25 research outputs found
The Mystery Deepens: Spitzer Observations of Cool White Dwarfs
We present 4.5m and 8m photometric observations of 18 cool white
dwarfs obtained with the Spitzer Space Telescope. Our observations demonstrate
that four white dwarfs with T_eff< 6000 K show slightly depressed mid-infrared
fluxes relative to white dwarf models. In addition, another white dwarf with a
peculiar optical and near-infrared spectral energy distribution (LHS 1126) is
found to display significant flux deficits in Spitzer observations. These
mid-infrared flux deficits are not predicted by the current white dwarf models
including collision induced absorption due to molecular hydrogen. We postulate
that either the collision induced absorption calculations are incomplete or
there are other unrecognized physical processes occuring in cool white dwarf
atmospheres. The spectral energy distribution of LHS 1126 surprisingly fits a
Rayleigh-Jeans spectrum in the infrared, mimicking a hot white dwarf with
effective temperature well in excess of 10 K. This implies that the source
of this flux deficit is probably not molecular absorption but some other
process.Comment: 17 pages, 4 figures, ApJ in press, 10 May 200
Photometric and Spectroscopic Analysis of Cool White Dwarfs with Trigonometric Parallax Measurements
A photometric and spectroscopic analysis of 152 cool white dwarf stars is
presented. The discovery of 7 new DA white dwarfs, 2 new DQ white dwarfs, 1 new
magnetic white dwarf, and 3 weak magnetic white dwarf candidates, is reported,
as well as 19 known or suspected double degenerates. The photometric energy
distributions, the Halpha line profiles, and the trigonometric parallax
measurements are combined and compared to model atmosphere calculations to
determine the effective temperature and the radius of each object, and also to
constrain the atmospheric composition. New evolutionary sequences with C/O
cores with thin and thick hydrogen layers are used to derive masses and ages.
We confirm the existence of a range in Teff between 5000 and 6000K where almost
all white dwarfs have H-rich atmospheres. There is little evidence for mixed
H/He dwarfs, with the exception of 2 He-rich DA stars, and 5 C2H white dwarfs
which possibly have mixed H/He/C atmospheres. The DQ sequence terminates near
6500K, below which they are believed to turn into C2H stars. True DC stars
slightly above this temperature are found to exhibit H-like energy
distributions despite the lack of Halpha absorption. Attempts to interpret the
chemical evolution show the problem to be complex. Convective mixing is
necessary to account for the non-DA to DA ratio as a function of temperature.
The presence of helium in cool DA stars, the existence of the non-DA gap, and
the peculiar DC stars are also explained in terms of convective mixing,
although our understanding of how this mechanism works needs to be revised. The
oldest object in our sample is about 7.9 Gyr or 9.7 Gyr old depending on
whether thin or thick hydrogen layer models are used. The mean mass of our
sample is 0.65 +/- 0.20 Msun.Comment: Accepted by ApJ Suppl (~April 2001); 79 pages incl. 25 figure
The Limiting Effects of Dust in Brown Dwarf Model Atmospheres
We present opacity sampling model atmospheres, synthetic spectra and colors
for brown dwarfs and very low mass stars in two limiting case of dust grain
formation: 1) inefficient gravitational settling i.e. the dust is distributed
according to the chemical equilibrium predictions, 2) efficient gravitational
settling i.e. the dust forms and depletes refractory elements from the gas, but
their opacity does not affect the thermal structure. The models include the
formation of over 600 gas phase species, and 1000 liquids and crystals, and the
opacities of 30 different types of grains including corundum (AlO), the
magnesium aluminum spinel MgAlO, iron, enstatite (MgSiO),
forsterite (MgSiO), amorphous carbon, SiC, and a number of calcium
silicates. The models extend from the beginning of the grain formation regime
well into the condensation regime of water ice (\teff= 3000 - 100 K) and
encompasses the range of at solar metallicity.
We find that silicate dust grains can form abundantly in the outer
atmospheric layers of red and brown dwarfs with spectral type later than M8.
The greenhouse effects of dust opacities provide a natural explanation for the
peculiarly red spectroscopic distribution of the latest M dwarfs and young
brown dwarfs. The grainless (Cond) models on the other hand, correspond closely
to methane brown dwarfs such as Gliese 229B. We also recover that the
5891,5897\AA Na I D and 7687,7701\AA K I resonance doublets
plays a critical role in T dwarfs where their red wing define the
pseudo-continuum from the to the bandpass.Comment: 49 pages, ApJ, in press. 22 figures (included). Corrected nasty
typos. Also available at http:/phoenix.physast.uga.ed
Infrared Colors at the Stellar/Substellar Boundary
We present new infrared photometry for 61 halo and disk stars around the
stellar/substellar boundary. These data are combined with available optical
photometry and astrometric data to produce color--color and absolute
magnitude--color diagrams. The disk and halo sequences are compared to the
predictions of the latest model atmospheres and structural models.
