Time filter

Source Type

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.

Matthews L.D.,Massachusetts Institute of Technology | Le Bertre T.,Paris Observatory | Gerard E.,CNRS Galaxies, Stars, Physics and Instrumentation Laboratory | Johnson M.C.,University of Texas at Austin
Astronomical Journal | Year: 2013

We present an imaging study of a sample of eight asymptotic giant branch stars in the H I 21 cm line. Using observations from the Very Large Array, we have unambiguously detected H I emission associated with the extended circumstellar envelopes of six of the targets. The detected H I masses range from M H I ≈ 0.015-0.055 M ȯ. The H I morphologies and kinematics are diverse, but in all cases appear to be significantly influenced by the interaction between the circumstellar envelope and the surrounding medium. Four stars (RX Lep, Y UMa, Y CVn, and V1942 Sgr) are surrounded by detached H I shells ranging from 0.36 to 0.76 pc across. We interpret these shells as resulting from material entrained in a stellar outflow being abruptly slowed at a termination shock where it meets the local medium. RX Lep and TX Psc, two stars with moderately high space velocities (V space > 56 km s-1), exhibit extended gaseous wakes (∼0.3 and 0.6 pc in the plane of the sky), trailing their motion through space. The other detected star, R Peg, displays a peculiar "horseshoe- shaped" H I morphology with emission extended on scales up to ∼1.7 pc; in this case, the circumstellar debris may have been distorted by transverse flows in the local interstellar medium. We briefly discuss our new results in the context of the entire sample of evolved stars that has been imaged in H I to date. © 2013. The American Astronomical Society. All rights reserved.

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.

North P.,Ecole Polytechnique Federale de Lausanne | Gauderon R.,Ecole Polytechnique Federale de Lausanne | Barblan F.,University of Geneva | Royer F.,CNRS Galaxies, Stars, Physics and Instrumentation Laboratory
Astronomy and Astrophysics | Year: 2010

Aims. Our purpose is to provide reliable stellar parameters for a significant sample of eclipsing binaries, which are representative of a whole dwarf and metal-poor galaxy. We also aim at providing a new estimate of the mean distance to the SMC and of its depth along the line of sight for the observed field of view. Methods. We use radial velocity curves obtained with the ESO FLAMES facility at the VLT and light curves from the OGLE-II photometric survey. The radial velocities were obtained by least-squares fits of the observed spectra to synthetic ones, excluding the hydrogen Balmer lines. Results. Our sample contains 23 detached, 9 semi-detached and 1 overcontact systems. Most detached systems have properties consistent with stellar evolution calculations from single-star models at the standard SMC metalicity Z = 0.004, though they tend to be slightly overluminous. The few exceptions are probably due to third-light contribution or insufficient signal-to-noise ratio. The mass ratios are consistent with a flat distribution, both for detached and semi-detached/contact binaries. A mass-luminosity relation valid from ∼4 to ∼18 M⊙ is derived. The uncertainties are in the ±2 to ±11% range for the masses, in the ±2 to ±5% range for the radii and in the ±1 to ±6% range for the effective temperatures. The average distance modulus is 19.11 ± 0.03 (66.4 ± 0.9 kpc). The moduli derived from the V and from the I data are consistent within 0.01 mag. The 2σ depth of the SMC is, for our field, of 0.25 mag or 7.6 kpc under the assumption of a Gaussian distribution of stars along the line of sight. Three systems show significant apsidal motion, one of them with an apsidal period of 7.6 years, the shortest known to date for a detached system with main sequence stars. © 2010 ESO.

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.

Findeisen K.,California Institute of Technology | Findeisen K.,CNRS Galaxies, Stars, Physics and Instrumentation Laboratory | Cody A.M.,California Institute of Technology | Hillenbrand L.,California Institute of Technology
Astrophysical Journal | Year: 2015

Aperiodic variability is a characteristic feature of young stars, massive stars, and active galactic nuclei. With the recent proliferation of time-domain surveys, it is increasingly essential to develop methods to quantify and analyze aperiodic variability. We develop three timescale metrics that have been little used in astronomy - Δm-Δt plots, peak-finding, and Gaussian process regression - and present simulations comparing their effectiveness across a range of aperiodic light curve shapes, characteristic timescales, observing cadences, and signal to noise ratios. We find that Gaussian process regression is easily confused by noise and by irregular sampling, even when the model being fit reflects the process underlying the light curve, but that Δm-Δt plots and peak-finding can coarsely characterize timescales across a broad region of parameter space. We make public the software we used for our simulations, both in the spirit of open research and to allow others to carry out analogous simulations for their own observing programs. © 2015. The American Astronomical Society. All rights reserved..

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.

Lebreton Y.,CNRS Galaxies, Stars, Physics and Instrumentation Laboratory | Goupil M.J.,Rennes Institute of Physics | Montalban J.,CNRS Laboratory for Space Studies and Astrophysical Instrumentation
EAS Publications Series | Year: 2014

Among the various methods used to age-date stars, methods based on stellar model predictions are widely used, for nearly all kind of stars in large ranges of masses, chemical compositions and evolutionary stages. The precision and accuracy on the age determination depend on both the precision and number of observational constraints, and on our ability to correctly describe the stellar interior and evolution. The imperfect input physics of stellar models as well as the uncertainties on the initial chemical composition of stars are responsible for uncertainties in the age determination. We present an overview of the calculation of stellar models and discuss the impact on age of their numerous inputs. © EAS, EDP Sciences 2014.

Kielkopf J.F.,University of Louisville | Allard N.F.,CNRS Galaxies, Stars, Physics and Instrumentation Laboratory
Journal of Physics B: Atomic, Molecular and Optical Physics | Year: 2014

Time-resolved profiles of the Balmer series Hα line emitted by a laser-produced hydrogen plasma have been measured to determine the shift and width for electron densities from below 1018 to above 1020 cm-3 at an average temperature of 28000 K. Fits of the profiles that allow for self-absorption in the plasma yield shifts and widths that are consistent with experiments on lower density and cooler gas-liner pinch plasmas. The width scales as and the shift as between and cm-3. Hα shifts monotonically and nearly linearly to the red with increasing density under the reported conditions. A comparison to theory calculations using exact potentials for shows that an intrinsic asymmetry becomes significant only in the upper limit of this range when a satellite develops in the far red wing. © 2014 IOP Publishing Ltd.

Haywood M.,CNRS Galaxies, Stars, Physics and Instrumentation Laboratory
EAS Publications Series | Year: 2014

Present studies on the evolution of the Milky Way are driven and shaped by how we conceive its stellar populations, in an on going process started by W. Baade seventy years ago. Despite much progress and advances in our understanding of these populations, inspection of their main properties is however hardly indicative of the path the Milky Way has followed to build up its mass. This is not only a matter of (stellar) age measurement, but more so the consequence of how we interprete the structures that we see in our Galaxy, often through the filter of our definitions of stellar populations. The panorama presented in the following pages opens the possibility that the present "filter" is not fully adequate. I start these Lectures with a summary of the main properties of the disks, bulge, and halo, and then present some of the new directions in the interpretation of the structure and evolution of the disk(s), with emphasis on chemical evolution. I discuss recent results in our understanding of the bulge, its stellar components and chemical evolution. Finally, I present the ideas currently proposed to explain the formation of the Galactic stellar halo. I conclude by examining how deeply all these new results question our present definition of stellar populations. © EAS, EDP Sciences 2014.

Loading CNRS Galaxies, Stars, Physics and Instrumentation Laboratory collaborators
Loading CNRS Galaxies, Stars, Physics and Instrumentation Laboratory collaborators