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Zorec J.,University Pierre and Marie Curie | Royer F.,CNRS Galaxies, Stars, Physics and Instrumentation Laboratory
Astronomy and Astrophysics | Year: 2012

Context. In previous works of this series, we have shown that late B-and early A-type stars have genuine bimodal distributions of rotational velocities and that late A-type stars lack slow rotators. The distributions of the surface angular velocity ratio Ω/Ω crit (Ω crit is the critical angular velocity) have peculiar shapes according to spectral type groups, which can be caused by evolutionary properties. Aims. We aim to review the properties of these rotational velocity distributions in some detail as a function of stellar mass and age. Methods. We have gathered vsini for a sample of 2014 B6-to F2-type stars. We have determined the masses and ages for these objects with stellar evolution models. The (T eff,log L/L ⊙)-parameters were determined from the uvby-β photometry and the HIPPARCOS parallaxes. Results. The velocity distributions show two regimes that depend on the stellar mass. Stars less massive than 2.5 M ⊙ have a unimodal equatorial velocity distribution and show a monotonical acceleration with age on the main sequence (MS). Stars more massive have a bimodal equatorial velocity distribution. Contrarily to theoretical predictions, the equatorial velocities of stars from about 1.7 M ⊙ to 3.2 M ⊙ undergo a strong acceleration in the first third of the MS evolutionary phase, while in the last third of the MS they evolve roughly as if there were no angular momentum redistribution in the external stellar layers. The studied stars might start in the ZAMS not necessarily as rigid rotators, but with a total angular momentum lower than the critical one of rigid rotators. The stars seem to evolve as differential rotators all the way of their MS life span and the variation of the observed rotational velocities proceeds with characteristic time scales δt ≈ 0.2 t MS, where t MS is the time spent by a star in the MS. © 2012 ESO. Source

Lebreton Y.,CNRS Galaxies, Stars, Physics and Instrumentation Laboratory | Lebreton Y.,Rennes Institute of Physics | Goupil M.J.,CNRS Laboratory for Space Studies and Astrophysical Instrumentation
Astronomy and Astrophysics | Year: 2012

Aims. We aim at characterizing the inward transition from convective to radiative energy transport at the base of the convective envelope of the solar-like oscillator HD 52265 recently observed by the CoRoT satellite. Methods. We investigated the origin of one specific feature found in the HD 52265 frequency spectrum. We modelled the star to derive the internal structure and the oscillation frequencies that best match the observations and used a seismic indicator sensitive to the properties of the base of the envelope convection zone. Results. The seismic indicators clearly reveal that to best represent the observed properties of HD 52265, models must include penetrative convection below the outer convective envelope. The penetrative distance is estimated to be ∼0.95H P, which corresponds to an extent over a distance representing 6.0 per cents of the total stellar radius, significantly larger than what is found for the Sun. The inner boundary of the extra-mixing region is found at 0.800 ± 0.004 R where R = 1.3 R ⊙ is the stellar radius. Conclusions. These results contribute to the tachocline characterization in stars other than the Sun. © 2012 ESO. Source

Samadi R.,CNRS Laboratory for Space Studies and Astrophysical Instrumentation | Belkacem K.,CNRS Laboratory for Space Studies and Astrophysical Instrumentation | Ludwi H.-G.,University of Heidelberg | Ludwi H.-G.,CNRS Galaxies, Stars, Physics and Instrumentation Laboratory
Astronomy and Astrophysics | Year: 2013

Context. Solar granulation has been known for a long time to be a surface manifestation of convection. The space-borne missions CoRoT and Kepler enable us to observe the signature of this phenomena in disk-integrated intensity on a large number of stars. Aims. The space-based photometric measurements show that the global brightness fluctuations and the lifetime associated with granulation obeys characteristic scaling relations. We thus aimed at providing simple theoretical modeling to reproduce these scaling relations, and subsequently at inferring the physical properties of granulation across the Hertzsprung-Russell diagram. Methods. We developed a simple 1D theoretical model. The input parameters were extracted from 3D hydrodynamical models of the surface layers of stars, and the free parameters involved in the model were calibrated with solar observations. Two different prescriptions for representing the Fourier transform of the time-correlation of the eddy velocity were compared: a Lorentzian and an exponential form. Finally, we compared our theoretical prediction with 3D radiative hydrodynamical (RHD) numerical modeling of stellar granulation (hereafter ab initio approach). Results. Provided that the free parameters are appropriately adjusted, our theoretical model reproduces the observed solar granulation spectrum quite satisfactorily; the best agreement is obtained for an exponential form. Furthermore, our model results in granulation spectra that agree well with the ab initio approach using two 3D RHD models that are representative of the surface layers of an F-dwarf and a red-giant star. Conclusions. We have developed a theoretical model that satisfactory reproduces the solar granulation spectrum and gives results consistent with the ab initio approach. The model is used in a companion paper as theoretical framework for interpretating the observed scaling relations. © ESO 2013. Source

Wolak P.,Nicolaus Copernicus University | Szymczak M.,Nicolaus Copernicus University | Gerard E.,CNRS Galaxies, Stars, Physics and Instrumentation Laboratory
Monthly Notices of the Royal Astronomical Society | Year: 2013

OH 1612- and 1667-MHz masers from the well-known object OH127.8+0.0 were monitored in full polarization mode over a period of 6.5 yr and mapped with MERLIN at one epoch. The OH variability pattern of the star is typical of extremely long-period asymptotic giant branch stars. The distance determined from the 1612-MHz light curve and a new measurement of the angular radius is 3.87 ± 0.28 kpc. At both frequencies, the flux of polarized emission tightly follows the total flux variations while, the degrees of circular and linear polarization are constant within measurement accuracy. There is net polarization at both lines. The magnetic field strength estimated from a likely Zeeman pair is -0.6 mG at the distance of 5400 au from the star. At the near and far sides of the envelope, the polarization vectors are well aligned implying a regular structure of the magnetic field. The polarization characteristics of the OH maser emission suggest a radial magnetic field which is frozen in the stellar wind. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Casanellas J.,Max Planck Institute For Gravitationsphysik | Brandao I.,University of Porto | Lebreton Y.,CNRS Galaxies, Stars, Physics and Instrumentation Laboratory | Lebreton Y.,Rennes Institute of Physics
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2015

The recent detection of a convective core in a main-sequence solar-type star is used here to test particular models of dark matter (DM) particles, those with masses and scattering cross sections in the range of interest for the DM interpretation of the positive results in several DM direct detection experiments. If DM particles do not effectively self annihilate after accumulating inside low-mass stars (e.g. in the asymmetric DM scenario) their conduction provides an efficient mechanism of energy transport in the stellar core. For main-sequence stars with masses between 1.1 and 1.3M this mechanism may lead to the suppression of the inner convective region expected to be present in standard stellar evolution theory. The asteroseismic analysis of the acoustic oscillations of a star can prove the presence/absence of such a convective core, as it was demonstrated for the first time with the Kepler field main-sequence solarlike pulsator, KIC 2009505. Studying this star we found that the asymmetric DM interpretation of the results in the CoGeNT experiment is incompatible with the confirmed presence of a small convective core in KIC 2009505. © 2015 American Physical Society. Source

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