A. Palacios

Université de Montpellier, Montpelhièr, Languedoc-Roussillon, France

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Publications (60)110.96 Total impact

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    A. S. Brun · A Palacios
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    ABSTRACT: With the development of one-dimensional stellar evolution codes including rotation and the increasing number of observational data for stars of various evolutionary stages, it becomes more and more possible to follow the evolution of the rotation profile and angular momentum distribution in stars. In this context, understanding the interplay between rotation and convection in the very extended envelopes of giant stars is very important considering that all low-and intermediate-mass stars become red giants after the central hydrogen burning phase. In this paper, we analyze the interplay between rotation and convection in the envelope of red giant stars using three-dimensional numerical experiments. We make use of the Anelastic Spherical Harmonics code to simulate the inner 50% of the envelope of a low-mass star on the red giant branch. We discuss the organization and dynamics of convection, and put a special emphasis on the distribution of angular momentum in such a rotating extended envelope. To do so, we explore two directions of the parameter space, namely, the bulk rotation rate and the Reynolds number with a series of four simulations. We find that turbulent convection in red giant stars is dynamically rich, and that it is particularly sensitive to the rotation rate of the star. Reynolds stresses and meridional circulation establish various differential rotation profiles (either cylindrical or shellular) depending on the convective Rossby number of the simulations, but they all agree that the radial shear is large. Temperature fluctuations are found to be large and in the slowly rotating cases, a dominant = 1 temperature dipole influences the convective motions. Both baroclinic effects and turbulent advection are strong in all cases and mostly oppose one another.
    Preview · Article · Mar 2016 · The Astrophysical Journal
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    ABSTRACT: The surface rotation rates of young solar-type stars vary rapidly with age from the end of the pre-main sequence through the early main sequence. Important changes in the dynamos operating in these stars may result from this evolution, which should be observable in their surface magnetic fields. Here we present a study aimed at observing the evolution of these magnetic fields through this critical time period. We observed stars in open clusters and stellar associations of known ages, and used Zeeman Doppler imaging to characterize their complex magnetic large-scale fields. Presented here are results for 15 stars, from five associations, with ages from 20 to 250 Myr, masses from 0.7 to 1.2 M⊙, and rotation periods from 0.4 to 6 d. We find complex large-scale magnetic field geometries, with global average strengths from 14 to 140 G. There is a clear trend towards decreasing average large-scale magnetic field strength with age, and a tight correlation between magnetic field strength and Rossby number. Comparing the magnetic properties of our zero-age main-sequence sample to those of both younger and older stars, it appears that the magnetic evolution of solar-type stars during the pre-main sequence is primarily driven by structural changes, while it closely follows the stars’ rotational evolution on the main sequence.
    No preview · Article · Mar 2016 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We study the predicted rotational evolution of solar-type stars from the pre-main sequence to the solar age with 1D rotating evolutionary models including physical ingredients. We computed rotating evolution models of solar-type stars including an external stellar wind torque and internal transport of angular momentum following the method of Maeder and Zahn with the code STAREVOL. We explored different formalisms and prescriptions available from the literature. We tested the predictions of the models against recent rotational period data from extensive photometric surveys, lithium abundances of solar-mass stars in young clusters, and the helioseismic rotation profile of the Sun. We find a best-matching combination of prescriptions for both internal transport and surface extraction of angular momentum. This combination provides a very good fit to the observed evolution of rotational periods for solar-type stars from early evolution to the age of the Sun. Additionally, we show that fast rotators experience a stronger coupling between their radiative region and the convective envelope. Regardless of the set of prescriptions, however, we cannot simultaneously reproduce surface angular velocity and the internal profile of the Sun or the evolution of lithium abundance. We confirm the idea that additional transport mechanisms must occur in solar-type stars until they reach the age of the Sun. Whether these processes are the same as those needed to explain recent asteroseismic data in more advanced evolutionary phases is still an open question.
    No preview · Article · Jan 2016 · Astronomy and Astrophysics
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    ABSTRACT: Massive stars burn hydrogen through the CNO cycle during most of their evolution. When mixing is efficient, or when mass transfer in binary systems happens, chemically processed material is observed at the surface of O and B stars. ON stars show stronger lines of nitrogen than morphologically normal counterparts. Whether this corresponds to the presence of material processed through the CNO cycle or not is not known. Our goal is to answer this question. We perform a spectroscopic analysis of a sample of ON stars with atmosphere models. We determine the fundamental parameters as well as the He, C, N, and O surface abundances. We also measure the projected rotational velocities. We compare the properties of the ON stars to those of normal O stars. We show that ON stars are usually helium-rich. Their CNO surface abundances are fully consistent with predictions of nucleosynthesis. ON stars are more chemically evolved and rotate - on average - faster than normal O stars. Evolutionary models including rotation cannot account for the extreme enrichment observed among ON main sequence stars. Some ON stars are members of binary systems, but others are single stars as indicated by stable radial velocities. Hence, mass transfer is not a simple explanation for the observed chemical properties. We conclude that ON stars show extreme chemical enrichment at their surface, consistent with nucleosynthesis through the CNO cycle. Its origin is not clear at present.
    No preview · Article · Apr 2015 · Astronomy and Astrophysics

