623 research outputs found
Investigation of transit-selected exoplanet candidates from the MACHO survey
Context: Planets outside our solar system transiting their host star, i. e.
those with an orbital inclination near 90 degree, are of special interest to
derive physical properties of extrasolar planets. With the knowledge of the
host star's physical parameters, the planetary radius can be determined.
Combined with spectroscopic observations the mass and therefore the density can
be derived from Doppler-measurements. Depending on the brightness of the host
star, additional information, e. g. about the spin-orbit alignment between the
host star and planetary orbit, can be obtained.
Aims: The last few years have witnessed a growing success of transit surveys.
Among other surveys, the MACHO project provided nine potential transiting
planets, several of them with relatively bright parent stars. The photometric
signature of a transit event is, however, insufficient to confirm the planetary
nature of the faint companion. The aim of this paper therefore is a
determination of the spectroscopic parameters of the host stars as well as a
dynamical mass determination through Doppler-measurements.
Methods: We have obtained follow-up high-resolution spectra for five stars
selected from the MACHO sample, which are consistent with transits of
low-luminosity objects. Radial velocities have been determined by means of
cross-correlation with model spectra. The MACHO light curves have been compared
to simulations based on the physical parameters of the system derived from the
radial velocities and spectral analyses.
Aims: We show that all transit light curves of the exoplanet candidates
analysed in this work can be explained by eclipses of stellar objects, hence
none of the five transiting objects is a planet.Comment: 6 pages, 3 figures, 1 table, accepted for publication in A&
New evolutionary models for pre-main sequence and main sequence low-mass stars down to the hydrogen-burning limit
We present new models for low-mass stars down to the hydrogen-burning limit
that consistently couple atmosphere and interior structures, thereby
superseding the widely used BCAH98 models. The new models include updated
molecular linelists and solar abundances, as well as atmospheric convection
parameters calibrated on 2D/3D radiative hydrodynamics simulations. Comparison
of these models with observations in various colour-magnitude diagrams for
various ages shows significant improvement over previous generations of models.
The new models can solve flaws that are present in the previous ones, such as
the prediction of optical colours that are too blue compared to M dwarf
observations. They can also reproduce the four components of the young
quadruple system LkCa 3 in a colour-magnitude diagram with one single
isochrone, in contrast to any presently existing model. In this paper we also
highlight the need for consistency when comparing models and observations, with
the necessity of using evolutionary models and colours based on the same
atmospheric structures.Comment: 7 pages, 8 figures, Astronomy & Astrophysics in pres
High-precision stellar limb-darkening in exoplanetary transits
Characterization of the atmospheres of transiting exoplanets relies on
accurate measurements of the extent of the optically thick area of the planet
at multiple wavelengths with a precision 100 parts per million (ppm).
Next-generation instruments onboard the James Webb Space Telescope (JWST) are
expected to achieve 10 ppm precision for several tens of targets. A
similar precision can be obtained in modelling only if other astrophysical
effects, including the stellar limb-darkening, are accounted for properly. In
this paper, we explore the limits on precision due to the mathematical formulas
currently adopted to approximate the stellar limb-darkening, and to the use of
limb-darkening coefficients obtained either from stellar-atmosphere models or
empirically. We propose a new limb-darkening law with two coefficients,
`power-2', which outperforms other two-coefficient laws adopted in the
literature in most cases, and particularly for cool stars. Empirical
limb-darkening based on two-coefficient formulas can be significantly biased,
even if the light-curve residuals are nearly photon-noise limited. We
demonstrate an optimal strategy to fitting for the four-coefficients
limb-darkening in the visible, using prior information on the exoplanet orbital
parameters to break some of the degeneracies that otherwise would prevent the
convergence of the fit. Infrared observations taken with the James Webb Space
Telescope (JWST) will provide accurate measurements of the exoplanet orbital
parameters with unprecedented precision, which can be used as priors to improve
the stellar limb-darkening characterization, and therefore the inferred
exoplanet parameters, from observations in the visible, such as those taken
with Kepler/K2, JWST, other past and future instruments
Spectral analysis of 636 white dwarf - M star binaries from the Sloan Digital Sky Survey
We present a catalog of 857 white dwarf (WD)-M binaries from the sixth data
release (DR6) of the Sloan Digital Sky Survey (SDSS), most of which were
previously identified. For 636 of them, we complete a spectral analysis and
derive the basic parameters of their stellar constituents and their distances
from Earth. We attempt to measure fundamental parameters of these systems by
completing spectral analyses. We use a Chi^2 minimization technique to
decompose each combined spectrum and derive independent parameter estimates for
its components. Forty-one of the stellar duets in our spectroscopic sample are
optically resolved in their respective SDSS images. For these systems, we also
derive a minimum true spatial separation and a lower limit to their orbital
periods, typically which are some 10^4 yr. Spectra of 167 stellar duets show
significant hydrogen emission and in most cases no additional He i or He ii
features. We also find that 20 of the 636 WDs are fitted to be DOs, with 16
measured to have T_eff around 40,000 K. Furthermore, we identify 70 very
low-mass objects, which are secondaries of masses smaller than about 0.1 solar
masses, to be candidate substellar companions. Although various selection
effects may play a role, the fraction 6.4 % of WD-M star binaries with orbital
separations of around 500 AU is a criterion for evolutionary models of stellar
binary systems. Active M dwarfs are likely present in 155 Balmer-emitting
systems, corresponding to a fraction of 24.4 %. The excess of cool DOs is most
likely due to additional WDs in the DB-DO T_eff range, for which no detailed
fitting was completed. The trend of the M stars being closer to Earth than the
WD component is probably due to an underestimation of the theoretical M star
radii.Comment: accepted by A&A October 3, 2008, 15 pages, 16 figures, 3 tables; v2,
minor grammatical changes, essential changes in Sect. 5.
