Navegando por Autor "Lèbre, A."

Agora exibindo 1 - 5 de 5

Lithium abundances and extra mixing processes in evolved stars of M 67
(Astronomy & Astrophysics, 2011) Martins, B. L. Canto; Lèbre, A.; Palacios, A.; Laverny, P. de; Richard, O.; Melo, C. H. F.; Nascimento Jr, J. D. do; Medeiros, José Renan de
Aims. We present a spectroscopic analysis of a sample of evolved stars in M 67 (turn-off, subgiant and giant stars) in order to bring observational constraints to evolutionary models taking into account non-standard transport processes. Methods. We determined the stellar parameters (Teff, log g, [Fe/H]), microturbulent and rotational velocities and, lithium abundances (ALi) for 27 evolved stars of M 67 with the spectral synthesis method based on MARCS model atmospheres. We also computed non-standard stellar evolution models, taking into account atomic diffusion and rotation-induced transport of angular momentum and chemicals that were compared with this set of homogeneous data. Results. The lithium abundances that we derive for the 27 stars in our sample follow a clear evolutionary pattern ranging from the turn-off to the Red Giant Branch. Our abundance determination confirms the well known decrease of lithium content for evolved stars. For the first time, we provide a consistent interpretation of both the surface rotation velocity and of the lithium abundance patterns observed in an homogeneous sample of TO and evolved stars of M 67. We show that the lithium evolution is determined by the evolution of the angular momentum through rotation-induced mixing in low-mass stars, in particular for those with initial masses larger than 1.30 M when at solar metallicity.
Lithium abundances and rotational behavior for bright giant stars
(EDP Sciences, 2005-11-16) Lèbre, A.; de Laverny, P.; Nascimento Júnior, José Dias do; de Medeiros, J. R.
Aims.We study the links possibly existing between the lithium content of bright giant stars and their rotational velocity. Methods.We performed a spectral analysis of 145 bright giant stars (luminosity class II) spanning the spectral range from F3 to K5. All these stars have homogeneous rotational velocity measurements available in the literature. Results.For all the stars of the sample, we provide consistent lithium abundances (ALi), effective temperatures (Teff), projected rotational velocity (), mean metallicity ([Fe/H]), stellar mass, and an indication of the stellar multiplicity. The gradual decrease in lithium abundance with Teff is confirmed for bright giant stars, and it points to a dilution factor that is at least as significant as in giant stars. From the F to K spectral types, the ALi spans at least three orders of magnitude, reflecting the effects of stellar mass and evolution on dilution. Conclusions.We find that the behavior of ALi as a function of in bright giant stars presents the same trend as is observed in giants and subgiants: stars with high ALi are moderate or fast rotators, while stars with low ALi show a wide range of values
Lithium and magnetic fields in giant stars: HD 232 862: a magnetic and lithium-rich giant
(Astronomy & Astrophysics, 2009-07-22) Lèbre, A.; Palacios, A.; Nascimento Jr., J. D. do; Konstantinova-Antova, R.; Kolev, D.; Aurière, M.; Laverny, P. de; Medeiros, José Renan de
Aims. We report the detection of an unusually high lithium content in HD 232 862, a field giant classified as a G8II star, and hostinga magnetic field. Methods. With the spectropolarimeters ESPaDOnS at CFHT and NARVAL at TBL, we collected high resolution and high signalto-noise spectra of three giants: HD 232 862, KU Peg and HD 21 018. From spectral synthesis we inferred stellar parameters and measured lithium abundances that we compared to predictions from evolutionary models. We also analysed Stokes V signatures, looking for a magnetic field on these giants. Results. HD 232 862, presents a very high abundance of lithium (ALi = 2.45 ± 0.25 dex), far in excess of the theoretically value expected at this spectral type and for this luminosity class (i.e., G8II). The evolutionary stage of HD 232 862 was determined, and it suggests a mass in the lower part of the [1.0 M, 3.5 M] mass interval, likely 1.5–2.0 M, at the bottom of the red giant branch. Also, a time variable Stokes V signature was detected in the data of HD 232 862 and KU Peg, pointing to the presence of a magnetic field at the surface of these two rapidly rotating active stars.
S1242: a lithium-rich subgiant star in the open cluster M 67
(Astronomy & Astrophysics, 2006-05-04) Martins, B. L. Canto; Lèbre, A.; Laverny, P. de; Melo, C. H. F.; Nascimento Jr, J. D. do; Richard, O.; Medeiros, José Renan de
Aims. We report the serendipitous discovery of a lithium-rich subgiant star, S1242. This object is member of a large eccentricity spectroscopic binary system in the solar-age open cluster M 67. Methods. Using VLT/FLAMES-UVES observations and synthetic spectrum analysis, we derived for this star stellar parameters (temperature, gravity, metallicity and rotational velocity) and a surface Li abundance (ALi = 2.7) larger than the higher abundance of M 67 stars near the turn-off. Results. The high Li abundance we found in S1242 points to an excess of lithium at the subgiant stage. We discuss two possible explanations for this unusually high Li content in this binary system: a preservation of the Li at the turn-off stage due to tidal effects, or an efficient dredge-up of Li, hidden below the convective zone by atomic diffusion occuring in the post turn-off stage.
S1242: a lithium-rich subgiant star in the open cluster M 67
(EDP Sciences, 2005-12-29) Martins, B. L. Canto; Lèbre, A.; de Laverny, P.; Melo, C. H. F.; Nascimento Júnior, José Dias do; de Medeiros, J. R.
Aims.We report the serendipitous discovery of a lithium-rich subgiant star, S1242. This object is member of a large eccentricity spectroscopic binary system in the solar-age open cluster M 67. Methods.Using VLT/FLAMES-UVES observations and synthetic spectrum analysis, we derived for this star stellar parameters (temperature, gravity, metallicity and rotational velocity) and a surface Li abundance () larger than the higher abundance of M 67 stars near the turn-off. Results.The high Li abundance we found in S1242 points to an excess of lithium at the subgiant stage. We discuss two possible explanations for this unusually high Li content in this binary system: a preservation of the Li at the turn-off stage due to tidal effects, or an efficient dredge-up of Li, hidden below the convective zone by atomic diffusion occuring in the post turn-off stage

SIGAA

Navegando por Autor "Lèbre, A."