1,436 research outputs found
Bayesian mass and age estimates for transiting exoplanet host stars
The mean density of a star transited by a planet, brown dwarf or low mass
star can be accurately measured from its light curve. This measurement can be
combined with other observations to estimate its mass and age by comparison
with stellar models. Our aim is to calculate the posterior probability
distributions for the mass and age of a star given its density, effective
temperature, metallicity and luminosity. We computed a large grid of stellar
models that densely sample the appropriate mass and metallicity range. The
posterior probability distributions are calculated using a Markov-chain
Monte-Carlo method. The method has been validated by comparison to the results
of other stellar models and by applying the method to stars in eclipsing binary
systems with accurately measured masses and radii. We have explored the
sensitivity of our results to the assumed values of the mixing-length
parameter, , and initial helium mass fraction, Y. For a star
with a mass of 0.9 solar masses and an age of 4 Gyr our method recovers the
mass of the star with a precision of 2% and the age to within 25% based on the
density, effective temperature and metallicity predicted by a range of
different stellar models. The masses of stars in eclipsing binaries are
recovered to within the calculated uncertainties (typically 5%) in about 90% of
cases. There is a tendency for the masses to be underestimated by about 0.1
solar masses for some stars with rotation periods Pd. Our method
makes it straightforward to determine accurately the joint posterior
probability distribution for the mass and age of a star eclipsed by a planet or
other dark body based on its observed properties and a state-of-the art set of
stellar models.Comment: Accepted for publication in A&A. 9 pages, 4 figures. Source code for
the software described is available from
http://sourceforge.net/projects/bagemas
Orbital periods of the binary sdB stars PG0940+068 and PG1247+554
We have used the radial velocity variations of two sdB stars previously
reported to be binaries to establish their orbital periods. They are
PG0940+068, (P=8.33d) and PG1247+554 (P=0.599d). The minimum masses of the
unseen companions, assuming a mass of 0.5 solar masses for the sdB stars, are
0.090 +/- 0.003 solar masses for PG1247+554 and 0.63 +/- 0.02 solar masses for
PG0940+068. The nature of the companions is not constrained further by our
data.Comment: 5 pages, 2 figure
CWRML: representing crop wild relative conservation and use data in XML
Background
Crop wild relatives are wild species that are closely related to crops. They are valuable as potential gene donors for crop improvement and may help to ensure food security for the future. However, they are becoming increasingly threatened in the wild and are inadequately conserved, both in situ and ex situ. Information about the conservation status and utilisation potential of crop wild relatives is diverse and dispersed, and no single agreed standard exists for representing such information; yet, this information is vital to ensure these species are effectively conserved and utilised. The European Community-funded project, European Crop Wild Relative Diversity Assessment and Conservation Forum, determined the minimum information requirements for the conservation and utilisation of crop wild relatives and created the Crop Wild Relative Information System, incorporating an eXtensible Markup Language (XML) schema to aid data sharing and exchange.
Results
Crop Wild Relative Markup Language (CWRML) was developed to represent the data necessary for crop wild relative conservation and ensure that they can be effectively utilised for crop improvement. The schema partitions data into taxon-, site-, and population-specific elements, to allow for integration with other more general conservation biology schemata which may emerge as accepted standards in the future. These elements are composed of sub-elements, which are structured in order to facilitate the use of the schema in a variety of crop wild relative conservation and use contexts. Pre-existing standards for data representation in conservation biology were reviewed and incorporated into the schema as restrictions on element data contents, where appropriate.
Conclusion
CWRML provides a flexible data communication format for representing in situ and ex situ conservation status of individual taxa as well as their utilisation potential. The development of the schema highlights a number of instances where additional standards-development may be valuable, particularly with regard to the representation of population-specific data and utilisation potential. As crop wild relatives are intrinsically no different to other wild plant species there is potential for the inclusion of CWRML data elements in the emerging standards for representation of biodiversity data
A Survey for Spectroscopic Binaries Among Very Low-Mass Stars
We report on the results of a survey for radial velocity variability in a
heterogeneous sample of very low-mass stars and brown dwarfs. One
distinguishing characteristic of the survey is its timespan, which allows an
overlap between spectroscopic binaries and those which can be found by high
angular-resolution imaging. We are able to place a new constraint on the total
binary fraction in these objects, which suggests that they are more likely the
result of extending the same processes at work at higher masses into this mass
range, rather than a distinct mode of formation. Our basic result is that there
are out of 53, or % spectroscopic binaries in the
separation range 0-6 AU, nearly as many as resolved binaries. This leads to an
estimate of an upper limit of % for the binary fraction of VLM
objects (it is an upper limit because of the possible overlap between the
spectroscopic and resolved populations). A reasonable estimate for the very
low-mass binary fraction is %. We consider several possible separation
and frequency distributions, including the same one as found for GK stars, a
compressed version of that, a version of the compressed distribution truncated
at 15 AU, and a theoretical distribution which considers the evaporation of
small-N clusters. We conclude that the latter two bracket the observations,
which may mean that these systems form with intrinsically smaller separations
due to their smaller mass, and then are truncated due to their smaller binding
energy. We do not find support for the ``ejection hypothesis'' as their
dominant mode of formation, particularly in view of the similarity in the total
