351 research outputs found
Discriminating between overshooting and rotational mixing in massive stars: any help from asteroseismology?
Chemical turbulent mixing induced by rotation can affect the internal
distribution of mu near the energy-generating core of main-sequence stars,
having an effect on the evolutionary tracks similar to that of overshooting.
However, this mixing also leads to a smoother chemical composition profile near
the edge of the convective core, which is reflected in the behaviour of the
buoyancy frequency and, therefore, in the frequencies of gravity modes. We show
that for rotational velocities typical of main-sequence B-type pulsating stars,
the signature of a rotationally induced mixing significantly perturbs the
spectrum of gravity modes and mixed modes, and can be distinguished from that
of overshooting. The cases of high-order gravity modes in Slowly Pulsating B
stars and of low-order g modes and mixed modes in beta Cephei stars are
discussed.Comment: 6 pages, 4 figures, Comm. in Asteroseismology, Contribution to the
Proceedings of the 38th LIAC, HELAS-ESTA, BAG, 200
The Old Halo metallicity gradient: the trace of a self-enrichment process
Based on a model of globular cluster self-enrichment published in a previous
paper, we present an explanation for the metallicity gradient observed
throughout the galactic Old Halo. Our self-enrichment model is based on the
ability of globular cluster progenitor clouds to retain the ejecta of a first
generation of Type II Supernovae. The key point is that this ability depends on
the pressure exerted on the progenitor cloud by the surrounding protogalactic
medium and therefore on the location of the cloud in the protoGalaxy. Since
there is no significant (if any) metallicity gradient in the whole halo, we
also present a review in favour of a galactic halo partly build via accretions
and mergers of satellite systems. Some of them bear their own globular clusters
and therefore ``contaminate'' the system of globular clusters formed ``in
situ'', namely within the original potential well of the Galaxy. Therefore, the
comparison between our self-enrichment model and the observational data should
be limited to the genuine galactic globular clusters, the so-called Old Halo
group.Comment: 11 pages, 4 figures, accepted for publication in Astronomy and
Astrophysic
The self-enrichment of galactic halo globular clusters : a clue to their formation ?
We present a model of globular cluster self-enrichment. In the protogalaxy,
cold and dense clouds embedded in the hot protogalactic medium are assumed to
be the progenitors of galactic halo globular clusters. The massive stars of a
first generation of metal-free stars, born in the central areas of the
proto-globular cluster clouds, explode as Type II supernovae. The associated
blast waves trigger the expansion of a supershell, sweeping all the material of
the cloud, and the heavy elements released by these massive stars enrich the
supershell. A second generation of stars is born in these compressed and
enriched layers of gas. These stars can recollapse and form a globular cluster.
This work aims at revising the most often encountered argument against
self-enrichment, namely the presumed ability of a small number of supernovae to
disrupt a proto-globular cluster cloud. We describe a model of the dynamics of
the supershell and of its progressive chemical enrichment. We show that the
minimal mass of the primordial cluster cloud required to avoid disruption by
several tens of Type II supernovae is compatible with the masses usually
assumed for proto-globular cluster clouds. Furthermore, the corresponding
self-enrichment level is in agreement with halo globular cluster metallicities.Comment: 12 pages, 7 figures. Accepted for publication in Astronomy and
Astrophysic
Theoretical seismic properties of pre-main sequence gamma Doradus pulsators
Context. gamma Doradus (gamma Dor) are late A and F-type stars pulsating with
high order gravity modes (g-modes). The existence of different evolutionary
phases crossing the gamma Dor instability strip raises the question of the
existence of pre-main sequence (PMS) gamma Dor stars. Aims. We intend to study
the differences between the asteroseismic behaviour of PMS and main sequence
(MS) gamma Dor pulsators as it is predicted by the current theory of stellar
evolution and stability. Methods. We explore the adiabatic and non-adiabatic
properties of high order g-modes in a grid of PMS and MS models covering the
mass range 1.2 Msun < Mstar < 2.5 Msun. Results. We derive the theoretical
instability strip (IS) for the PMS gamma Dor pulsators. This IS covers the same
effective temperature range as the MS gamma Dor one. Nevertheless, the
frequency domain of unstable modes in PMS models with a fully radiative core is
larger than in MS models, even if they present the same number of unstable
modes. Moreover, the differences between MS and PMS internal structures are
reflected on the average values of the period spacing as well as on the
dependence of the period spacing on the radial order of the modes, opening the
window to the determination of the evolutionary phase of gamma Dor stars from
their pulsation spectra.Comment: 9 pages, 17 figures, accepted for publication in A&
Effects of rotation on the evolution and asteroseismic properties of red giants
The influence of rotation on the properties of red giants is studied in the
context of the asteroseismic modelling of these stars. While red giants exhibit
low surface rotational velocities, we find that the rotational history of the
star has a large impact on its properties during the red giant phase. In
particular, for stars massive enough to ignite He burning in non-degenerate
conditions, rotational mixing induces a significant increase of the stellar
luminosity and shifts the location of the core helium burning phase to a higher
luminosity in the HR diagram. This of course results in a change of the seismic
properties of red giants at the same evolutionary state. As a consequence the
inclusion of rotation significantly changes the fundamental parameters of a red
giant star as determined by performing an asteroseismic calibration. In
particular rotation decreases the derived stellar mass and increases the age.
