1,015 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
Instability strips of main sequence B stars: a parametric study of iron enhancement
The discovery of beta Cephei stars in low metallicity environments, as well
as the difficulty to theoretically explain the excitation of the pulsation
modes observed in some beta Cephei and SPB stars, suggest that the iron opacity
``bump'' provided by standard models could be underestimated. We investigate,
by means of a parametric study, the effect of a local iron enhancement on the
location of the beta Cephei and SPB instability strips.Comment: 2 pages, to appear in the proceedings of "Vienna Workshop on the
Future of Asteroseismology", September 20-22, 200
Instability strips of SPB and beta Cephei stars: the effect of the updated OP opacities and of the metal mixture
The discovery of Cephei stars in low metallicity environments, as
well as the difficulty in theoretically explaining the excitation of the
pulsation modes observed in some Cephei and hybrid SPB- Cephei
pulsators, suggest that the ``iron opacity bump'' provided by stellar models
could be underestimated. We analyze the effect of uncertainties in the opacity
computations and in the solar metal mixture, on the excitation of pulsation
modes in B-type stars. We carry out a pulsational stability analysis for four
grids of main-sequence models with masses between 2.5 and 12
computed with OPAL and OP opacity tables and two different metal mixtures.
We find that in a typical Cephei model the OP opacity is 25% larger
than OPAL in the region where the driving of pulsation modes occurs.
Furthermore, the difference in the Fe mass fraction between the two metal
mixtures considered is of the order of 20%. The implication on the excitation
of pulsation modes is non-negligible: the blue border of the SPB instability
strip is displaced at higher effective temperatures, leading to a larger number
of models being hybrid SPB- Cephei pulsators. Moreover, higher overtone
p-modes are excited in Cephei models and unstable modes are found in a
larger number of models for lower metallicities, in particular Cephei
pulsations are also found in models with Z=0.01.Comment: Accepted for publication in MNRAS Letter
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
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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