1,945 research outputs found
The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets
There is no universally acknowledged criterion to distinguish brown dwarfs
from planets. Numerous studies have used or suggested a definition based on an
object's mass, taking the ~13-Jupiter mass (M_J) limit for the ignition of
deuterium. Here, we investigate various deuterium-burning masses for a range of
models. We find that, while 13 M_J is generally a reasonable rule of thumb, the
deuterium fusion mass depends on the helium abundance, the initial deuterium
abundance, the metallicity of the model, and on what fraction of an object's
initial deuterium abundance must combust in order for the object to qualify as
having burned deuterium. Even though, for most proto-brown dwarf conditions,
50% of the initial deuterium will burn if the object's mass is ~(13.0 +/-
0.8)M_J, the full range of possibilities is significantly broader. For models
ranging from zero-metallicity to more than three times solar metallicity, the
deuterium burning mass ranges from ~11.0 M_J (for 3-times solar metallicity,
10% of initial deuterium burned) to ~16.3 M_J (for zero metallicity, 90% of
initial deuterium burned).Comment: "Models" section expanded, references added, accepted by Ap
The Evolution of L and T Dwarfs in Color-Magnitude Diagrams
We present new evolution sequences for very low mass stars, brown dwarfs and
giant planets and use them to explore a variety of influences on the evolution
of these objects. We compare our results with previous work and discuss the
causes of the differences and argue for the importance of the surface boundary
condition provided by atmosphere models including clouds.
The L- to T-type ultracool dwarf transition can be accommodated within the
Ackerman & Marley (2001) cloud model by varying the cloud sedimentation
parameter. We develop a simple model for the evolution across the L/T
transition. By combining the evolution calculation and our atmosphere models,
we generate colors and magnitudes of synthetic populations of ultracool dwarfs
in the field and in galactic clusters. We focus on near infrared color-
magnitude diagrams (CMDs) and on the nature of the ``second parameter'' that is
responsible for the scatter of colors along the Teff sequence. Variations in
metallicity and cloud parameters, unresolved binaries and possibly a relatively
young population all play a role in defining the spread of brown dwarfs along
the cooling sequence. We find that the transition from cloudy L dwarfs to
cloudless T dwarfs slows down the evolution and causes a pile up of substellar
objects in the transition region, in contradiction with previous studies. We
apply the same model to the Pleiades brown dwarf sequence. Taken at face value,
the Pleiades data suggest that the L/T transition occurs at lower Teff for
lower gravity objects. The simulated populations of brown dwarfs also reveal
that the phase of deuterium burning produces a distinctive feature in CMDs that
should be detectable in ~50-100 Myr old clusters.Comment: Accepted for publication in the ApJ. 52 pages including 20 figure
Near-Infrared Spectroscopy of the Y0 WISEP J173835.52+273258.9 and the Y1 WISE J035000.32-565830.2: the Importance of Non-Equilibrium Chemistry
We present new near-infrared spectra, obtained at Gemini Observatory, for two
Y dwarfs: WISE J035000.32-565830.2 (W0350) and WISEP J173835.52+273258.9
(W1738). A FLAMINGOS-2 R=540 spectrum was obtained for W0350, covering 1.0 <
lambda um < 1.7, and a cross-dispersed GNIRS R=2800 spectrum was obtained for
W1738, covering 0.993-1.087 um, 1.191-1.305 um, 1.589-1.631 um, and 1.985-2.175
um, in four orders. We also present revised YJH photometry for W1738, using new
NIRI Y and J imaging, and a re-analysis of the previously published NIRI H band
images. We compare these data, together with previously published data for
late-T and Y dwarfs, to cloud-free models of solar metallicity, calculated both
in chemical equilibrium and with disequilibrium driven by vertical transport.
