ap
AMI Large Array radio continuum observations of Spitzer c2d small clouds and cores
We perform deep 1.8 cm radio continuum imaging towards thirteen protostellar
regions selected from the Spitzer c2d small clouds and cores programme at high
resolution (25") in order to detect and quantify the cm-wave emission from
deeply embedded young protostars. Within these regions we detect fifteen
compact radio sources which we identify as radio protostars including two
probable new detections. The sample is in general of low bolometric luminosity
and contains several of the newly detected VeLLO sources. We determine the 1.8
cm radio luminosity to bolometric luminosity correlation, L_rad -L_bol, for the
sample and discuss the nature of the radio emission in terms of the available
sources of ionized gas. We also investigate the L_rad-L_IR correlation and
suggest that radio flux density may be used as a proxy for the internal
luminosity of low luminosity protostars.Comment: submitted MNRA
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