516 research outputs found
Deep imaging survey of the environment of Alpha Centauri - II. CCD imaging with the NTT-SUSI2 camera
Context: The nearby pair of solar-type stars Alpha Centauri is a favorable
target for an imaging search for extrasolar planets. Indications exist that the
gravitational mass of Alpha Cen B could be higher than its modeled mass, the
difference being consistent with a substellar companion of a few tens of
Jupiter masses. However, Alpha Centauri usually appears in star catalogues
surrounded by a large void area, due to the strong diffused light. Aims: We
searched for faint comoving companions to Alpha Cen located at angular
distances of the order of a few tens of arcseconds, up to 2-3 arcmin. As a
secondary objective, we built a catalogue of the detected background sources.
Methods: In order to complement our adaptive optics search at small angular
distances (Paper I), we used atmosphere limited CCD imaging from the NTT-SUSI2
instrument in the Bessel V, R, I, and Z bands. Results: We present the results
of our search in the form of a catalogue of the detected objects inside a 5.5
arcmin box around this star. A total of 4313 sources down to mV~24 and mI~22
were detected from this wide-field survey. We extracted the infrared photometry
of part of the detected sources from archive images of the 2MASS survey (JHK
bands). We investigate briefly the nature of the detected sources, many of them
presenting extremely red color indices (V-K > 14). Conclusions: We did not
detect any companion to Alpha Centauri between 100 and 300 AU, down to a
maximum mass of ~15 times Jupiter. We also mostly exclude the presence of a
companion more massive than 30 MJup between 50 and 100 AU.Comment: Accepted for publication as a Research Note in A&
New determination of abundances and stellar parameters for a set of weak G-band stars
Weak G-band (wGb) stars are very peculiar red giants almost devoided of
carbon and often mildly enriched in lithium. Despite their very puzzling
abundance patterns, very few detailed spectroscopic studies existed up to a few
years ago, preventing any clear understanding of the wGb phenomenon. We
recently proposed the first consistent analysis of published data for 28 wGb
stars and identified them as descendants of early A-type to late B-type stars,
without being able to conclude on their evolutionary status or the origin of
their peculiar abundance pattern.
We used newly obtained high-resolution and high SNR spectra for 19 wGb stars
in the southern and northern hemisphere to homogeneously derive their
fundamental parameters, metallicities, as well as the spectroscopic abundances
for Li, C, N, O, Na, Sr, and Ba. We also computed dedicated stellar evolution
models that we used to determine the masses and to investigate the evolutionary
status and chemical history of the stars in our sample. We confirm that the wGb
stars are stars in the mass range 3.2 to 4.2 M. We suggest that a large
fraction could be mildly evolved stars on the SGB currently undergoing the 1st
DUP, while a smaller number of stars are more probably in the core He burning
phase at the clump. After analysing their abundance pattern, we confirm their
strong N enrichment anti-correlated with large C depletion, characteristic of
material fully processed through the CNO cycle to an extent not known in other
evolved intermediate-mass stars. However, we demonstrate here that such a
pattern is very unlikely due to self-enrichment. In the light of the current
observational constraints, no solid self-consistent pollution scenario can be
presented either, leaving the wGb puzzle largely unsolved.Comment: 19 pages , 14 figures, accepted for publication in Astronomy &
Astrophysic
Gaia FGK Benchmark Stars: Effective temperatures and surface gravities
Large Galactic stellar surveys and new generations of stellar atmosphere
models and spectral line formation computations need to be subjected to careful
calibration and validation and to benchmark tests. We focus on cool stars and
aim at establishing a sample of 34 Gaia FGK Benchmark Stars with a range of
different metallicities. The goal was to determine the effective temperature
and the surface gravity independently from spectroscopy and atmospheric models
as far as possible. Fundamental determinations of Teff and logg were obtained
in a systematic way from a compilation of angular diameter measurements and
bolometric fluxes, and from a homogeneous mass determination based on stellar
evolution models. The derived parameters were compared to recent spectroscopic
and photometric determinations and to gravity estimates based on seismic data.
Most of the adopted diameter measurements have formal uncertainties around 1%,
which translate into uncertainties in effective temperature of 0.5%. The
measurements of bolometric flux seem to be accurate to 5% or better, which
contributes about 1% or less to the uncertainties in effective temperature. The
comparisons of parameter determinations with the literature show in general
good agreements with a few exceptions, most notably for the coolest stars and
for metal-poor stars. The sample consists of 29 FGK-type stars and 5 M giants.
