602 research outputs found
Asteroseismology of the visual binary 70 Ophiuchi
Convection in stars excites resonant acoustic waves. The frequencies of these
oscillations depend on the sound speed inside the star, which in turn depends
on density, temperature, gas motion, and other properties of the stellar
interior. Therefore, analysis of the oscillations provides an unrivaled method
to probe the internal structure of a star. Solar-like oscillations in the
primary of the visual binary 70 Ophiuchi are investigated. 70 Ophiuchi A was
observed with the Harps spectrograph mounted on the 3.6-m telescope at the ESO
La Silla Observatory (Chile) during 6 nights in July 2004 allowing us to
collect 1758 radial velocity measurements with a standard deviation of about
1.39 m s-1. The power spectrum of the high precision velocity time series
clearly presents several identifiable peaks between 3 and 6 mHz showing
regularity with a large spacing of Delta_nu = 161.7 +- 0.3 uHz. Fourteen
individual modes were identified with amplitudes in the range 11 to 14 cm s-1.Comment: 5 pages, A&A in pres
Impact of rotation and disc lifetime on pre-main sequence lithium depletion of solar-type stars
Aims: We study the influence of rotation and disc lifetime on lithium
depletion of pre-main sequence (PMS) solar-type stars. Methods: The impact of
rotational mixing and of the hydrostatic effects of rotation on lithium
abundances are investigated by computing non-rotating and rotating PMS models
that include a comprehensive treatment of shellular rotation. The influence of
the disc lifetime is then studied by comparing the lithium content of PMS
rotating models experiencing different durations of the disc-locking phase
between 3 and 9 Myr. Results: The surface lithium abundance at the end of the
PMS is decreased when rotational effects are included. During the beginning of
the lithium depletion phase, only hydrostatic effects of rotation are at work.
This results in a decrease in the lithium depletion rate for rotating models
compared to non-rotating ones. When the convective envelope recedes from the
stellar centre, rotational mixing begins to play an important role due to
differential rotation near the bottom of the convective envelope. This mixing
results in a decrease in the surface lithium abundance with a limited
contribution from hydrostatic effects of rotation, which favours lithium
depletion during the second part of the PMS evolution. The impact of rotation
on PMS lithium depletion is also found to be sensitive to the duration of the
disc-locking phase. When the disc lifetime increases, the PMS lithium abundance
of a solar-type star decreases owing to the higher efficiency of rotational
mixing in the radiative zone. A relationship between the surface rotation and
lithium abundance at the end of the PMS is then obtained: slow rotators on the
zero-age main sequence are predicted to be more lithium-depleted than fast
rotators due to the increase in the disc lifetime.Comment: 8 pages, 11 figures, A&
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
Understanding angular momentum transport in red giants: the case of KIC 7341231
Context. Thanks to recent asteroseismic observations, it has been possible to
infer the radial differential rotation profile of subgiants and red giants.
Aims. We want to reproduce through modeling the observed rotation profile of
the early red giant KIC 7341231 and constrain the physical mechanisms
responsible for angular momentum transport in stellar interiors.
Methods. We compute models of KIC 7341231 including a treatment of shellular
rotation and we compare the rotation profiles obtained with the one derived by
Deheuvels et al. (2012). We then modify some modeling parameters in order to
quantify their effect on the obtained rotation profile. Moreover, we mimic a
powerful angular momentum transport during the Main Sequence and study its
effect on the evolution of the rotation profile during the subgiant and red
giant phases.
