259 research outputs found
A posteriori detection of the planetary transit of HD189733b in the Hipparcos photometry
Thanks to observations performed at the Haute-Provence Observatory, Bouchy et
al. recently announced the detection of a 2.2-day orbital period extra-solar
planet that transits the disk of its parent star, HD189733. With high level of
confidence, we find that Hipparcos likely observed one transit of HD189733b in
October 1991, and possibly two others in February 1991 and February 1993. Using
the range of possible periods for HD189733b, we find that the probability that
none of those events are due to planetary transits but are instead all due to
artifacts is lower than 0.15%. Thanks to the 15-year temporal baseline
available, we can measure the orbital period of the planet HD189733b with a
particularly high accuracy. We obtain a period of 2.218574
(+0.000006/-0.000010) days, corresponding to an accuracy of ~1 second. Such
accurate measurements might provide clues for companions presence.Comment: 7 pages, to be published in Astronomy & Astrophysic
Regulation of ABCA1 expression and cholesterol efflux during adipose differentiation of 3T3-L1 cells.
Adipose cells specialized in energy storage, contain large intracellular triglyceride-rich lipid droplets, are enriched with free cholesterol, and express sterol-regulated transcription factors such as liver X receptor (LXR). The recent identification of the LXR-dependent ATP binding cassette transporter A1 (ABCA1) pathway for cholesterol release from peripheral cells has led us to address the question of the expression and function of ABCA1 in adipocytes. In 3T3-L1 adipose cells, we observed a strong induction of ABCA1 mRNA during adipose differentiation, but only limited variations in ABCA1 protein. Lipid efflux onto apolipoprotein A-I (apoA-I), which depends on ABCA1, was comparable in adipocytes and preadipocytes, demonstrating a differential regulation of ABCA1 mRNA and cholesterol efflux. We also found that total cell cholesterol remained stable during differentiation of 3T3-L1 cells, but membrane cholesterol was lower in adipocytes than in preadipocytes, suggesting redistribution of cholesterol to the lipid droplet. Finally, we show that under standard lipolytic stimulation, 3T3-L1 adipocytes do not release cholesterol onto apoA-I, a process that required long exposures to lipolytic agents (24 h). In conclusion, despite large induction of ABCA1 mRNA during differentiation, cholesterol efflux through the ABCA1 pathway remains limited in adipocytes and requires prolonged lipolysis. This is consistent with the view of the adipocyte behaving as a cholesterol sink, with plasma cholesterol-buffering properties
HD 209458: Physical Parameters of the Parent Star and the Transiting Planet
The Sun-like star HD209458 harbors a close-in giant planet which transits
across the star's disk, and thus allows an unprecedented access to the basic
parameters of the planet, given a certain knowledge of the basic parameters of
the star, namely its mass and radius. We present theoretical stellar evolution
model calculations for HD 209458 and discuss the uncertainties involved in
deriving the stellar mass and radius. We derive the mass, M=1.06 Msun, radius,
R=1.18 Rsun,and age, t=5.2 Gyr of the star with uncertainties of 10% or more.
The dominant sources of uncertainty remain to be the helium abundance estimate
and the treatment of convection, even after an optimistic estimate for the
effective temperature of the star. However, we find that in deriving the radius
of the planet, R_p, the relevant stellar model input is the M/R relation, which
runs orthogonal to a degeneracy in the transit light curve solution and greatly
improves the estimate of R_p. Theoretically the M/R relation has a lower
uncertainty than the M and R separately. We estimate the planet radius and mass
to be R_p =1.42 +0.10/-0.13 R_J and M_p =0.69 +/- 0.02 M_J.Comment: 24 pages, 5 figures, accepted by the Astrophysical Journal; added
correct version of figure 5 and fixed reference
The HARPS search for southern extra-solar planets. XX. Planets around the active star BD-08:2823
We report the detection of a planetary system around BD-08:2823, that
includes at least one Uranus-mass planet and one Saturn-mass planet. This
discovery serendipitously originates from a search for planetary transits in
the Hipparcos photometry database. This program preferentially selected active
stars and did not allow the detection of new transiting planets. It allowed
however the identification of the K3V star BD-08:2823 as a target harboring a
multiplanet system, that we secured and characterized thanks to an intensive
monitoring with the HARPS spectrograph at the 3.6-m ESO telescope in La Silla.
The stellar activity level of BD-08:2823 complicates the analysis but does not
prohibit the detection of two planets around this star. BD-08:2823b has a
minimum mass of 14.4+/-2.1 M_Earth and an orbital period of 5.60 days, whereas
BD-08:2823c has a minimum mass of 0.33+/-0.03 M_Jup and an orbital period of
237.6 days. This new system strengthens the fact that low-mass planets are
preferentially found in multiplanetary systems, but not around high-metallicity
stars as this is the case for massive planets. It also supports the belief that
active stars should not be neglected in exoplanet searches, even when searching
for low-mass planets.Comment: 10 pages, 8 figures, 3 tables, accepted for publication in A&
Projected Rotational Velocities and Stellar Characterization of 350 B Stars in the Nearby Galactic Disk
Projected rotational velocities (vsini) are presented for a sample of 350
early B-type main sequence stars in the nearby Galactic disk. The stars are
located within ~1.5 kpc from the Sun, and the great majority within 700 pc. The
analysis is based on high-resolution spectra obtained with the MIKE
spectrograph on the Magellan Clay 6.5-m telescope at the Las Campanas
Observatory in Chile.Spectral types were estimated based on relative
intensities of some key line absorption ratios and comparisons to synthetic
spectra. Effective temperatures were estimated from the reddening-free Q index,
and projected rotational velocities were then determined via interpolation on a
published grid that correlates the synthetic full width at half maximum of the
He I lines at 4026, 4388 and 4471 A with vsini. As the sample has been selected
solely on the basis of spectral types it contains an selection of B stars in
the field, in clusters, and in OB associations. The vsini distribution obtained
for the entire sample is found to be essentially flat for vsini values between
0-150 km/s, with only a modest peak at low projected rotational velocities.
