348 research outputs found
Core properties of alpha Cen A using asteroseismology
A set of long and nearly continuous observations of alpha Centauri A should
allow us to derive an accurate set of asteroseismic constraints to compare to
models, and make inferences on the internal structure of our closest stellar
neighbour. We intend to improve the knowledge of the interior of alpha Centauri
A by determining the nature of its core. We combined the radial velocity time
series obtained in May 2001 with three spectrographs in Chile and Australia:
CORALIE, UVES, and UCLES. The resulting combined time series has a length of
12.45 days and contains over 10,000 data points and allows to greatly reduce
the daily alias peaks in the power spectral window. We detected 44 frequencies
that are in good overall agreement with previous studies, and found that 14 of
these show possible rotational splittings. New values for the large and small
separations have been derived. A comparison with stellar models indicates that
the asteroseismic constraints determined in this study allows us to set an
upper limit to the amount of convective-core overshooting needed to model stars
of mass and metallicity similar to those of alpha Cen A.Comment: 8 pages, 11 figures, A&A accepte
Improved stellar parameters of CoRoT-7
Accurate parameters of the host stars of exoplanets are important for the
interpretation of the new planet systems that continue to emerge. The CoRoT
satellite recently discovered a transiting rocky planet with a density similar
to the inner planets in our solar system, a so-called Super Earth. This planet
is orbiting a relatively faint G9V star called CoRoT-7, and we wish to refine
its physical properties, which are important for the interpretation of the
properties of the planet system. We used spectra from [email protected] and
[email protected]. From the analysis of Fe-1 and Fe-2 lines we determine Teff, log
g and microturbulence. We use the Balmer lines to constrain Teff and pressure
sensitive Mg-1b and Ca lines to constrain log g. From the analysis we find
Teff=5250+-60K, log g = 4.47+-0.05, [M/H]=+0.12+-0.06, and vsini = 1.1 km/s. We
compared the L/M ratio with isochrones to constrain the evolutionary status.
Using the age estimate of 1.2-2.3 Gyr based on stellar activity, we determine
the mass and radius 0.91+-0.03 Msun and 0.82+-0.04 Rsun. With these updated
constraints we fitted the CoRoT transit light curve for CoRoT-7b. We revise the
planet radius to be slightly smaller, R = 1.58+-0.10 Rearth, and the density
becomes higher, rho = 7.2+-1.8 g/cm3. The host star CoRoT-7 is a slowly
rotating, metal rich, unevolved type G9V star. The star is relatively faint
(V=11.7) and its fundamental parameters can only be determined through indirect
methods. Our methods rely on detailed spectral analyses that depend on the
adopted model atmospheres. From the analysis of spectra of stars with
well-known parameters with similar parameters to CoRoT-7 (the Sun and alpha Cen
B) we demonstrate that our methods are robust within the claimed uncertainties.
Therefore our methods can be reliably used in subsequent analyses of similar
exoplanet host stars.Comment: Accepted by A&A; 10 pages; abstract abridged; resolution decreased in
Fig.
A Planet at 5 AU Around 55 Cancri
We report precise Doppler shift measurements of 55 Cancri (G8V) obtained from
1989 to 2002 at Lick Observatory. The velocities reveal evidence for an outer
planetary companion to 55 Cancri orbiting at 5.5 AU. The velocities also
confirm a second, inner planet at 0.11 AU. The outer planet is the first
extrasolar planet found that orbits near or beyond the orbit of Jupiter. It was
drawn from a sample of ~50 stars observed with sufficient duration and quality
to detect a giant planet at 5 AU, implying that such planets are not rare. The
properties of this jupiter analog may be compared directly to those of the
Jovian planets in our Solar System. Its eccentricity is modest, e=0.16,
compared with e=0.05 for both Jupiter and Saturn. Its mass is at least 4.0
jupiter masses (M sin i). The two planets do not perturb each other
significantly. Moreover, a third planet of sub-Jupiter mass could easily
survive in between these two known planets. Indeed a third periodicity remains
in the velocity measurements with P = 44.3 d and a semi-amplitude of 13 m/s.
