36 research outputs found
The new Toulouse-Geneva Stellar Evolution Code including radiative accelerations of heavy elements
Atomic diffusion has been recognized as an important process that has to be
considered in any computations of stellar models. In solar-type and cooler
stars, this process is dominated by gravitational settling, which is now
included in most stellar evolution codes. In hotter stars, radiative
accelerations compete with gravity and become the dominant ingredient in the
diffusion flux for most heavy elements. Introducing radiative accelerations
into the computations of stellar models modifies the internal element
distribution and may have major consequences on the stellar structure. Coupling
these processes with hydrodynamical stellar motions has important consequences
that need to be investigated in detail. We aim to include the computations of
radiative accelerations in a stellar evolution code (here the TGEC code) using
a simplified method (SVP) so that it may be coupled with sophisticated
macroscopic motions. We also compare the results with those of the Montreal
code in specific cases for validation and study the consequences of these
coupled processes on accurate models of A- and early-type stars. We implemented
radiative accelerations computations into the Toulouse-Geneva stellar evolution
code following the semi-analytical prescription proposed by Alecian and
LeBlanc. This allows more rapid computations than the full description used in
the Montreal code. We present results for A-type stellar models computed with
this updated version of TGEC and compare them with similar published models
obtained with the Montreal evolution code. We discuss the consequences for the
coupling with macroscopic motions, including thermohaline convection.Comment: 12 pages, 13 figures, published in A&
The convective envelope in gamma Doradus stars: theoretical uncertainties
The depth of the convective envelope plays a fundamental role in the driving
mechanism proposed by Guzik et al. (2000) to explain the high-order g modes of
gamma Dor pulsators. In this poster we study the sensitivity of the convective
envelope depth to the description of convective transport, to relevant physical
processes, such as microscopic diffusion, and to other uncertainties in
theoretical stellar models.Comment: 2 pags. 1 fig. Poster in Vienna Workshop on the Future of
Asteroseismology, September 20-22, 2006. To be published by CoAs
On the Coupling between Helium Settling and Rotation-Induced Mixing in Stellar Radiative Zones: II- Application to light elements in population I main-sequence stars
In the two previous papers of this series, we have discussed the importance
of t he -gradients due to helium settling on rotation-induced mixing,
first in a n approximate analytical way, second in a 2D numerical simulation.
We have found that, for slowly rotating low mass stars, a process of ``creeping
paralysis" in which the circulation and the diffusion are nearly frozen may
take place below the convective zone. Here we apply this theory to the case of
lithium and beryll ium in galactic clusters and specially the Hyades. We take
into account the rota tional braking with rotation velocities adjusted to the
present observations. We find that two different cells of meridional
circulation appear on the hot side of the "lithium dip" and that the "creeping
paralysis" process occurs, not dir ectly below the convective zone, but deeper
inside the radiative zone, at the to p of the second cell. As a consequence,
the two cells are disconnected, which ma y be the basic reason for the lithium
increase with effective temperature on thi s side of the dip. On the cool side,
there is just one cell of circulation and t he paralysis has not yet set down
at the age of the Hyades; the same modelisatio n accounts nicely for the
beryllium observations as well as for the lithium ones .Comment: 13 printed pages, 10 figures. ApJ, in press (April 20, 2003
Asteroseismology of exoplanets host stars: the special case of Horologii (HD17051)
{This paper presents detailed analysis and modelisation of the star HD17051
(alias Hor), which appears as a specially interesting case among
exoplanet host stars. As most of these stars, Hor presents a
metallicity excess which has been measured by various observers who give
different results, ranging from [Fe/H] = 0.11 to 0.26, associated with
different atmospheric parameters. Meanwhile the luminosity of the star may be
determined owing to Hipparcos parallax. Although in the southern hemisphere,
this star belongs to the Hyades stream and its external parameters show that it
could even be one of the Hyades stars ejected during cluster formation. The aim
of this work was to gather and analyse our present knowledge on this star and
to prepare seismic tests for future observations with the HARPS spectrometer
(planned for November 2006).} {We have computed evolutionary tracks with
various metallicities, in the two frameworks of primordial overmetallicity and
accretion. We have concentrated on models inside the error boxes given by the