We find that for the Y dwarfs the non-equilibrium models reproduce the
near-infrared data better than the equilibrium models. The remaining
discrepancies suggest that fine-tuning the CH_4/CO and NH_3/N_2 balance is
needed. Improved trigonometric parallaxes would improve the analysis. Despite
the uncertainties and discrepancies, the models reproduce the observed
near-infrared spectra well. We find that for the Y0, W1738, T_eff = 425 +/- 25
K and log g = 4.0 +/- 0.25, and for the Y1, W0350, T_eff = 350 +/- 25 K and log
g = 4.0 +/- 0.25. W1738 may be metal-rich. Based on evolutionary models, these
temperatures and gravities correspond to a mass range for both Y dwarfs of 3-9
Jupiter masses, with W0350 being a cooler, slightly older, version of W1738;
the age of W0350 is 0.3-3 Gyr, and the age of W1738 is 0.15-1 Gyr.Comment: Accepted on March 30 2016 for publication in Ap
A new equation of state for dense hydrogen-helium mixtures
This is the final version. Available from American Astronomical Society via the DOI in this recordWe present a new equation of state (EOS) for dense hydrogen/helium mixtures that covers a range of densities
from 10−8 to 106 g cm-3, pressures from 10−9 to 1013 GPa, and temperatures from 102 to 108 K. The calculations
combine the EOS of Saumon, Chabrier & van Horn in the low-density, low-temperature molecular/atomic domain,
the EOS of Chabrier & Potekhin in the high-density, high-temperature fully ionized domain, the limits of which
differ for H and He, and ab initio quantum molecular dynamics calculations in the regime of intermediate density
and temperature, characteristic of pressure dissociation and ionization. The EOS for the H/He mixture is based on
the so-called additive volume law and thus does not take into account the interactions between the two species. A
major improvement of the present calculations over existing ones is that we calculate the entropy over the entire
density–temperature domain, a necessary quantity for calculations of stellar or planetary evolution. The EOS
results are compared with existing experimental data, namely Hugoniot shock experiments for pure H and He, and
with first-principles numerical simulations for both the single elements and the mixture. This new EOS covers a
wide range of physical and astrophysical conditions, from Jovian planets to solar-type stars, and recovers the
existing relativistic EOS at very high densities, in the domains of white dwarfs and neutron stars. All the tables are
made publicly available.Programme National de Planétologie (PNP
A new model for mixing by double-diffusive convection (semi-convection): I. The conditions for layer formation
The process referred to as "semi-convection" in astrophysics and
"double-diffusive convection in the diffusive regime" in Earth and planetary
sciences, occurs in stellar and planetary interiors in regions which are stable
according to the Ledoux criterion but unstable according to the Schwarzschild
criterion. In this series of papers, we analyze the results of an extensive
suite of 3D numerical simulations of the process, and ultimately propose a new
1D prescription for heat and compositional transport in this regime which can
be used in stellar or planetary structure and evolution models.
In a preliminary study of the phenomenon, Rosenblum et al. (2011) showed
that, after saturation of the primary instability, a system can evolve in one
of two possible ways: the induced turbulence either remains homogeneous, with
very weak transport properties, or transitions into a thermo-compositional
staircase where the transport rate is much larger (albeit still smaller than in
standard convection).
In this paper, we show that this dichotomous behavior is a robust property of
semi-convection across a wide region of parameter space. We propose a simple
semi-analytical criterion to determine whether layer formation is expected or
not, and at what rate it proceeds, as a function of the background
stratification and of the diffusion parameters (viscosity, thermal diffusivity
and compositional diffusivity) only. The theoretical criterion matches the
outcome of our numerical simulations very adequately in the numerically
accessible "planetary" parameter regime, and can easily be extrapolated to the
stellar parameter regime.
Subsequent papers will address more specifically the question of quantifying
transport in the layered case and in the non-layered case.Comment: Submitted to Ap
Fingering convection and cloudless models for cool brown dwarf atmospheres
This work aims to improve the current understanding of the atmospheres of
brown dwarfs, especially cold ones with spectral type T and Y, whose modeling
is a current challenge. Silicate and iron clouds are believed to disappear at
the photosphere at the L/T transition, but cloudless models fail to reproduce
correctly the spectra of T dwarfs, advocating for the addition of more physics,
e.g. other types of clouds or internal energy transport mechanisms. We use a
one-dimensional (1D) radiative/convective equilibrium code ATMO to investigate
this issue. This code includes both equilibrium and out-of-equilibrium
chemistry and solves consistently the PT structure. Included opacity sources
are H2-H2, H2-He, H2O, CO, CO2, CH4, NH3, K, Na, and TiO, VO if they are
present in the atmosphere. We show that the spectra of Y dwarfs can be
accurately reproduced with a cloudless model if vertical mixing and NH3
quenching are taken into account. T dwarf spectra still have some reddening in
e.g. J - H compared to cloudless models. This reddening can be reproduced by
slightly reducing the temperature gradient in the atmosphere. We propose that
this reduction of the stabilizing temperature gradient in these layers, leading
to cooler structures, is due to the onset of fingering convection, triggered by
the destabilizing impact of condensation of very thin dust.Comment: Accepted in ApJ
Equation of state of fully ionized electron-ion plasmas
Thermodynamic quantities of Coulomb plasmas consisting of point-like ions
immersed in a compressible, polarizable electron background are calculated for
ion charges Z=1 to 26 and for a wide domain of plasma parameters ranging from
the Debye-Hueckel limit to the crystallization point and from the region of
nondegenerate to fully degenerate nonrelativistic or relativistic electrons.