Among the FGK stars, 21 have reliable parameters suitable for testing,
validation, or calibration purposes. For four stars, future adjustments of the
fundamental Teff are required, and for five stars the logg determination needs
to be improved. Future extensions of the sample of Gaia FGK Benchmark Stars are
required to fill gaps in parameter space, and we include a list of suggested
candidates.Comment: Accepted by A&A; 34 pages (printer format), 14 tables, 13 figures;
language correcte
The Zeta Herculis binary system revisited. Calibration and seismology
We have revisited the calibration of the visual binary system Zeta Herculis
with the goal to give the seismological properties of the G0 IV sub-giant Zeta
Her A. We have used the most recent physical and observational data. For the
age we have obtained 3387 Myr, for the masses respectively 1.45 and 0.98 solar
mass, for the initial helium mass fraction 0.243, for the initial mass ratio of
heavy elements to hydrogen 0.0269 and for the mixing-length parameters
respectively 0.92 and 0.90 using the Canuto & Mazitelli (1991, 1992) convection
theory. Our results do not exclude that Zeta Her A is itself a binary
sub-system; the mass of the hypothetical unseen companion would be smaller than
0.05 solar mass. The adiabatic oscillation spectrum of Zeta Her A is found to
be a complicated superposition of acoustic and gravity modes; some of them have
a dual character. This greatly complicates the classification of the non-radial
modes. The echelle diagram used by the observers to extract the frequencies
will work for ell=0, 2, 3. The large difference is found to be of the order of
42 mu Hz, in agreement with the Martic et al. (2001) seismic observations.Comment: 12 pages, A&A in pres
A large sample of calibration stars for Gaia: log g from Kepler and CoRoT
Asteroseismic data can be used to determine surface gravities with precisions
of < 0.05 dex by using the global seismic quantities Deltanu and nu_max along
with Teff and [Fe/H]. Surface gravity is also one of the four stellar
properties to be derived by automatic analyses for 1 billion stars from Gaia
data (workpackage GSP_Phot). We explore seismic data from MS F, G, K stars
(solar-like stars) observed by Kepler as a potential calibration source for
methods that Gaia will use for object characterisation (log g). We calculate
log g for bright nearby stars for which radii and masses are known, and using
their global seismic quantities in a grid-based method, we determine an
asteroseismic log g to within 0.01 dex of the direct calculation, thus
validating the accuracy of our method. We find that errors in Teff and mainly
[Fe/H] can cause systematic errors of 0.02 dex. We then apply our method to a
list of 40 stars to deliver precise values of surface gravity, i.e. sigma <
0.02 dex, and we find agreement with recent literature values. Finally, we
explore the precision we expect in a sample of 400+ Kepler stars which have
their global seismic quantities measured. We find a mean uncertainty
(precision) on the order of <0.02 dex in log g over the full explored range 3.8
< log g < 4.6, with the mean value varying only with stellar magnitude (0.01 -
0.02 dex). We study sources of systematic errors in log g and find possible
biases on the order of 0.04 dex, independent of log g and magnitude, which
accounts for errors in the Teff and [Fe/H] measurements, as well as from using
a different grid-based method. We conclude that Kepler stars provide a wealth
of reliable information that can help to calibrate methods that Gaia will use,
in particular, for source characterisation with GSP_Phot where excellent
precision (small uncertainties) and accuracy in log g is obtained from seismic
data.Comment: Accepted MNRAS, 15 pages (10 figures and 3 tables), v2=some rewording
of two sentence
On the metallicity distribution of classical Cepheids in the Galactic inner disk
We present homogeneous and accurate iron abundances for almost four dozen
(47) of Galactic Cepheids using high-spectral resolution (R40,000) high
signal-to-noise ratio (S/N 100) optical spectra collected with UVES at
VLT. A significant fraction of the sample (32) is located in the inner disk (RG
6.9 kpc) and for half of them we provide new iron abundances. Current
findings indicate a steady increase in iron abundance when approaching the
innermost regions of the thin disk. The metallicity is super-solar and ranges
from 0.2 dex for RG 6.5 kpc to 0.4 dex for RG 5.5 kpc. Moreover,
we do not find evidence of correlation between iron abundance and distance from
the Galactic plane. We collected similar data available in the literature and
ended up with a sample of 420 Cepheids. Current data suggest that the mean
metallicity and the metallicity dispersion in the four quadrants of the
Galactic disk attain similar values. The first-second quadrants show a more
extended metal-poor tail, while the third-fourth quadrants show a more extended
metal-rich tail, but the bulk of the sample is at solar iron abundance.