This periodicity is caused either by a third planet at a=0.24 AU or by
inhomogeneities on the stellar surface that rotates with period 42 d. The
planet interpretation is more likely, as the stellar surface is quiet,
exhibiting log(R'_{HK}) = -5.0 and brightness variations less than 1 millimag,
and any hypothetical surface inhomogeneity would have to persist in longitude
for 14 yr. Even with all three planets, an additional planet of
terrestrial--mass could orbit stably at ~1 AU. The star 55 Cancri is apparently
a normal, middle-aged main sequence star with a mass of 0.95 solar masses, rich
in heavy elements ([Fe/H] = +0.27). This high metallicity raises the issue of
the relationship between its age, rotation, and chromosphere.Comment: 47 pages, 4 tables, 12 figures, uses AASTE
The VAST Survey - I. Companions and the unexpected X-ray detection of B6-A7 stars
With an adaptive optics imaging survey of 148 B6-A7 stars, we have tested the
hypothesis that unresolved lower-mass companions are the source of the
unexpected X-ray detections of stars in this spectral type range. The sample is
composed of 63 stars detected in X-rays within the ROSAT All-Sky Survey and 85
stars that form a control sample; both subsets have the same restricted
distribution of spectral type, age, X-ray sensitivity and separation coverage.
A total of 68 companion candidates are resolved with separations ranging from
0.3" to 26.2", with 23 new detections. The multiple star frequency of the X-ray
sample based on companions resolved within the ROSAT error ellipse is found to
be 43 (+6,-6)%. The corresponding control sample multiple star frequency is
three times lower at 12 (+4,-3)% -- a difference of 31\pm7%. These results are
presented in the first of a series of papers based on our Volume-limited A-Star
(VAST) survey -- a comprehensive study of the multiplicity of A-type stars.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Societ
Double-blind test program for astrometric planet detection with Gaia
We use detailed simulations of the Gaia observations of synthetic planetary
systems and develop and utilize independent software codes in double-blind mode
to analyze the data, including statistical tools for planet detection and
different algorithms for single and multiple Keplerian orbit fitting that use
no a priori knowledge of the true orbital parameters of the systems. 1) Planets
with astrometric signatures times the single-measurement error
and period yr can be detected reliably, with a very
small number of false positives. 2) At twice the detection limit, uncertainties
in orbital parameters and masses are typically . 3) Over 70% of
two-planet systems with well-separated periods in the range
yr, , and eccentricity are
correctly identified. 4) Favorable orbital configurations have orbital elements
measured to better than 10% accuracy of the time, and the value of the
mutual inclination angle determined with uncertainties \leq 10^{\degr}. 5)
Finally, uncertainties obtained from the fitting procedures are a good estimate
of the actual errors. Extrapolating from the present-day statistical properties
of the exoplanet sample, the results imply that a Gaia with = 8
as, in its unbiased and complete magnitude-limited census of planetary
systems, will measure several thousand giant planets out to 3-4 AUs from stars
within 200 pc, and will characterize hundreds of multiple-planet systems,
including meaningful coplanarity tests. Finally, we put Gaia into context,
identifying several areas of planetary-system science in which Gaia can be
expected to have a relevant impact, when combined with data coming from other
ongoing and future planet search programs.Comment: 32 pages, 24 figures, 6 tables. Accepted for pubolication in A&
A catalogue of young runaway Hipparcos stars within 3kpc from the Sun
Traditionally runaway stars are O and B type stars with large peculiar
velocities.We want to extend this definition to young stars (up to ~50 Myr) of
any spectral type and identify those present in the Hipparcos catalogue
applying different selection criteria such as peculiar space velocities or
peculiar one-dimensional velocities. Runaway stars are important to study the
evolution of multiple star systems or star clusters as well as to identify
