239 research outputs found
The CoRoT discovery of a unique triple-mode cepheid in the galaxy
The exploitation of the CoRoT treasure of stars observed in the exoplanetary
field allowed the detection of a unusual triple-mode Cepheid in the Milky Way,
CoRoT 0223989566. The two modes with the largest amplitudes and period ratio of
0.80 are identified with the first (P1=1.29 d) and second (P2=1.03 d) radial
overtones. The third period, which has the smallest amplitude but able to
produce combination terms with the other two, is the longest one (P3=1.89 d).
The ratio of 0.68 between the first-overtone period and the third period is the
unusual feature. Its identification with the fundamental radial or a nonradial
mode is discussed with respect to similar cases in the Magellanic Clouds. In
both cases the period triplet and the respective ratios make the star unique in
our Galaxy. The distance derived from the period-luminosity relation and the
galactic coordinates put CoRoT~0223989566 in the metal-rich environment of the
"outer arm" of the Milky Way.Comment: Published as ApJ Lette
Comparative seismology of pre- and main sequence stars in the instability strip
Pulsational properties of 1.8 M stellar models covering the latest
stages of contraction toward the main sequence up to early hydrogen burning
phases are investigated by means of linear nonadiabatic analyses. Results
confirm that pre-main sequence stars (pms) which cross the classical
instability strip on their way toward the main sequence are pulsationally
unstable with respect to the classical opacity mechanisms. For both pms and
main sequence types of models in the lower part of the instability strip, the
unstable frequency range is found to be roughly the same. Some non-radial
unstable modes are very sensitive to the deep internal structure of the star.
It is shown that discrimination between pms and main sequence stages is
possible using differences in their oscillation frequency distributions in the
low frequency range.Comment: 8 pages, 9 figures, accepted for publication in A&
CoRoT reveals a magnetic activity cycle in a Sun-like star
The 11-year activity cycle of the Sun is a consequence of a dynamo process
occurring beneath its surface. We analyzed photometric data obtained by the
CoRoT space mission, showing solar-like oscillations in the star HD49933, for
signatures of stellar magnetic activity. Asteroseismic measurements of global
changes in the oscillation frequencies and mode amplitudes reveal a modulation
of at least 120 days, with the minimum frequency shift corresponding to maximum
amplitude as in the Sun. These observations are evidence of a stellar magnetic
activity cycle taking place beneath the surface of HD49933 and provide
constraints for stellar dynamo models under conditions different from those of
the Sun.Comment: Brevia text and supporting online material, published in Scienc
Seismology and activity of the F type star HD 49933
A 10-night asteroseismic observation programme has been conducted in January
2004 with the spectrometer Harps at the ES0 3.6-m telescope. The selected
target, the 6th magnitude F5V star HD 49933, was chosen among the prime
candidates of Corot, the European space mission dedicated to characterize
stellar oscillations mode with high precision photometry measurements. This
star shows important line profiles variations, indicating a surprisingly high
activity with respect to its low rotation rate. However, with the help of tools
developed for disentangling the signatures of activity and oscillations, we are
able to observe its oscillation spectrum in the frequency range [1.2, 2.2 mHz].
We measure the large separation (88.7+-0.4 microHz) and the maximum amplitude
(around 0.4+-0.1 m/s rms), respectively in agreement and marginal agreement
with the predicted values.Comment: accepted in A&A Letter
CoRoT: harvest of the exoplanet program
One of the objectives of the CoRoT mission is the search for transiting
extrasolar planets using high-precision photometry, and the accurate
characterization of their fundamental parameters. The CoRoT satellite
consecutively observes crowded stellar fields since February 2007, in
high-cadence precise photometry; periodic eclipses are detected and analysed in
the stellar light curves. Then complementary observations using ground-based
facilities allows establishing the nature of the transiting body and its mass.
CoRoT has acquired more than 163,000 light curves and detected about 500 planet
candidates. A fraction of them (5%) are confirmed planets whose masses are
independently measured. Main highlights of the CoRoT discoveries are: i) the
variety of internal structures in close-in giant planets, ii) the
characterisation of the first known transiting rocky planet, CoRoT-7 b, iii)
multiple constraints on the formation, evolution, role of tides in planetary
systems.Comment: Icarus, in press, special issue on Exoplanet
How Much do we Trust Stellar Models? Foreword
peer reviewe
Monitoring a high--amplitude Delta Sct star for 152 days: discovery of 12 additional modes and modulation effects in the light curve of CoRoT 101155310
The detection of small-amplitude nonradial modes in high-amplitude Delta Sct
(HADS) variables has been very elusive until at least five of them were
detected in the light curve of V974 Oph obtained from ground-based
observations. The combination of radial and nonradial modes has a high
asteroseismic potential, thanks to the strong constraints we can put in the
modelling. The continuous monitoring of ASAS 192647-0030.0=CoRoT 101155310
(P=0.1258 d, V=13.4) ensured from space by the CoRoT (Convection, Rotation and
planetary Transits) mission constitutes a unique opportunity to exploit such
potential. The 22270 CoRoT measurements were performed in the chromatic mode.
They span 152 d and cover 1208 consecutive cycles. After the correction for one
jump and the long-term drift, the level of the noise turned out to be 29
micromag. The phase shifts and amplitude ratios of the coloured CoRoT data, the
HARPS spectra, and the period-luminosity relation were used to determine a
self-consistent physical model. In turn, it allowed us to model the oscillation
spectrum, also giving feedback on the internal structure of the star. In
addition to the fundamental radial mode f1=7.949 c/d with harmonics up to 10f1,
we detected 12 independent terms. Linear combinations were also found and the
light curve was solved by means of 61 frequencies (smallest amplitude 0.10
mmag). The newest result is the detection of a periodic modulation of the f1
mode (triplets at +/-0.193 c/d centred on f1 and 2f1), discussed as a
rotational effect or as an extension of the Blazhko effect to HADS stars. The
physical model suggests that CoRoT 101155310 is an evolved star, with a slight
subsolar metallic abundance, close to the terminal age main sequence. All the
12 additional terms are identified with mixed modes in the predicted overstable
region.Comment: 10 pages, 6 figures, Tables 1 and 2 available at CDS after
publication. Accepted for publication in Astronomy and Astrophysic
Seismic constraints on rotation of Sun-like star and mass of exoplanet
Rotation is thought to drive cyclic magnetic activity in the Sun and Sun-like
stars. Stellar dynamos, however, are poorly understood owing to the scarcity of
observations of rotation and magnetic fields in stars. Here, inferences are
drawn on the internal rotation of a distant Sun-like star by studying its
global modes of oscillation. We report asteroseismic constraints imposed on the
rotation rate and the inclination of the spin axis of the Sun-like star HD
52265, a principal target observed by the CoRoT satellite that is known to host
a planetary companion. These seismic inferences are remarkably consistent with
an independent spectroscopic observation (rotational line broadening) and with
the observed rotation period of star spots. Furthermore, asteroseismology
constrains the mass of exoplanet HD 52265b. Under the standard assumption that
the stellar spin axis and the axis of the planetary orbit coincide, the minimum
spectroscopic mass of the planet can be converted into a true mass of 1.85
(+0.52,-0.42) M_Jupiter, which implies that it is a planet, not a brown dwarf.Comment: Published in Proceedings of the National Academy of Sciences (5
pages, 5 figures, 3 tables). Available at
http://www.pnas.org/cgi/doi/10.1073/pnas.130329111
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