Finally, we found a significant difference between the iron abundance of
Cepheids located close to the edge of the inner disk ([Fe/H]0.4) and
young stars located either along the Galactic bar or in the nuclear bulge
([Fe/H]0). Thus suggesting that the above regions have had different
chemical enrichment histories. The same outcome applies to the metallicity
gradient of the Galactic bulge, since mounting empirical evidence indicates
that the mean metallicity increases when moving from the outer to the inner
bulge regions.Comment: 10 pages, 5 figures; Corrected typos, corrected Table
VLT Observations of Turnoff stars in the Globular Cluster NGC 6397
VLT-UVES high resolution spectra of seven turnoff stars in the metal-poor
globular cluster NGC 6397 have been obtained. Atmospheric parameters and
abundances of several elements (Li, Na, Mg, Ca, Sc, Ti, Cr, Fe, Ni, Zn and Ba)
were derived for program stars. The mean iron abundance is [Fe/H] = -2.02, with
no star-to-star variation. The mean abundances of the alpha-elements (Ca, Ti)
and of the iron-peak elements (Sc, Cr, Ni) are consistent with abundances
derived for field stars of similar metallicity. Magnesium is also almost solar,
consistent with the values found by Idiart & Th\'evenin (2000) when non-LTE
effects (NLTE hereafter) are taken into account. The sodium abundance derived
for five stars is essentially solar, but one object (A447) is clearly Na
deficient. These results are compatible with the expected abundance range
estimated from the stochastic evolutionary halo model by Argast et al. (2000)
when at the epoch of [Fe/H] -2 the interstellar medium is supposed to
become well-mixed.Comment: to appear in A&
The Carina Project IX: on Hydrogen and helium burning variables
We present new multi-band (UBVI) time-series data of helium burning variables
in the Carina dwarf spheroidal galaxy. The current sample includes 92 RR
Lyrae-six of them are new identifications-and 20 Anomalous Cepheids, one of
which is new identification. The analysis of the Bailey diagram shows that the
luminosity amplitude of the first overtone component in double-mode variables
is located along the long-period tail of regular first overtone variables,
while the fundamental component is located along the short-period tale of
regular fundamental variables. This evidence further supports the transitional
nature of these objects. Moreover, the distribution of Carina double-mode
variables in the Petersen diagram (P_1/P_0 vs P_0) is similar to metal-poor
globulars (M15, M68), to the dwarf spheroidal Draco and to the Galactic Halo.
This suggests that the Carina old stellar population is metal-poor and affected
by a small spread in metallicity. We use trigonometric parallaxes for five
field RR Lyrae stars to provide an independent estimate of the Carina distance
using the observed reddening free Period--Wesenheit [PW, (BV)] relation. Theory
and observations indicate that this diagnostic is independent of metallicity.
We found a true distance modulus of \mu=20.01\pm0.02 (standard error of the
mean) \pm0.05 (standard deviation) mag. We also provided independent estimates
of the Carina true distance modulus using four predicted PW relations (BV, BI,
VI, BVI) and we found: \mu=(20.08\pm0.007\pm0.07) mag,
\mu=(20.06\pm0.006\pm0.06) mag, \mu=(20.07\pm0.008\pm0.08) mag and
\mu=(20.06\pm0.006\pm0.06) mag. Finally, we identified more than 100 new SX
Phoenicis stars that together with those already known in the literature (340)
make Carina a fundamental laboratory to constrain the evolutionary and
pulsation properties of these transitional variables.Comment: 44 pages, 13 tables, 13 figures, Accepted for publication in Ap
A grid of NLTE corrections for magnesium and calcium in late-type giant and supergiant stars: application to Gaia
We investigate NLTE effects for magnesium and calcium in the atmospheres of
late-type giant and supergiant stars. The aim of this paper is to provide a
grid of NLTE/LTE equivalent width ratios W/W* of Mg and Ca lines for the
following range of stellar parameters: Teff in [3500, 5250] K, log g in [0.5,
2.0] dex and [Fe/H] in [-4.0, 0.5] dex. We use realistic model atoms with the
best physics available and taking into account the fine structure. The Mg and
Ca lines of interest are in optical and near IR ranges. A special interest
concerns the lines in the Gaia spectrograph (RVS) wavelength domain [8470,
8740] A. The NLTE corrections are provided as function of stellar parameters in
an electronic table as well as in a polynomial form for the Gaia/RVS lines.Comment: 27 pages, 19 figures and 11 tables. Also on-line data; MNRAS (2011)
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