origins of neutron stars. We compile distances, proper motions, spectral types,
luminosity classes, V magnitudes and B-V colours and utilise evolutionary
models from different authors to obtain star ages and study a sample of 7663
young Hipparcos stars within 3 kpc from the Sun. Radial velocities are obtained
from the literature. We investigate the distributions of the peculiar spatial
velocity, the peculiar radial velocity as well as the peculiar tangential
velocity and its one-dimensional components and obtain runaway star
probabilities for each star in the sample. In addition, we look for stars that
are situated outside any OB association or OB cluster and the Galactic plane as
well as stars of which the velocity vector points away from the median velocity
vector of neighbouring stars or the surrounding local OB association/ cluster
although the absolute velocity might be small. We find a total of 2547 runaway
star candidates (with a contamination of normal Population I stars of 20 per
cent at most). Thus, after subtraction of those 20 per cent, the runaway
frequency among young stars is about 27 per cent. We compile a catalogue of
runaway stars which will be available via VizieR.Comment: 12 pages, 8 figures, 7 tables, accepted for publication in MNRAS old
version replaced due to change of the title after journal proof-readin
VLT multi-object spectroscopy of 33 eclipsing binaries in the Small Magellanic Cloud. New distance and depth of the SMC, and a record-breaking apsidal motion
Aim: Our purpose is to provide reliable stellar parameters for a significant
sample of eclipsing binaries, which are representative of a whole dwarf and
metal-poor galaxy. We also aim at providing a new estimate of the mean distance
to the SMC and of its depth along the line of sight for the observed field of
view. Method: We use radial velocity curves obtained with the ESO FLAMES
facility at the VLT and light curves from the OGLE-II photometric survey. The
radial velocities were obtained by least-squares fits of the observed spectra
to synthetic ones, excluding the hydrogen Balmer lines. Results: Our sample
contains 23 detached, 9 semi-detached and 1 overcontact systems. Most detached
systems have properties consistent with stellar evolution calculations from
single-star models at the standard SMC metallicity Z = 0.004, though they tend
to be slightly overluminous. The few exceptions are probably due to third light
contribution or insufficient signal-to-noise ratio. The mass ratios are
consistent with a flat distribution, both for detached and
semi-detached/contact binaries. A mass-luminosity relation valid from ~4 to ~18
Msol is derived. The uncertainties are in the +-2 to +-11% range for the
masses, in the +-2 to +-5% range for the radii and in the +-1 to +-6% range for
the effective temperatures. The average distance modulus is 19.11+-0.03
(66.4+-0.9 kpc). The moduli derived from the V and from the I data are
consistent within 0.01 mag. The 2-sigma depth of the SMC is, for our field, of
0.25 mag or 7.6 kpc under the assumption of a gaussian distribution of stars
along the line of sight. Three systems show significant apsidal motion, one of
them with an apsidal period of 7.6 years, the shortest known to date for a
detached system with main sequence stars.Comment: 61 pages, 41 figures; accepted for publication in Astronomy &
Astrophysic
Magnetic field measurements and wind-line variability of OB-type stars
Context. The first magnetic fields in O- and B-type stars that do not belong
to the Bp-star class, have been discovered. The cyclic UV wind-line
variability, which has been observed in a significant fraction of early-type
stars, is likely to be related to such magnetic fields. Aims. We attempt to
improve our understanding of massive-star magnetic fields, and observe
twenty-five carefully-selected, OB-type stars. Methods. Of these stars we
obtain 136 magnetic field strength measurements. We present the UV wind-line
variability of all selected targets and summarise spectropolarimetric
observations acquired using the MUSICOS spectropolarimeter, mounted at the TBL,
Pic du Midi, between December 1998 and November 2004. From the average Stokes I
and V line profiles, derived using the LSD method, we measure the magnetic
field strengths, radial velocities, and first moment of the line profiles.