Depending on the rotation law assumed in the convective envelope and on the
initial velocity of the star, non-negligible values of rotational splitting can
be reached, which may complicate the observation and identification of
non-radial oscillation modes for red giants exhibiting moderate surface
rotational velocities. By comparing the effects of rotation and overshooting,
we find that the main-sequence widening and the increase of the H-burning
lifetime induced by rotation (Vini=150 km/s) are well reproduced by
non-rotating models with an overshooting parameter of 0.1, while the increase
of luminosity during the post-main sequence evolution is better reproduced by
non-rotating models with overshooting parameters twice as large. This is due to
the fact that rotation not only increases the size of the convective core but
also changes the chemical composition of the radiative zone.Comment: 9 pages, 13 figures, accepted for publication in A&
Asteroseismology of red-clump stars with CoRoT and Kepler
The availability of asteroseismic constraints for a large number of red
giants with CoRoT and in the near future with Kepler, paves the way for
detailed studies of populations of galactic-disk red giants. We investigate
which information on the observed population can be recovered by the
distribution of the observed seismic constraints: the frequency of maximum
power of solar-like oscillations () and the large frequency
separation (). We use the distribution of and of
observed by CoRoT in nearly 800 red giants in the first long
observational run, as a tool to investigate the properties of galactic
red-giant stars through the comparison with simulated distributions based on
synthetic stellar populations.
We can clearly identify the bulk of the red giants observed by CoRoT as
red-clump stars, i.e. post-flash core-He-burning stars. The distribution of
and of give us access to the distribution of the
stellar radius and mass, and thus represent a most promising probe of the age
and star formation rate of the disk, and of the mass-loss rate during the
red-giant branch.
This approach will be of great utility also in the interpretation of
forthcoming surveys of variability of red giants with CoRoT and Kepler. In
particular, an asteroseismic mass estimate of clump stars in the old-open
clusters observed by Kepler, would represent a most valuable observational test
of the poorly known mass-loss rate on the giant branch, and of its dependence
on metallicity.Comment: 5 pages, 6 figures, proceeding for "Stellar Pulsation: Challenges for
Theory and Observation", Santa Fe 200
Hybrid gamma Doradus/delta Scuti Stars: Comparison Between Observations and Theory
Gamma Doradus are F-type stars pulsating with high order g-modes. Their
instability strip (IS) overlaps the red edge of the delta Scuti one. This
observation has led to search for objects in this region of the HR diagram
showing p and g-modes simultaneously. The existence of such hybrid pulsators
has recently been confirmed (Handler 2009) and the number of candidates is
increasing (Matthews 2007). From a theoretical point of view, non-adiabatic
computations including a time-dependent treatment of convection (TDC) predict
the existence of gamma Dor/delta Sct hybrid pulsators (Dupret et al. 2004;
Grigahcene et al. 2006). Our aim is to confront the properties of the observed
hybrid candidates with the theoretical predictions from non-adiabatic
computations of non-radial pulsations including the convection-pulsation
interaction.Comment: 3 pages, 3 figures, Poster at "Stellar Pulsation: challenges for
theory and observation", Santa Fe, June 200
V.3 Present and future space missions for ultra-precision photometry
This book is dedicated to all the people interested in the CoRoT mission and the beautiful data that were delivered during its six year duration. Either amateurs, professional, young or senior researchers, they will find treasures not only at the time of this publication but also in the future twenty or thirty years. It presents the data in their final version, explains how they have been obtained, how to handle them, describes the tools necessary to understand them, and where to find them. It also highlights the most striking first results obtained up to now. CoRoT has opened several unexpected directions of research and certainly new ones still to be discovered
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