various observers in the log g - log T diagram. We then computed the
adiabatic oscillation frequencies of these models to prepare future
observations.} {The detailed analysis of Hor presented in this paper
already allowed to constrain its external parameters, mass and age. Some values
given in the literature could be rejected as inconsistent with the overall
analysis. We found that a model computed with the Hyades parameters (age,
metallicity) was clearly acceptable, but other ones were possible too. We are
confident that observations with HARPS will allow for a clear conclusion about
this star and that it will bring important new light on the physics of
exoplanet host stars.}Comment: to be published in Astronomy and Astrophysic
On the Coupling between Helium Settling and Rotation-Induced Mixing in Stellar Radiative Zones: I- Analytical Approach
In the presence of rotation-induced mixing, element diffusion still occurs in
stellar radiative zones, although at a slower rate than in the case of a
complete stability of the stellar gas. As a consequence, helium settling leads
to vertical -gradients which, due to the meridional
circulation, turn into horizontal fluctuations. Up to now, the feed-back effect
of this process on the rotation-induced mixing was currently neglected in the
computations of abundance variations in stellar surfaces, or artificially
reduced. Here we analyse its consequences and derive an approximate analytical
solution in a quasi-stationary case. We also discuss the relative importance of
the various physical effects which influence the meridional circulation
velocity. In a second paper (Th\'eado and Vauclair 2002a), we will present a
complete 2D numerical simulation of this process while a third paper (Th\'eado
and Vauclair 2002b) will be devoted to special applications to Pop I stars.Comment: 7 printed pages, 1 figure. ApJ, in press (April 20, 2003
Recent Advances in Modeling Stellar Interiors
Advances in stellar interior modeling are being driven by new data from
large-scale surveys and high-precision photometric and spectroscopic
observations. Here we focus on single stars in normal evolutionary phases; we
will not discuss the many advances in modeling star formation, interacting
binaries, supernovae, or neutron stars. We review briefly: 1) updates to input
physics of stellar models; 2) progress in two and three-dimensional evolution
and hydrodynamic models; 3) insights from oscillation data used to infer
stellar interior structure and validate model predictions (asteroseismology).
We close by highlighting a few outstanding problems, e.g., the driving
mechanisms for hybrid gamma Dor/delta Sct star pulsations, the cause of giant
eruptions seen in luminous blue variables such as eta Car and P Cyg, and the
solar abundance problem.Comment: Proceedings for invited talk at conference High Energy Density
Laboratory Astrophysics 2010, Caltech, March 2010, submitted for special
issue of Astrophysics and Space Science; 7 pages; 5 figure
s-Process in Low Metallicity Stars. III. Individual analysis of CEMP-s and CEMP-s/r with AGB models
We provide an individual analysis of 94 carbon enhanced metal-poor stars
showing an s-process enrichment (CEMP-s) collected from the literature. The
s-process enhancement observed in these stars is ascribed to mass transfer by
stellar winds in a binary system from a more massive companion evolving faster
toward the asymptotic giant branch (AGB) phase. The theoretical AGB
nucleosynthesis models have been presented in Paper I. Several CEMP-s stars
show an enhancement in both s and r-process elements (CEMP-s/r). In order to
explain the peculiar abundances observed in CEMP-s/r stars, we assume that the
molecular cloud from which CEMP-s formed was previously enriched in r-elements
by Supernovae pollution.
A general discussion and the method adopted in order to interpret the
observations have been provided in Paper II. We present in this paper a
detailed study of spectroscopic observations of individual stars. We consider
all elements from carbon to bismuth, with particular attention to the three
s-process peaks, ls (Y, Zr), hs (La, Nd, Sm) and Pb, and their ratios [hs/ls]
and [Pb/hs]. The presence of an initial r-process contribution may be typically
evaluated by the [La/Eu] ratio. We found possible agreements between
theoretical predictions and spectroscopic data. In general, the observed
[Na/Fe] (and [Mg/Fe]) provide information on the AGB initial mass, while
[hs/ls] and [Pb/hs] are mainly indicators of the s-process efficiency. A range
of 13C-pocket strengths is required to interpret the observations. However,
major discrepancies between models and observations exist. We highlight star by
star the agreements and the main problems encountered and, when possible, we
suggest potential indications for further studies. These discrepancies provide
starting points of debate for unsolved problems ...Comment: 40 pages, 54 figures, 3 tables (Appendix A as supplementary
material), accepted Monthly Notices of the Royal Astronomical Society, 201