Considering subsamples of stars, there appears to be a gradation in the vsini
distribution with the field stars presenting a larger fraction of the slow
rotators and the cluster stars distribution showing an excess of stars with
vsini between 70 and 130 km/s. Furthermore, for a subsample of potential
runaway stars we find that the vsini distribution resembles the distribution
seen in denser environments, which could suggest that these runaway stars have
been subject to dynamical ejection mechanisms.Comment: 38 pages, 11 figures. Complete sample table. AJ accepte
Hipparcos open clusters and stellar evolution
By relying on recently improved Hipparcos parallaxes for the Hyades, Pleiades
and Ursa Major clusters we find that stellar models with updated physical
inputs nicely reproduce the location in the color magnitude diagram of main
sequence stars of different metallicities. Stars in the helium burning phase
are also discussed, showing that the luminosity of giants in the Hyades,
Praesepe and Ursa Major clusters appears to be in reasonable agreement with
theoretical predictions. A short discussion concerning the current evolutionary
scenarios closes the paper.Comment: 5 pages, 6 Postscript figures, accepted by MNRA
Debris Disks in NGC 2547
We have surveyed the 30 Myr-old cluster NGC 2547 for planetary debris disks
using Spitzer. At 4.5-8 um we are sensitive to the photospheric level down to
mid-M stars (0.2 Msol) and at 24 um to early-G stars (1.2 Msol). We find only
two to four stars with excesses at 8 um out of ~400-500 cluster members,
resulting in an excess fraction <~1 percent at this wavelength. By contrast,
the excess fraction at 24 um is ~40 percent (for B-F types). Out of four
late-type stars with excesses at 8 um two marginal ones are consistent with
asteroid-like debris disks. Among stars with strong 8 um excesses one is
possibly from a transitional disk, while another one can be a result of a
catastrophic collision. Our survey demonstrates that the inner 0.1-1 AU parts
of disks around solar-type stars clear out very thoroughly by 30 Myrs of age.
Comparing with the much slower decay of excesses at 24 and 70 um, disks clear
from the inside out, of order 10 Myr for the inner zones probed at 8 um
compared with a hundred or more Myr for those probed with the two longer
wavelengths.Comment: Accepted to ApJ, 29 pages, 13 figs. A Note in Proof concerning
cluster's age was added in the original submission of 2007 July 19. Full
Tables 1 and 2 in the electronic form together with the article with full
resolution figures are available at
http://www.astro.ufl.edu/~ngorlova/disksNGC2547
Photometric Monitoring of Open Clusters I. The Survey
Open clusters, which have age, abundance, and extinction information from
studies of main-sequence turn off stars, are the ideal location in which to
determine the mass-luminosity-radius relation for low-mass stars. We have
undertaken a photometric monitoring survey of open clusters in the Galaxy
designed to detect low-mass eclipsing binary systems through variations in
their relative light curves. Our aim is to provide an improved calibration of
the mass-luminosity-radius relation for low-mass stars and brown dwarfs, to
test stellar structure and evolution models, and to help quantify the
contribution of low-mass stars to the global mass census in the Galaxy. In this
paper we present our survey, describing the data and outlining the analysis
techniques. We study six nearby open clusters, with a range of ages from to 4 Gyr and metallicities from approximately solar to -0.2dex. We monitor
a field-of-view of > 1 square degree per target cluster, well beyond the
characteristic cluster radius, over timescales of hours, days, and months with
a sampling rate optimised for the detection of eclipsing binaries with periods
of hours to days. Our survey depth is designed to detect eclipse events in a
binary with a primary star of \lesssim 0.3~M_{\sun}. Our data have a
photometric precision of mmag at .Comment: 50 pages, 18 figures, accepted for publication in A
The effect of rotation on the abundances of the chemical elements of the A-type stars in the Praesepe cluster
We study how chemical abundances of late B-, A- and early F-type stars evolve
with time, and we search for correlations between the abundance of chemical
elements and other stellar parameters, such as effective temperature and Vsini.
We have observed a large number of B-, A- and F-type stars belonging to open
clusters of different ages. In this paper we concentrate on the Praesepe
cluster (log t = 8.85), for which we have obtained high resolution, high
signal-to-noise ratio spectra of sixteen normal A- and F-type stars and one Am
star, using the SOPHIE spectrograph of the Observatoire de Haute-Provence. For
all the observed stars, we have derived fundamental parameters and chemical
abundances. In addition, we discuss another eight Am stars belonging to the
same cluster, for which the abundance analysis had been presented in a previous
paper. We find a strong correlation between peculiarity of Am stars and Vsini.
The abundance of the elements underabundant in Am stars increases with Vsini,
while it decreases for the overabundant elements. Chemical abundances of
various elements appear correlated with the iron abundance.Comment: Accepted for publication on A&
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