Progress in Modeling Very Low Mass Stars, Brown Dwarfs, and Planetary Mass Objects
We review recent advancements in modeling the stellar to substellar
transition. The revised molecular opacities, solar oxygen abundances and cloud
models allow to reproduce the photometric and spectroscopic properties of this
transition to a degree never achieved before, but problems remain in the
important M-L transition characteristic of the effective temperature range of
characterizable exoplanets. We discuss of the validity of these classical
models. We also present new preliminary global Radiation HydroDynamical M
dwarfs simulations.Comment: Submitted to Mem. S. A. It. Supp
High resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia Progenitor Survey III. DA white dwarfs
Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO) DOI: 10.1051/0004-6361/200912531Context. The ESO Supernova Ia Progenitor Survey (SPY) took high-resolution spectra of more than 1000 white dwarfs and pre-white dwarfs. About two thirds of the stars observed are hydrogen-dominated DA white dwarfs. Here we present a catalog and detailed spectroscopic analysis of the DA stars in the SPY. Aims. Atmospheric parameters effective temperature and surface gravity are determined for normal DAs. Double-degenerate binaries, DAs with magnetic fields or dM companions, are classified and discussed. Methods. The spectra are compared with theoretical model atmospheres using a fitting technique. Results. Our final sample contains 615 DAs, which show only hydrogen features in their spectra, although some are double-degenerate binaries. 187 are new detections or classifications. We also find 10 magnetic DAs (4 new) and 46 DA+dM pairs (10 new).Peer reviewe
High-contrast imaging of Sirius~A with VLT/SPHERE: Looking for giant planets down to one astronomical unit
Sirius has always attracted a lot of scientific interest, especially after
the discovery of a companion white dwarf at the end of the 19th century. Very
early on, the existence of a potential third body was put forward to explain
some of the observed properties of the system. We present new coronagraphic
observations obtained with VLT/SPHERE that explore, for the very first time,
the innermost regions of the system down to 0.2" (0.5 AU) from Sirius A. Our
observations cover the near-infrared from 0.95 to 2.3 m and they offer the
best on-sky contrast ever reached at these angular separations. After detailing
the steps of our SPHERE/IRDIFS data analysis, we present a robust method to
derive detection limits for multi-spectral data from high-contrast imagers and
spectrographs. In terms of raw performance, we report contrasts of 14.3 mag at
0.2", ~16.3 mag in the 0.4-1.0" range and down to 19 mag at 3.7". In physical
units, our observations are sensitive to giant planets down to 11 at
0.5 AU, 6-7 in the 1-2 AU range and ~4 at 10 AU. Despite
the exceptional sensitivity of our observations, we do not report the detection
of additional companions around Sirius A. Using a Monte Carlo orbital analysis,
we show that we can reject, with about 50% probability, the existence of an 8
planet orbiting at 1 AU. In addition to the results presented in the
paper, we provide our SPHERE/IFS data reduction pipeline at
http://people.lam.fr/vigan.arthur/ under the MIT license.Comment: 16 pages, 10 figures, accepted for publication in MNRA
Subdwarf B Stars from the ESO Supernova Ia Progenitor Survey -- Observation versus Theory
Original paper can be found at: http://www.astrosociety.org/pubs/cs/328.html--Copyright Astronomical Society of the PacificWe present the analysis of a high-quality sample of optical spectra for 76 sdB stars from the ESO Supernova Ia Progenitor Survey. Effective temperature, surface gravity, and photospheric helium abundance were derived from line profile fits. We demonstrate that our subsample of 52 single-lined sdB stars is a useful tool to compare observation and theory. The predictions of population synthesis models for close binary evolution are compared to our data. We show that the simulations cover the observed parameter range of sdBs, but fail to reproduce the observed distribution in detail
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