We find good agreement between observation and theory except for known
problems in the V and H passbands probably due to incomplete molecular data for
TiO, metal hydrides and HO. The metal--poor M subdwarfs are well matched by
the models as oxide opacity sources are less important in this case. The known
extreme M subdwarfs have metallicities about one--hundredth solar, and the
coolest subdwarfs have T K with masses 0.09M/M. The grainless models are not able to reproduce the flux
distributions of disk objects with T 2500 K, however a preliminary
version of the NextGen--Dusty models which includes homogeneous formation and
extinction by dust grains {\it is} able to match the colors of these very cool
objects. The least luminous objects in this sample are GD165B, three DENIS
objects --- DBD0205, DBD1058 and DBD1228 --- and Kelu-1. These have
T 2000 K and are at or below the stellar limit with masses
0.075M/M. Photometry alone cannot constrain these parameters
further as the age is unknown, but published lithium detections for two of
these objects (Kelu-1 and DBD1228) imply that they are young (aged about 1 Gyr)
and substellar (mass 0.06M/M).Comment: ApJ, in press. 18 pages. Also available at
ftp://ftp.jach.hawaii.edu/pub/ukirt/skl/dM_preprint
The First Substellar Subdwarf? Discovery of a Metal-poor L Dwarf with Halo Kinematics
We present the discovery of the first L-type subdwarf, 2MASS
J05325346+8246465. This object exhibits enhanced collision-induced H
absorption, resulting in blue NIR colors (). In
addition, strong hydride bands in the red optical and NIR, weak TiO absorption,
and an optical/J-band spectral morphology similar to the L7 DENIS 02051159AB
imply a cool, metal-deficient atmosphere. We find that 2MASS 0532+8246 has both
a high proper motion, = 2\farcs60\pm0\farcs15 yr, and a
substantial radial velocity, km s, and its
probable proximity to the Sun (d = 10--30 pc) is consistent with halo
membership. Comparison to subsolar-metallicity evolutionary models strongly
suggests that 2MASS 0532+8246 is substellar, with a mass of 0.077 M
0.085 M_{\sun} for ages 10--15 Gyr and metallicities Z_{\sun}. The discovery of this object clearly indicates that star
formation occurred below the Hydrogen burning mass limit at early times,
consistent with prior results indicating a flat or slightly rising mass
function for the lowest-mass stellar subdwarfs. Furthermore, 2MASS 0532+8246
serves as a prototype for a new spectral class of subdwarfs, additional
examples of which could be found in NIR proper motion surveys.Comment: 9 pages, 3 figures, accepted to Ap
First light of the VLT planet finder SPHERE IV : Physical and chemical properties of the planets around HR8799
This is a pre-copyedited, author produced PDF of an article accepted for publication in Astronomy & Astrophysics following peer review. Subject to 12 month's embargo period. Embargo end date: 16 February 2017. The version of record [Bonnefoy, M., 'First light of the VLT planet finder SPHERE, IV. Physical and chemical properties of the planets around HR8799', A&A, 587, A58 (2016), first published online 16 February 2016] is available online at doi http://dx.doi.org/10.1051/0004-6361/201526906Context. The system of four planets discovered around the intermediate-mass star HR8799 offers a unique opportunity to test planet formation theories at large orbital radii and to probe the physics and chemistry at play in the atmospheres of self-luminous young (∼30 Myr) planets. We recently obtained new photometry of the four planets and low-resolution (R∼30) spectra of HR8799 d and e with the SPHERE instrument (paper III). Aims. In this paper (paper IV), we aim to use these spectra and available photometry to determine how they compare to known objects, what the planet physical properties are, and how their atmospheres work. Methods. We compare the available spectra, photometry, and spectral-energy distribution (SED) of the planets to field dwarfs and young companions. In addition, we use the extinction from corundum, silicate (enstatite and forsterite), or iron grains likely to form in the atmosphere of the planets to try to better understand empirically the peculiarity of their spectrophotometric properties. To conclude, we use three sets of atmospheric models (BT-SETTL14, Cloud-AE60, Exo-REM) to determine which ingredients are critically needed in the models to represent the SED of the objects, and to constrain their atmospheric parameters (T eff , log g, M/H). Results. We find that HR8799d and e properties are well reproduced by those of L6-L8 dusty dwarfs discovered in the field, among which some are candidate members of young nearby associations. No known object reproduces well the properties of planets b and c. Nevertheless, we find that the spectra and WISE photometry of peculiar and/or young early-T dwarfs reddened by submicron grains made of corundum, iron, enstatite, or forsterite successfully reproduce the SED of these planets. Our analysis confirms that only the Exo-REM models with thick clouds fit (within 2σ) the whole set of spectrophotometric datapoints available for HR8799 d and e for T eff = 1200 K, log g in the range 3.0-4.5, and M/H=+0.5. The models still fail to reproduce the SED of HR8799c and b. The determination of the metallicity, log g, and cloud thickness are degenerate. Conclusions. Our empirical analysis and atmospheric modelling show that an enhanced content in dust and decreased CIA of H2 is certainly responsible for the deviation of the properties of the planet with respect to field dwarfs. The analysis suggests in addition that HR8799c and b have later spectral types than the two other planets, and therefore could both have lower masses.Peer reviewe