The calculations are based on the linear-response theory for the electron-ion
interaction, including the local-field corrections in the electronic dielectric
function. The thermodynamic quantities are calculated in the framework of the
N-body hypernetted-chain equations and fitted by analytic expressions. We
present also accurate analytic approximations for the free energy of the ideal
electron gas at arbitrary degeneracy and relativity and for the excess free
energy of the one-component plasma of ions (OCP) derived from Monte Carlo
simulations.
The extension to multi-ionic mixtures is discussed within the framework of
the linear mixing rule. These formulae provide a completely analytic, accurate
description of the thermodynamic quantities of fully ionized electron-ion
Coulomb plasmas, a useful tool for various applications from liquid state
theory to dense stellar matter.Comment: 13 pages, 2 tables, 7 figures, REVTeX using epsf.sty. To be published
in Phys. Rev. E, vol. 58 (1998
Ab initio based equation of state of dense water for planetary and exoplanetary modeling
This is the author accepted manuscript. The final version is available from EDP Sciences via the DOI in this record.As a first step toward a multi-phase equation of state for dense water, we develop a temperature-dependent equation of state for dense water covering the liquid and plasma regimes and extending to the super-ionic and gas regimes. This equation of state covers the complete range of conditions encountered in planetary modeling. We use first principles quantum molecular dynamics simulations and its Thomas-Fermi extension to reach the highest pressures encountered in giant planets several times the size of Jupiter. Using these results, as well as the data available at lower pressures, we obtain a parametrization of the Helmholtz free energy adjusted over this extended temperature and pressure domain. The parametrization ignores the entropy and density jumps at phase boundaries but we show that it is sufficiently accurate to model interior properties of most planets and exoplanets. We produce an equation of state given in analytical form that is readily usable in planetary modeling codes and dynamical simulations {\bf (a fortran implementation can be found at http://www.ioffe.ru/astro/H2O/)}. The EOS produced is valid for the entire density range relevant to planetary modeling, {\bf for densities where quantum effects for the ions can be neglected, and for temperatures below 50,000K. We use this equation of state to calculate the mass-radius relationship of exoplanets up to 5,000M_Earth, explore temperature effects in ocean and wet Earth-like planets, and quantify the influence of the water EOS for the core on the gravitational moments of Jupiter.s. Part of this work was supported by the SNR grant PLANETLAB
12-BS04-0015 and the Programme National de Planetologie (PNP) of
CNRS-INSU co-funded by CNES. Funding and support from Paris Sciences et
Lettres (PSL) university through the project origins and conditions for the emergence
of life is also acknowledged. This work was performed using HPC resources
from GENCI- TGCC (Grant 2017- A0030406113
Forever Young: High Chromospheric Activity in M subdwarfs
We present spectroscopic observations of two halo M subdwarfs which have H
alpha emission lines. We show that in both cases close companions are the most
likely cause of the chromospheric activity in these old, metal-poor stars. We
argue that Gl 781 A's unseen companion is most likely a cool helium white
dwarf. Gl 455 is a near-equal-mass M subdwarf (sdM) system. Gl 781 A is rapidly
rotating with v sin i = 30 km/s. The properties of the chromospheres and X-ray
coronae of these systems are compared to M dwarfs with emission (dMe). The
X-ray hardness ratios and optical chromospheric lines emission ratios are
consistent with those seen in dMe stars. Comparison to active near-solar
metallicity stars indicates that despite their low metallicity ([m/H] = -1/2),
the sdMe stars are roughly as active in both X-rays and chromospheric emission.
Measured by L_X/L_bol, the activity level of Gl 781 A is no more than a factor
of 2.5 subluminous with respect to near-solar metallicity stars.Comment: 16 pages including 1 figure, AASTeX, to appear in May 1998 A.
A photometric and astrometric investigation of the brown dwarfs in Blanco 1
We present the results of a photometric and astrometric study of the low mass
stellar and substellar population of the young open cluster Blanco 1. We have
exploited J band data, obtained recently with the Wide Field Camera (WFCAM) on
the United Kingdom InfraRed Telescope (UKIRT), and 10 year old I and z band
optical imaging from CFH12k and Canada France Hawaii Telescope (CFHT), to
identify 44 candidate low mass stellar and substellar members, in an area of 2
sq. degrees, on the basis of their colours and proper motions. This sample
includes five sources which are newly discovered. We also confirm the lowest
mass candidate member of Blanco 1 unearthed so far (29MJup). We determine the
cluster mass function to have a slope of alpha=+0.93, assuming it to have a
power law form. This is high, but nearly consistent with previous studies of
the cluster (to within the errors), and also that of its much better studied
northern hemisphere analogue, the Pleiades.Comment: 8 Pages, 5 Figures, 2 Tables and 1 Appendix. Accepted for publication
in MNRA
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