204 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
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
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
Spectroscopic monitoring of the Herbig Ae star HD 104237. II. Non-radial pulsations, mode analysis and fundamental stellar parameters
Herbig Ae/Be stars are intermediate-mass pre-main sequence (PMS) stars
showing signs of intense activity and strong stellar winds, whose origin is not
yet understood in the frame of current theoretical models of stellar evolution
for young stars. The evolutionary tracks of the earlier Herbig Ae stars cross a
recently discovered PMS instability strip. Many of these stars exhibit
pulsations of delta Scuti type. HD 104237 is a well-known pulsating Herbig Ae
star. In this article, we reinvestigated an extensive high-resolution
quasi-continuous spectroscopic data set in order to search for very faint
indications of non-radial pulsations in the line profile. To do this, we worked
on dynamical spectra of equivalent photospheric (LSD) profiles of HD 104237. A
2D Fourier analysis (F2D) was performed of the entire profile and the temporal
variation of the central depth of the line was studied with the time-series
analysis tools Period04 and SigSpec. We present a mode identification
corresponding to the detected dominant frequency. We perform a new accurate
determination of the fundamental stellar parameters in view of a forthcoming
asteroseismic modeling. Following the previous studies on this star, our
analysis of the dynamical spectrum of recentered LSD profiles corresponding to
the 22nd -25th of April 1999 nights spectra has confirmed the presence of
multiple oscillation modes of low-degree l in HD 104237 and led to the first
direct detection of a non-radial pulsation mode in this star: the dominant mode
F1 was identified by the Fourier 2D method having a degree l value comprised
between 1 and 2, the symmetry of the pattern variation indicating an azimuthal
order of +1 or -1. The detailed study of the fundamental stellar parameters has
provided a Teff, log g and iron abundance of 8550 +/- 150K, 3.9 +/- 0.3 and
-4.38 +/- 0.19 (i.e. [Fe/H]=+0.16 +/- 0.19), respectively
Comparative actions of progesterone, medroxyprogesterone acetate, drospirenone and nestorone on breast cancer cell migration and invasion
<p>Abstract</p> <p>Background</p> <p>Limited information is available on the effects of progestins on breast cancer progression and metastasis. Cell migration and invasion are central for these processes, and require dynamic cytoskeletal and cell membrane rearrangements for cell motility to be enacted.</p> <p>Methods</p> <p>We investigated the effects of progesterone (P), medroxyprogesterone acetate (MPA), drospirenone (DRSP) and nestorone (NES) alone or with 17β-estradiol (E2) on T47-D breast cancer cell migration and invasion and we linked some of these actions to the regulation of the actin-regulatory protein, moesin and to cytoskeletal remodeling.</p> <p>Results</p> <p>Breast cancer cell horizontal migration and invasion of three-dimensional matrices are enhanced by all the progestins, but differences are found in terms of potency, with MPA being the most effective and DRSP being the least. This is related to the differential ability of the progestins to activate the actin-binding protein moesin, leading to distinct effects on actin cytoskeleton remodeling and on the formation of cell membrane structures that mediate cell movement. E2 also induces actin remodeling through moesin activation. However, the addition of some progestins partially offsets the action of estradiol on cell migration and invasion of breast cancer cells.</p> <p>Conclusion</p> <p>These results imply that P, MPA, DRSP and NES alone or in combination with E2 enhance the ability of breast cancer cells to move in the surrounding environment. However, these progestins show different potencies and to some extent use distinct intracellular intermediates to drive moesin activation and actin remodeling. These findings support the concept that each progestin acts differently on breast cancer cells, which may have relevant clinical implications.</p
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