1,721,031 research outputs found
Spectral analysis of 636 white dwarf-M star binaries from the sloan digital sky survey
No full textContext. 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. Aims. We attempt to measure fundamental parameters of these systems by completing spectral analyses. We propose to test models typically applied in fitting procedures and constrain likely and appropriate evolutionary scenarios for the systems. Methods. We use chi(2) minimization technique to decompose each combined spectrum and derive independent parameter estimates for its components. The possibility of alignment by chance is demoted to statistical insignificance, hence, we use physical interaction of the binary constituents as input parameter. Additionally, we check the corresponding photometric data from the SDSS to find optically resolved systems. Results. 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)(WD) around 40 000 K. Furthermore, we identify 70 very low-mass objects (VLMOs), which are secondaries of masses smaller than about 0.1 M(circle dot), to be candidate substellar companions. Conclusions. 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. Of the 167 spectra with hydrogen emission, 8 had already been found to be post-common envelope binaries (PCEBs) and 4 are systems with strong irradiation processes on the Mdwarf. The remaining 155 Balmer-emitting binaries probably harbor an active Mdwarf (dM), 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
Spectral types of planetary host star candidates from OGLE III
No full textContext.The Optical Gravitational Lensing Experiment project has
recently provided the OGLE III list of low-luminosity object
transits from campaigns #3 and #4, reporting 40 new objects
exhibiting the low-amplitude photometric eclipses expected for
exoplanets. Compared to previous OGLE targets, these OGLE III
candidates have been more restrictively selected and may contain
low-mass planets.
Aims.We have secured follow-up low-resolution
spectroscopy for 28 candidates out of this list (and one from the
OGLE Carina fields) to obtain an independent characterization of the
primary stars by spectral classification and thus better constrain
the parameters of their companions.
Methods.We fed the constraints from
these results back into an improved light curve solution. Together
with the radius ratios from the transit measurements, we derived the
radii of the low-luminosity companions. This allows us to examine
the possible sub-stellar nature of these objects.
Results.Sixteen of the
companions can be clearly identified as low-mass stars orbiting
a main sequence primary, while 10 more objects are likely
to have red giant primaries and therefore also host a stellar companion; 3
possibly have a sub-stellar nature ().
Conclusions.The
planetary nature of these objects should therefore be confirmed by
dynamical mass determinations
On the possibility of detecting extrasolar planets' atmospheres with the Rossiter-McLaughlin effect
No full textContext. The detection of extrasolar planets' atmospheres requires
very demanding observations. For planets that cannot be spatially
separated from their host stars, i.e. the vast majority of planets,
the transiting planets are the only ones to allow their atmospheres
to be probed. This is possible from transmission spectroscopy or
from measurements taken during the secondary eclipse. An alternative
is to measure of the Rossiter-McLaughlin effect, which is sensitive
to the size of the planetary radius. Since the radius is
wavelength-dependent due to contributions of strong planetary
absorption lines, this opens a path toward also probing planetary
atmospheres with ground-based high-resolution spectroscopy.
Aims. The
major goal of our numerical simulations is to provide a reliable
estimate of the amplitude of the wavelength-dependent
Rossiter-McLaughlin effect.
Methods. Our numerical simulations provide
detailed phase-resolved synthetic spectra modeling the partly
eclipsed stellar surface during the transit. With these spectra we
can obtain Rossiter-McLaughlin curves for different wavelength
regions and for a wavelength-dependent planetary radius. Curves from
regions with high and low contributions of absorption lines within
the planetary atmosphere can be compared. Observable quantities are
derived from these differential effects.
Results. We applied our
simulations to HD 209458. Our numerical simulations show that a
detailed treatment of the limb-darkening for the synthetic spectra
is important for a precise analysis. Compared to a parameterized
limb-darkening law, systematic errors of 6 m s-1 occur. The
wavelength dependency of the planetary atmospheres over the
NaD-doublet produces a differential effect in the
Rossiter-McLaughlin curve of 1.5 m s-1 for a star with a
rotation velocity of 4.5 km s-1, which increases to
4 m s-1 for twice the rotation velocity.
Conclusions. As a tool for
probing planetary atmospheres the Rossiter-McLaughlin effect
requires phase-resolved, high signal-to-noise, high-resolution
spectra taken with a stabilized spectrograph in order to obtain
reliable results for slowly rotating (<10 m s-1) planet
host stars. Stars with spectral type earlier than about F5 are a bit
less demanding since the typically higher rotation velocity
increases the amplitude of the effect to about 15 m s-1 for
a star with v sin i = 25 km s-1
A high resolution spectral atlas of brown dwarfs
No full textWe present a UVES/VLT high resolution atlas of three L
dwarfs and one T dwarf system, spectral classes at which most of the
objects are brown dwarfs. Our atlas covers the optical region from
Hα up to the near infrared at 1 μm. We present spectral
details of ultra-cool atmospheres at very high resolution () and compare the spectra to model calculations. Our
comparison shows that molecular features from VO and CaH, and atomic
features from Cs and Rb are reasonably well fit by current models.
On the other hand, features due to TiO, CrH, and water, and atomic
Na and K reveal large discrepancies between model calculations and
our observations
Investigation of transit-selected exoplanet candidates from the MACHO survey
No full textContext.Planets outside our solar system transiting their host star,
i.e. those with an orbital inclination near 90°, 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 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 were determined by means of
cross-correlation with model spectra. The MACHO light-curves were
compared to simulations based on the physical parameters of the system
derived from the radial velocities and spectral analyses.