  • No preview · Article · Oct 2014
  • T. Decressin · C. Charbonnel · L. Amard · A. Palacios · S. Talon
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    ABSTRACT: In stellar interior rotation profile can be changed by stellar contraction, meridional circulation, shear turbulence and internal gravity waves. These waves are generated at the edge of the convective zones and propagate inside the radiative zone where they damped by thermal diffusivity and viscosity in corotation resonance. The differential damping between prograde and retrograde let its imprint on the rotation profile along with other hydrodynamic transport processes. This interplay will be discussed for low-mass stars along the PMS.
    No preview · Article · Nov 2013
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    C. Charbonnel · T. Decressin · Louis AMARD · Ana PALACIOS · S. Talon
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    ABSTRACT: We study the impact of internal gravity waves (IGW), meridional circulation, shear turbulence, and stellar contraction on the internal rotation profile and surface velocity evolution of solar metallicity low-mass pre-main sequence stars. We compute a grid of rotating stellar evolution models with masses between 0.6 and 2.0Msun taking these processes into account for the transport of angular momentum, as soon as the radiative core appears and assuming no more disk-locking from that moment on.IGW generation along the PMS is computed taking Reynolds-stress and buoyancy into account in the bulk of the stellar convective envelope and convective core (when present). Redistribution of angular momentum within the radiative layers accounts for damping of prograde and retrograde IGW by thermal diffusivity and viscosity in corotation resonance. Over the whole mass range considered, IGW are found to be efficiently generated by the convective envelope and to slow down the stellar core early on the PMS. In stars more massive than ~ 1.6Msun, IGW produced by the convective core also contribute to angular momentum redistribution close to the ZAMS. Overall, IGW are found to significantly change the internal rotation profile of PMS low-mass stars.
    Full-text · Article · Apr 2013 · Astronomy and Astrophysics
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    ABSTRACT: Thermohaline mixing has been recently identified as the probable dominating process that governs the photospheric composition of low-mass bright red giant stars. Here, we present the predictions of stellar models computed with the code STAREVOL including this process together with rotational mixing. We compare our theoretical predictions with recent observations, and discuss the effects of both mechanisms on asteroseismic diagnostics.
    No preview · Article · Mar 2013 · The European Physical Journal Conferences
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    ABSTRACT: Context. Rotational splittings are currently measured for several main sequence stars and a large number of red giants with the space mission Kepler. This will provide stringent constraints on rotation profiles. Aims: Our aim is to obtain seismic constraints on the internal transport and surface loss of the angular momentum of oscillating solar-like stars. To this end, we study the evolution of rotational splittings from the pre-main sequence to the red-giant branch for stochastically excited oscillation modes. Methods: We modified the evolutionary code CESAM2K to take rotationally induced transport in radiative zones into account. Linear rotational splittings were computed for a sequence of 1.3 M⊙ models. Rotation profiles were derived from our evolutionary models and eigenfunctions from linear adiabatic oscillation calculations. Results: We find that transport by meridional circulation and shear turbulence yields far too high a core rotation rate for red-giant models compared with recent seismic observations. We discuss several uncertainties in the physical description of stars that could have an impact on the rotation profiles. For instance, we find that the Goldreich-Schubert-Fricke instability does not extract enough angular momentum from the core to account for the discrepancy. In contrast, an increase of the horizontal turbulent viscosity by 2 orders of magnitude is able to significantly decrease the central rotation rate on the red-giant branch. Conclusions: Our results indicate that it is possible that the prescription for the horizontal turbulent viscosity largely underestimates its actual value or else a mechanism not included in current stellar models of low mass stars is needed to slow down the rotation in the radiative core of red-giant stars.
    Full-text · Article · Jan 2013 · Astronomy and Astrophysics