Results. We show that meridional circulation and shear mixing alone produce a
rotation profile for KIC 7341231 too steep compared to the observed one. An
additional mechanism is then needed to increase the internal transport of
angular momentum. We find that this undetermined mechanism has to be efficient
not only during the Main Sequence but also during the much quicker subgiant
phase. Moreover, we point out the importance of studying the whole rotational
history of a star in order to explain its rotation profile during the red giant
evolution.Comment: 8 pages, 8 figures, 5 table
Constraining angular momentum transport processes in stellar interiors with red-giant stars in the open cluster NGC6819
Clusters are excellent test benches for verification and improvement of
stellar evolution theory. The recent detection of solar-like oscillations in
G-K giants in the open cluster NGC6819 with Kepler provides us with independent
constraints on the masses and radii of stars on the red giant branch, as well
as on the distance to clusters and their ages. We present, for NGC6819,
evolutionary models by considering rotation-induced mixing ; and the
theoretical low-l frequencies of our stellar models.Comment: Submitted to EPJ Web of Conferences, to appear in the Proceedings of
the 3rd CoRoT Symposium, Kepler KASC7 joint meeting; 2 pages, 1 figur
Asteroseismology of red giants to constrain angular momentum transport
Asteroseismic data obtained by the Kepler spacecraft have led to the recent detection and characterization of rotational frequency splittings of mixed modes in red-giant stars. This has opened the way to the determination of the core rotation rates for these stars, which is of prime importance to progress in our understanding of internal angular momentum transport. In this contribution, we discuss which constraints can be brought by these asteroseismic measurements on the modelling of angular momentum transport in stellar radiative zone
Massive star evolution in close binaries:conditions for homogeneous chemical evolution
We investigate the impact of tidal interactions, before any mass transfer, on
various properties of the stellar models. We study the conditions for obtaining
homogeneous evolution triggered by tidal interactions, and for avoiding any
Roche lobe overflow during the Main-Sequence phase. We consider the case of
rotating stars computed with a strong coupling mediated by an interior magnetic
field. In models without any tidal interaction (single stars and wide
binaries), homogeneous evolution in solid body rotating models is obtained when
two conditions are realized: the initial rotation must be high enough, the loss
of angular momentum by stellar winds should be modest. This last point favors
metal-poor fast rotating stars. In models with tidal interactions, homogeneous
evolution is obtained when rotation imposed by synchronization is high enough
(typically a time-averaged surface velocities during the Main-Sequence phase
above 250 km s), whatever the mass losses. In close binaries, mixing is
stronger at higher than at lower metallicities. Homogeneous evolution is thus
favored at higher metallicities. Roche lobe overflow avoidance is favored at
lower metallicities due to the fact that stars with less metals remain more
compact. We study also the impact of different processes for the angular
momentum transport on the surface abundances and velocities in single and close
binaries. In models where strong internal coupling is assumed, strong surface
enrichments are always associated to high surface velocities in binary or
single star models. In contrast, models computed with mild coupling may produce
strong surface enrichments associated to low surface velocities. Close binary
models may be of interest for explaining homogeneous massive stars, fast
rotating Wolf-Rayet stars, and progenitors of long soft gamma ray bursts, even
at high metallicities.Comment: 21 pages, 13 figures, 3 tables, accepted for publication in Astronomy
and Astrophysic
Effects of rotation and magnetic fields on the lithium abundance and asteroseismic properties of exoplanet-host stars
Aims: The effects of rotation and magnetic fields on the surface abundances
of solar-type stars are studied in order to investigate whether the reported
difference in lithium content of exoplanet-host stars can be related to their
rotational history. Moreover, the asteroseismic properties predicted for stars
with and without exoplanets are compared to determine how such a scenario,
which relates the lithium abundances and the rotational history of the star,
can be further challenged by observations of solar-like oscillations. Methods:
Based on observations of rotational periods of solar-type stars, slow rotators
on the zero age main sequence (ZAMS) are modelled with a comprehensive
treatment of only the shellular rotation, while fast rotators are modelled
including both shellular rotation and magnetic fields. Assuming a possible link
between low rotation rates on the ZAMS and the presence of planets as a result
of a longer disc-locking phase during the pre-main sequence (PMS), we compare
the surface abundances and asteroseismic properties of slow and fast rotating
models, which correspond to exoplanet-host stars and stars without detected
planets, respectively. Results: We confirm previous suggestions that the
difference in the lithium content of stars with and without detected planets
can be related to their different rotational history. The larger efficiency of
rotational mixing predicted in exoplanet-host stars explains their lithium
depletion and also leads to changes in the structure and chemical composition
of the central stellar layers. Asteroseismic observations can reveal these
changes and can help us distinguish between different possible explanations for
the lower lithium content of exoplanet-host stars.Comment: 4 pages, 4 figures, A&A lette
Thermohaline instability and rotation-induced mixing. III - Grid of stellar models and asymptotic asteroseismic quantities from the pre-main sequence up to the AGB for low- and intermediate-mass stars at various metallicities
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.Comment: 15 pages, 11 figures, accepted for publication in A&
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