83 research outputs found
Exploration of the BaSeL stellar library for 9 F-type stars COROT potential targets
The Basel Stellar Library (BaSeL models) is constituted of the merging of
various synthetic stellar spectra libraries, with the purpose of giving the
most comprehensive coverage of stellar parameters. It has been corrected for
systematic deviations detected in respect to UBVRIJHKLM photometry at solar
metallicity, and can then be considered as the state-of-the-art knowledge of
the broad band content of stellar spectra. In this paper, we consider a sample
of 9 F-type stars with detailed spectroscopic analysis to investigate the Basel
Stellar Library in two photometric systems simultaneously, Johnson (B-V, U-B)
and Stromgren (b-y, m_1, and c_1). The sample corresponds to potential targets
of the central seismology programme of the COROT space experiment, which have
been recently observed at OHP. The atmospheric parameters T_eff, [Fe/H], and
log g obtained from the BaSeL models are compared with spectroscopic
determinations as well as with results of other photometric calibrations. For a
careful interpretation of the BaSeL solutions, we computed confidence regions
around the best ^2-estimates and projected them on T_eff-[Fe/H],
T_eff-log g, and log g-[Fe/H] diagrams. (Abridged)Comment: 16 pages, LaTeX2e; version accepted for publication in the new A&A
Journal: minor changes + figures in black and white for better readabilit
Study of KIC 8561221 observed by Kepler: an early red giant showing depressed dipolar modes
The continuous high-precision photometric observations provided by the CoRoT
and Kepler space missions have allowed us to better understand the structure
and dynamics of red giants using asteroseismic techniques. A small fraction of
these stars shows dipole modes with unexpectedly low amplitudes. The reduction
in amplitude is more pronounced for stars with higher frequency of maximum
power. In this work we want to characterize KIC 8561221 in order to confirm
that it is currently the least evolved star among this peculiar subset and to
discuss several hypotheses that could help explain the reduction of the dipole
mode amplitudes. We used Kepler short- and long-cadence data combined with
spectroscopic observations to infer the stellar structure and dynamics of KIC
8561221. We then discussed different scenarios that could contribute to the
reduction of the dipole amplitudes such as a fast rotating interior or the
effect of a magnetic field on the properties of the modes. We also performed a
detailed study of the inertia and damping of the modes. We have been able to
characterize 37 oscillations modes, in particular, a few dipole modes above
nu_max that exhibit nearly normal amplitudes. We have inferred a surface
rotation period of around 91 days and uncovered the existence of a variation in
the surface magnetic activity during the last 4 years. As expected, the
internal regions of the star probed by the l = 2 and 3 modes spin 4 to 8 times
faster than the surface. With our grid of standard models we are able to
properly fit the observed frequencies. Our model calculation of mode inertia
and damping give no explanation for the depressed dipole modes. A fast rotating
core is also ruled out as a possible explanation. Finally, we do not have any
observational evidence of the presence of a strong deep magnetic field inside
the star.Comment: Accepted in A&A. 17 pages, 16 figure
The rapid rotation and complex magnetic field geometry of Vega
The recent discovery of a weak surface magnetic field on the normal
intermediate-mass star Vega raises the question of the origin of this magnetism
in a class of stars that was not known to host magnetic fields. We aim to
confirm the field detection and provide additional observational constraints
about the field characteristics, by modelling the magnetic geometry of the star
and by investigating the seasonal variability of the reconstructed field. We
analyse a total of 799 circularly-polarized spectra collected with the NARVAL
and ESPaDOnS spectropolarimeters during 2008 and 2009. We employ a
cross-correlation procedure to compute, from each spectrum, a mean polarized
line profile with a signal-to-noise ratio of about 20,000. The technique of
Zeeman-Doppler Imaging is then used to determine the rotation period of the
star and reconstruct the large-scale magnetic geometry of Vega at two different
epochs. We confirm the detection of circularly polarized signatures in the mean
line profiles. The amplitude of the signatures is larger when spectral lines of
higher magnetic sensitivity are selected for the analysis, as expected for a
signal of magnetic origin. The short-term evolution of polarized signatures is
consistent with a rotational period of 0.732 \pm 0.008 d. The reconstructed
magnetic topology unveils a magnetic region of radial field orientation,
closely concentrated around the rotation pole. This polar feature is
accompanied by a small number of magnetic patches at lower latitudes. No
significant variability in the field structure is observed over a time span of
one year. The repeated observation of a weak photospheric magnetic field on
Vega suggests that a previously unknown type of magnetic stars exists in the
intermediate-mass domain. Vega may well be the first confirmed member of a much
larger, as yet unexplored, class of weakly-magnetic stars.Comment: Accepted by Astronomy & Astrophysics. Abstract shortened to respect
the arXiv limit of 1920 character
Why are some A stars magnetic, while most are not?
A small fraction of intermediate-mass main sequence (A and B type) stars have strong, organised magnetic fields. The large majority of such stars, however, show no evidence for magnetic fields, even when observed with very high precision. In this paper we describe a simple model, motivated by qualitatively new observational results, that provides a natural physical explanation for the small fraction of observed magnetic stars
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