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    ABSTRACT: Rotational splittings are currently measured for several main sequence stars and a large number of red giants with the space mission Kepler. This will provide stringent constraints on rotation profiles. Our aim is to obtain seismic constraints on the internal transport and surface loss of angular momentum of oscillating solar-like stars. To this end, we study the evolution of rotational splittings from the pre-main sequence to the red-giant branch for stochastically excited oscillation modes. We modified the evolutionary code CESAM2K to take rotationally induced transport in radiative zones into account. Linear rotational splittings were computed for a sequence of $1.3 M_{\odot}$ models. Rotation profiles were derived from our evolutionary models and eigenfunctions from linear adiabatic oscillation calculations. We find that transport by meridional circulation and shear turbulence yields far too high a core rotation rate for red-giant models compared with recent seismic observations. We discuss several uncertainties in the physical description of stars that could have an impact on the rotation profiles. For instance, we find that the Goldreich-Schubert-Fricke instability does not extract enough angular momentum from the core to account for the discrepancy. In contrast, an increase of the horizontal turbulent viscosity by 2 orders of magnitude is able to significantly decrease the central rotation rate on the red-giant branch. Our results indicate that it is possible that the prescription for the horizontal turbulent viscosity largely underestimates its actual value or else a mechanism not included in current stellar models of low mass stars is needed to slow down the rotation in the radiative core of red-giant stars.
    Full-text · Article · Nov 2012
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    ABSTRACT: Grid of stellar models and asymptotic asteroseismic quantities for four metallicities (Z= 0.0001, 0.002, 0.004, and 0.014) in the mass range from 0.85 to 6.0 Msun. The models are computed either with standard prescriptions or including both thermohaline convection and rotation-induced mixing. For the whole grid, we provide the usual stellar parameters (luminosity, effective temperature, lifetimes, ...), together with the global seismic parameters, i.e. the large frequency separation and asymptotic relation (microHz)s, the frequency corresponding to the maximum oscillation power νmax, the maximal amplitude Amax, the asymptotic period spacing of g-modes, and different acoustic radii. (4 data files).
    No preview · Article · Sep 2012
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    ABSTRACT: The availability of asteroseismic constraints for a large sample of stars from the missions CoRoT and Kepler paves the way for various statistical studies of the seismic properties of stellar populations. In this paper, we evaluate the impact of rotation-induced mixing and thermohaline instability on the global asteroseismic parameters at different stages of the stellar evolution from the Zero Age Main Sequence to the Thermally Pulsating Asymptotic Giant Branch to distinguish stellar populations. We present a grid of stellar evolutionary models for four metallicities (Z = 0.0001, 0.002, 0.004, and 0.014) in the mass range between 0.85 to 6.0 Msun. The models are computed either with standard prescriptions or including both thermohaline convection and rotation-induced mixing. For the whole grid we provide the usual stellar parameters (luminosity, effective temperature, lifetimes, ...), together with the global seismic parameters, i.e. the large frequency separation and asymptotic relations, the frequency corresponding to the maximum oscillation power {\nu}_{max}, the maximal amplitude A_{max}, the asymptotic period spacing of g-modes, and different acoustic radii. We discuss the signature of rotation-induced mixing on the global asteroseismic quantities, that can be detected observationally. Thermohaline mixing whose effects can be identified by spectroscopic studies cannot be caracterized with the global seismic parameters studied here. But it is not excluded that individual mode frequencies or other well chosen asteroseismic quantities might help constraining this mixing.
    Full-text · Article · Apr 2012 · Astronomy and Astrophysics
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    ABSTRACT: Aims. We present a spectroscopic analysis of a sample of evolved stars in M67 (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 M67 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 M67. 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_\odot at solar metallicity.
    Full-text · Article · Jan 2011 · Astronomy and Astrophysics
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    ABSTRACT: Helioseismology puts strong constraints on the internal sound speed and on the rotation profile in the radiative zone. Young stars of solar type are more active and faster rotators than the Sun. So we begin to build models which include different rotation histories and compare the results with all the solar observations. The profiles of the rotation we get have interesting consequence for the introduction of magnetic field in the radiative zone. We discuss also the impact of mass loss deduced from measured flux of young stars. We deduce from these comparisons some quantitative effect of the dynamical processes (rotation, magnetic field and mass loss) of these early stages on the present sound speed and density. We show finally how we can improve our present knowledge of the radiative zone with PICARD and GOLFNG.
    Full-text · Article · Jan 2011 · Journal of Physics Conference Series
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    ABSTRACT: Using spectropolarimetric data acquired with the ESPaDOnS and NARVAL instruments at CFHT and at TBL, we present a detailed spectral synthesis analysis of HD 232 862, a field giant classified as a G8II star hosting a magnetic field. This star is the first lithium-rich field giant hosting a magnetic field. Stellar evolution models suggest that HD 232 862 should be a 1.5 to 2.0 M&sun; star at the bottom of the red giant branch. Its unsually high lithium content (A(Li) = 2.45 ± 0.25 dex) is even more puzzling and challenges our understanding of the evolution of this star.