binary fraction compared with slightly more massive stars, and the difficulty
this mechanism has in producing numerous binary systems.Comment: 36 pages, accepted for publication in AJ, abstract shortened for
arXiv.or
SDSS J080531.84+481233.0: An Unresolved L Dwarf/T Dwarf Binary
SDSS J080531.84+481233.0 is a peculiar L-type dwarf that exhibits unusually
blue near-infrared and mid-infrared colors and divergent optical (L4) and
near-infrared (L9.5) spectral classifications. These peculiar spectral traits
have been variously attributed to condensate cloud effects or subsolar
metallicity. Here I present an improved near-infrared spectrum of this source
which further demonstrates the presence of weak CH4 absorption at 1.6 micron
but no corresponding band at 2.2 micron. It is shown that these features can be
collectively reproduced by the combined light spectrum of a binary with L4.5
and T5 components, as deduced by spectral template matching. Thus, SDSS
J080531.84+481233.0 appears to be a new low-mass binary straddling the L
dwarf/T dwarf transition, an evolutionary phase for brown dwarfs that remains
poorly understood by current theoretical models. The case of SDSS
J080531.84+481233.0 further illustrates how a select range of L dwarf/T dwarf
binaries could be identified and characterized without the need for high
angular resolution imaging or radial velocity monitoring, potentially
alleviating some of the detection biases and limitations inherent to such
techniques.Comment: 11 pages, 4 figures, accepted by A
Absolute dimensions of detached eclipsing binaries. I. The metallic-lined system WW Aurigae
WW Aurigae is a detached eclipsing binary composed of two metallic-lined
A-type stars orbiting each other every 2.5 days. We have determined the masses
and radii of both components to accuracies of 0.4 and 0.6 percent,
respectively. From a cross-correlation analysis of high-resolution spectra we
find masses of 1.964 +/- 0.007 Msun for the primary star and 1.814 +/- 0.007
Msun for the secondary star. From an analysis of photoelectric uvby and UBV
light curves we find the radii of the stars to be 1.927 +/- 0.011 Rsun and
1.841 +/- 0.011 Rsun, where the uncertainties have been calculated using a
Monte Carlo algorithm. Fundamental effective temperatures of the two stars have
been derived, using the Hipparcos parallax of WW Aur and published ultraviolet,
optical and infrared fluxes, and are 7960 +/- 420 and 7670 +/- 410 K. The
masses, radii and effective temperatures of WW Aur are only matched by
theoretical evolutionary models for a fractional initial metal abundance, Z, of
approximately 0.06 and an age of roughly 90 Myr. This seems to be the highest
metal abundance inferred for a well-studied detached eclipsing binary, but we
find no evidence that it is related to the metallic-lined nature of the stars.
The circular orbit of WW Aur is in conflict with the circularization timescales
of both the Tassoul and the Zahn tidal theories and we suggest that this is due
to pre-main-sequence evolution or the presence of a circular orbit when the
stars were formed.Comment: Accepted for publication in MNRAS (14 pages, 8 figures). Photometric
data will be made available at the CDS once the final version appear
Two new hot subdwarf binaries in the GALEX survey
We report the discovery of two new hot, hydrogen-rich subdwarfs (sdB) in
close binary systems. The hot subdwarfs, GALEX J0321+4727 and GALEX J2349+3844,
were selected from a joint optical-ultraviolet catalogue of hot sub-luminous
stars based on GSC2.3.2 and the Galaxy Evolution Explorer all-sky survey. Using
high-dispersion spectra of the Halpha core obtained using the 2m telescope at
Ondrejov Observatory we measured the radial velocities of the sdB primaries and
determined orbital periods of 0.26584+/-0.00004 days and 0.46249+/-0.00007 days
for GALEX J0321+4727 and GALEX J2349+3844, respectively. The time series
obtained from the Northern Sky Variability Survey with an effective wavelength
near the R band show that GALEX J0321+4727 is a variable star (Delta m=0.12
mag) while no significant variations are observed in GALEX J2349+3844. The
period of variations in GALEX J0321+4727 coincides with the orbital period and
the variability is probably caused by a reflection effect on a late-type
secondary star. Lack of photometric variations in GALEX J2349+3844 probably
indicates that the companion is a white dwarf star. Using all available
photometry and spectroscopy, we measured the atmospheric properties of the two
sdB stars and placed limits on the mass and luminosity of the companion stars.Comment: Accepted for publication in MNRA
Irradiated brown dwarfs
We have observed the post common envelope binary WD0137-349 in the near
infrared , and bands and have determined that the photometry varies
on the system period (116 min). The amplitude of the variability increases with
increasing wavelength, indicating that the brown dwarf in the system is likely
being irradiated by its 16500 K white dwarf companion. The effect of the
(primarily) UV irradiation on the brown dwarf atmosphere is unknown, but it is
possible that stratospheric hazes are formed. It is also possible that the
brown dwarf (an L-T transition object) itself is variable due to patchy cloud
cover. Both these scenarios are discussed, and suggestions for further study
are made.Comment: 5 pages, 2 figures. Proceedings from "Brown dwarfs come of age"
meeting in Fuerteventura 201
Rotational period of WD1953-011 - a magnetic white dwarf with a star spot
WD1953-011 is an isolated, cool (7920 +/- 200K, Bergeron, Legget & Ruiz,
2001) magnetic white dwarf (MWD) with a low average field strength (~70kG,
Maxted et al. 2000) and a higher than average mass (~0.74 M_sun, Bergeron et
al. 2001). Spectroscopic observations taken by Maxted et al. (2000) showed
variations of equivalent width in the Balmer lines, unusual in a low field
white dwarf. Here we present V band photometry of WD1953-011 taken at 7 epochs
over a total of 22 months. All of the datasets show a sinusoidal variation of
approximately 2% peak-to-peak amplitude. We propose that these variations are
due to a star spot on the MWD, analogous to a sunspot, which is affecting the
temperature at the surface, and therefore its photometric magnitude. The
variations have a best-fit period over the entire 22 months of 1.4418 days,
which we interpret as the rotational period of the WD.Comment: (1) University of Southampton, (2) University of Warwick, (3)
University of Nijmegen, (4) Keele University, (5) University of Leicester. 6
pages, 5 figs, accepted MNRA
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