Results. No significant magnetic field is detected in any OB-type star that we
observed. Typical 1{\sigma} errors are between 15 and 200 G. A possible
magnetic-field detection for the O9V star 10 Lac remains uncertain, because the
field measurements depend critically on the fringe- effect correction in the
Stokes V spectra. We find excess emission in UV-wind lines, centred about the
rest wavelength, to be a new indirect indicator of the presence of a magnetic
field in early B-type stars. The most promising candidates to host magnetic
fields are the B-type stars {\delta} Cet and 6 Cep, and a number of O stars.
Conclusions. Although some O and B stars have strong dipolar field, which cause
periodic variability in the UV wind-lines, such strong fields are not
widespread. If the variability observed in the UV wind-lines of OB stars is
generally caused by surface magnetic fields, these fields are either weak
(<~few hundred G) or localised.Comment: A&A publishe
The Alpha Centauri Binary System: Atmospheric Parameters and Element Abundances
The alpha Centauri binary system, owing to its duplicity, proximity and
brightness, and its components' likeness to the Sun, is a fundamental
calibrating object for the theory of stellar structure and evolution and the
determination of stellar atmospheric parameters. This role, however, is
hindered by a considerable disagreement in the published analyses of its
atmospheric parameters and abundances. We report a new spectroscopic analysis
of both components of the alpha Centauri binary system and compare published
analyses of the system. The analysis is differential with respect to the Sun,
based on high-quality spectra, and employed spectroscopic and photometric
methods to obtain as many independent Teff determinations as possible. The
atmospheric parameters are also checked for consistency against the results of
the dynamical analysis and the positions of the components in a theoretical HR
diagram. We discuss possible origins of discrepancies, concluding that the
presence of NLTE effects is a probable candidate, but we note that there is as
yet no consensus on the existence and cause of an offset between the
spectroscopic and photometric Teff scales of cool dwarfs. The spectroscopic
surface gravities also agree with those derived from directly measured masses
and radii. The abundance pattern can be deemed normal in the context of recent
data on metal-rich stars. The position of alpha Cen A in an up-to-date
theoretical evolutionary diagrams yields a good match of the evolutionary mass
and age with those from the dynamical solution and seismology.Comment: 17 pages, 10 figures, accepted by A&A. Replacing the older version
with this new, refereed, one. Main modifications concern an updated
discussion of the impact of systematic offsets between different temperature
scales and non-LTE effects on the determination of abundances and atmospheric
parameters. The abstract is abridge
Accurate fundamental parameters for 23 bright solar-type stars
We combine results from interferometry, asteroseismology and spectroscopy to
determine accurate fundamental parameters of 23 bright solar-type stars, from
spectral type F5 to K2 and luminosity classes III to V. For some stars we can
use direct techniques to determine the mass, radius, luminosity and effective
temperature, and we compare with indirect methods that rely on photometric
calibrations or spectroscopic analyses. We use the asteroseismic information
available in the literature to infer an indirect mass with an accuracy of 4-15
percent. From indirect methods we determine luminosity and radius to 3 percent.
For Teff we find a slight offset of -40+-20 K between the spectroscopic method
and the direct method, meaning the spectroscopic temperatures are too high.
From the spectroscopic analysis we determine the detailed chemical
composition for 13 elements, including Li, C and O. We find no significant
offset between the spectroscopic surface gravity and the value from combining
asteroseismology with radius estimates. From the spectroscopy we also determine
vsini and we present a new calibration of macro- and microturbulence. From the
comparison between the results from the direct and spectroscopic methods we
claim that we can determine Teff, log g, and [Fe/H] with absolute accuracies of
80 K, 0.08 dex, and 0.07 dex. The indirect methods are important to obtain
reliable estimates of the fundamental parameters of relatively faint stars when
interferometry cannot be used. Our study is the first to compare direct and
indirect methods for a large sample of stars, and we conclude that indirect
methods are valid, although slight corrections may be needed.Comment: Accepted by MNRAS. Abstract abridge
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