Results.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
HS 0146+1847 – a DAZB white dwarf of very unusual composition
No full textHS 0146+1847, originally identified as a white dwarf candidate in
proper motion surveys, was rediscovered as a candidate in the
Hamburg Quasar Survey. Spectra obtained for the SPY (ESO
Supernova Ia Progenitor) survey show strong Balmer and Ca II lines,
suggesting a classification as DAZ white dwarf. Contrary to the
objects known so far in this class, HS 0146+1847 has a helium-rich
atmosphere at = 11 500 K. This is confirmed by very weak He
lines, changing the classification to DAZB. Mg and Fe lines are
also detected. We discuss the physics of Balmer line broadening by
neutral helium, present a spectral analysis and note some
implications for the accretion/diffusion scenario of heavy elements
in cool white dwarfs
Tidal effects on brown dwarfs: application to the eclipsing binary 2MASS J05352184-0546085
No full textContext. 2MASS J05352184-0546085 (2M0535-05) is the only known eclipsing brown dwarf (BD) binary, and so may serve as a benchmark for models
of BD formation and evolution. However, theoretical predictions of the system's properties seem inconsistent with observations: i) the
more massive (primary) component is observed to be cooler than the less massive (secondary) one;
ii) the secondary is more luminous (by ≈1024 W) than expected. Previous explanations for the temperature reversal have
invoked reduced convective efficiency in the structure of the primary, connected to magnetic activity and to surface spots, but these
explanations cannot account for the enhanced luminosity of the secondary. Previous studies also considered the possibility that the
secondary is younger than the primary.
Aims. We study the impact of tidal heating to the energy budget of both components to determine if it can account for the observed temperature
reversal and the high luminosity of the secondary. We also compare various plausible tidal models to determine a range of predicted
properties.
Methods. We apply two versions of two different, well-known models for tidal interaction, respectively: i) the “constant-phase-lag” model;
and ii) the “constant-time-lag” model and incorporate the predicted tidal heating into a model of BD structure. The four models differ
in their assumptions about the rotational behavior of the bodies, the system's eccentricity and putative misalignments ψ between the
bodies' equatorial planes and the orbital plane of the system.
Results. The contribution of heat from tides in 2M0535-05 alone may only be large enough to account for the discrepancies between observation
and theory in an unlikely region of the parameter space. The tidal quality factor of BDs would have to be 10
and the secondary needs a spin-orbit misalignment of ≳. However, tidal synchronization time scales for 2M0535-05
restrict the tidal dissipation function to log() ≳ 4.5 and rule out intense tidal heating in 2M0535-05. We
provide the first constraint on Q for BDs.
Conclusions. Tidal heating alone is unlikely to be responsible for the surprising temperature reversal within 2M0535-05. But an evolutionary
embedment of tidal effects and a coupled treatment with the structural evolution of the BDs is necessary to corroborate or refute
this result. The heating could have slowed down the BDs' shrinking and cooling processes after the birth of the system ≈1 Myr
ago, leading to a feedback between tidal inflation and tidal heating. Observations of old BD binaries and measurements of the
Rossiter-McLaughlin effect for 2M0535-05 can provide further constraints on
A ground-based transmission spectrum of the super-Earth exoplanet GJ 1214b
No full textIn contrast to planets with masses similar to that of Jupiter and higher, the bulk compositions of planets in the so-called super-Earth regime (masses 2-10 times that of the Earth) cannot be uniquely determined from a measurement of mass and radius alone. For these planets, there is a degeneracy between the mass and composition of both the interior and a possible atmosphere in theoretical models(1,2). The recently discovered transiting super-Earth exoplanet GJ 1214b is one example of this problem(3). Three distinct models for the planet that are consistent with its mass and radius have been suggested(4). Breaking the degeneracy between these models requires obtaining constraints on the planet's atmospheric composition(5,6). Here we report a ground-based measurement of the transmission spectrum of GJ 1214b between wavelengths of 780 and 1,000 nm. The lack of features in this spectrum rules out (at 4.9 sigma confidence) cloud-free atmospheres composed primarily of hydrogen. If the planet's atmosphere is hydrogen-dominated, then it must contain clouds or hazes that are optically thick at the observed wavelengths at pressures less than 200 mbar. Alternatively, the featureless transmission spectrum is also consistent with the presence of a dense, water vapour atmosphere.European Commissions; NAS
Radiative transfer in circumstellar disks
No full textWe present a new code for the calculation of the 1D
structure and synthetic spectra of accretion disks. The code is an
extension of the general purpose stellar atmosphere code
PHOENIX and is therefore capable of including extensive lists
of atomic and molecular lines as well as dust in the calculations. We
assume that the average viscosity can be represented by a critical
Reynolds number in a geometrically thin disk and solve the structure
and radiative transfer equations for a number of disk rings in the
vertical direction. The combination of these rings provides the total
disk structure and spectrum. Since the warm inner regions of
protoplanetary disks show a rich molecular spectrum, they are well
suited for a spectral analysis with our models. In this paper we test
our code by comparing our models with high-resolution VLT CRIRES
spectra of the T Tauri star GQ Lup
Two new ZZ Ceti pulsators from the HS and HE surveys
No full textWe report the detection of nonradial g-mode oscillations in the DA white
dwarfs HS 1039+4112 () and HE 1429-0343 () from time-series
photometry made at the Loiano 1.5 m telescope.
The two stars were previously selected as probable pulsators based on
two-color photometry and spectral analysis respectively.
Following our temperature and surface gravity determinations, HS 1039+4112
(Teff K, \mbox{\log g} =8.2) is located near the red edge of the
ZZ Ceti instability strip, whereas HE 1429-0343 (Teff K,
\mbox{\log g}=7.8) falls in the middle of the strip.
Both stars show a multi-mode behavior with the main periods at about 850
and 970 s respectively, and relatively large amplitudes (~7% and ~2.5%)
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