569 research outputs found
A-type stars: evolution, rotation and binarity
We discuss the internal structure of stars in the mass range 1.5 to 4 M_sun
from the PMS to the subgiant phase with a particular emphasis on the convective
core and the convective superficial layers. Different physical aspects are
considered such as overshooting, treatment of convection, microscopic diffusion
and rotation. Their influence on the internal structure and on the photospheric
chemical abundances is briefly described. The role of binarity in determining
the observed properties and as a tool to constrain the internal structure is
also introduced and the current limits of theories of orbital evolution and of
available binary data--sets are discussed. keywords{stars: evolution, stars:
binaries: general, stars: rotation}Comment: 11 pages, 7 figures, conference: The A-star Puzzle, IAU Simp. 224,
200
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
Modelling the components of binaries in Hyades: The dependence of the mixing-length parameter on stellar mass
We present our findings based on a detailed analysis for the binaries of the
Hyades, in which the masses of the components are well known. We fit the models
of components of a binary system to the observations so as to give the observed
total V and B-V of that system and the observed slope of the main-sequence in
the corresponding parts. According to our findings, there is a very definite
relationship between the mixing-length parameter and the stellar mass. The
fitting formula for this relationship can be given as , which is valid for stellar masses greater than
0.77 M_sun. While no strict information is gathered for the chemical
composition of the cluster, as a result of degeneracy in the colour-magnitude
diagram, by adopting Z=0.033 and using models for the components of 70 Tau and
theta^2 Tau we find the hydrogen abundance to be X=0.676 and the age to be 670
Myr. If we assume that Z=0.024, then X=0.718 and the age is 720 Myr. Our
findings concerning the mixing length parameter are valid for both sets of the
solution. For both components of the active binary system V818 Tau, the
differences between radii of the models with Z=0.024 and the observed radii are
only about 4 percent. More generally, the effective temperatures of the models
of low mass stars in the binary systems studied are in good agreement with
those determined by spectroscopic methods.Comment: 11 pages, 7 figures, accepted for publication in MNRA
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
Asteroseismology of Massive Stars : Some Words of Caution
Although playing a key role in the understanding of the supernova phenomenon,
the evolution of massive stars still suffers from uncertainties in their
structure, even during their "quiet" main sequence phase and later on during
their subgiant and helium burning phases. What is the extent of the mixed
central region? In the local mixing length theory (LMLT) frame, are there
structural differences using Schwarzschild or Ledoux convection criterion?
Where are located the convective zone boundaries? Are there intermediate
convection zones during MS and post-MS phase, and what is their extent and
location? We discuss these points and show how asteroseismology could bring
some light on these questions.Comment: 10 pages, 5 figures, IAU Symposium 307, New windows on massive stars:
asteroseismology, interferometry, and spectropolarimetry, G. Meynet, C.
Georgy, J.H. Groh & Ph. Stee, ed
Can an underestimation of opacity explain B-type pulsators in the SMC?
Slowly Pulsating B and Cephei are mechanism driven pulsating
B stars. That mechanism works since a peak in the opacity due to a
high number of atomic transitions from iron-group elements occurs in the area
of . Theoretical results predict very few SPBs and no
Cep to be encountered in low metallicity environments such as the Small
Magellanic Cloud. However recent variability surveys of B stars in the SMC
reported the detection of a significant number of SPB and Cep
candidates. Though the iron content plays a major role in the excitation of
Cep and SPB pulsations, the chemical mixture representative of the SMC
B stars such as recently derived does not leave room for a significant increase
of the iron abundance in these stars. Whilst abundance of iron-group elements
seems reliable, is the opacity in the iron-group elements bump underestimated?
We determine how the opacity profile in B-type stars should change to excite
SPB and Cep pulsations in early-type stars of the SMC.Comment: 5 pages, 7 figures, to appear under electronic form in : Proceedings
of the 4th HELAS International Conference: Seismological Challenges for
Stellar Structur
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&
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