    Preview · Article · Nov 2010 · Proceedings of the International Astronomical Union
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    A. Palacios · M. Gebran · E. Josselin · F. Martins · B. Plez · M. Belmas · A. Lèbre
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    ABSTRACT: Aims: Synthetic spectra are needed to determine fundamental stellar and wind parameters of all types of stars. They are also used for the construction of theoretical spectral libraries helpful for stellar population synthesis. Therefore, a database of theoretical spectra is required to allow rapid and quantitative comparisons to spectroscopic data. We provide such a database offering an unprecedented coverage of the entire Hertzsprung-Russell diagram. Methods: We present the POLLUX database of synthetic stellar spectra. For objects with Teff
    Full-text · Article · Jun 2010 · Astronomy and Astrophysics
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    ABSTRACT: Solar activity and helioseismology show the limitation of the standard solar model and call for the inclusion of dynamical processes in both convective and radiative zones. We concentrate here on the radiative zone and first show the sensitivity of boron neutrinos to the microscopic physics included in solar models. We confront the neutrino predictions of the seismic model to all the detected neutrino fluxes. Then we compute new models of the Sun including a detailed transport of angular momentum and chemicals due to internal rotation that includes meridional circulation and shear induced turbulence. We use two stellar evolution codes: CESAM and STAREVOL to estimate the different terms. We follow three temporal evolutions of the internal rotation differing by their initial conditions: very slow, moderate and fast rotation, with magnetic braking at the arrival on the main sequence for the last two. We find that the meridional velocity in the present solar radiative zone is extremely small in comparison with those of the convective zone, smaller than 10^-6 cm/s instead of m/s. All models lead to a radial differential rotation profile but with a significantly different contrast. We compare these profiles to the presumed solar internal rotation and show that if meridional circulation and shear turbulence were the only mechanisms transporting angular momentum within the Sun, a rather slow rotation in the young Sun is favored. The transport by rotation slightly influence the sound speed profile but its potential impact on the chemicals in the transition region between radiation and convective zones. This work pushes us to pursue the inclusion of the other dynamical processes to better reproduce the present observable and to describe the young active Sun. We also need to get a better knowledge of solar gravity mode splittings to use their constraints. Comment: 39 pages, 9 figures, accepted in Astrophysical Journal
    Full-text · Article · Apr 2010 · The Astrophysical Journal
  • A. Palacios · M. Gebran · E. Josselin · F. Martins · B. Plez · M. Belmas · A. Lebre
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    ABSTRACT: A description of the data (high resolution synthetic spectra and spectral energy distributions) in the POLLUX database is presented in the form of an ascii table providing parameters that can be queried (Teff, logg, code, metallicity) and giving the filename and URL where a fits file can be retrieved. (1 data file).
    No preview · Article · Apr 2010
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    ABSTRACT: From spectropolarimetric data and spectral synthesis analysis, we report the serendipitous discovery of an unusually high lithium content field giant. HD 232862, classified as a G8II star, appears to be the first Lithium-rich field giant star hosting a surface magnetic field.
    Preview · Article · Nov 2009 · Proceedings of the International Astronomical Union
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    ABSTRACT: Aims: We report the detection of an unusually high lithium content in HD 232 862, a field giant classified as a G8II star, and hosting a magnetic field. Methods: With the spectropolarimeters ESPaDOnS at CFHT and NARVAL at TBL, we collected high resolution and high signal-to-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 (A_Li = 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_&sun;, 3.5 M_&sun;] mass interval, likely 1.5-2.0 M&sun; , 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. Based on spectropolarimetric observations obtained at the Canada-France-Hawaii Telescope (CFHT, operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii), and at the Télescope Bernard Lyot (TBL at Observatoire du Pic du Midi, CNRS and Université de Toulouse, France).
    Preview · Article · Sep 2009 · Astronomy and Astrophysics

Publication Stats

783 Citations
110.96 Total Impact Points

Institutions

  • 2007-2016
    • Université de Montpellier
      Montpelhièr, Languedoc-Roussillon, France
  • 2001-2015
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 2013
    • Université Montpellier 2 Sciences et Techniques
      Montpelhièr, Languedoc-Roussillon, France
  • 2010
    • Cea Leti
      Grenoble, Rhône-Alpes, France
  • 2003-2005
    • Université Libre de Bruxelles
      • Institute of Astronomy and Astrophysics
      Bruxelles, Brussels Capital, Belgium
    • Vrije Universiteit Brussel
      Bruxelles, Brussels Capital, Belgium