29 research outputs found
12 Bootis: a test bed for extra-mixing processes in stars
12 Bootis is a spectroscopic binary whose visual orbit has been resolved by
interferometry. Though the physical parameters of the system have been
determined with an excellent precision, the theoretical modelling of the
components is still uncertain. We study the capability of solar-like
oscillations to distinguish between calibrated models of the system obtained by
including in the stellar modelling different mixing processes. We consider
different scenarios for the chemical transport processes: classical
overshooting, microscopic diffusion and turbulent mixing. For each of them we
calibrate the stellar models of 12 Boo A and B by fitting the available
observational constraints by means of a Levenberg-Marquardt minimization
algorithm, and finally, we analyze the asteroseismic properties of different
calibrated models. Several solutions with 12 Boo A in (or close to) post-main
sequence and 12 Boo B on main sequence are found by assuming a thickness of the
overshooting layer between 0.06 and 0.23 the pressure scale height. Solutions
with both components on the main sequence can be found only by assuming an
overshoot larger in the primary than in the secondary, or a more efficient
central mixing for 12 Boo A than for 12 Boo B. We show that the detection of
solar-like oscillations expected in these stars would allow to distinguish
between different scenarios and provide therefore an estimation of the
overshooting parameters and of the properties of extra-mixing processes.Comment: 12 pages, 11 figures. Accepted for publication in MNRA
Revised instability domains of SPB and beta Cephei stars
The excitation of pulsation modes in beta Cephei and Slowly Pulsating B stars
is known to be very sensitive to opacity changes in the stellar interior where
T~2 10^5 K. In this region differences in opacity up to ~50% can be induced by
the choice between OPAL and OP opacity tables, and between two different metal
mixtures (Grevesse and Noels 1993 and Asplund et al. 2005). We have extended
the non-adiabatic computations presented in Miglio et al. (2007) towards models
of higher mass and pulsation modes of degree l=3, and we present here the
instability domains in the HR- and log(P)-log(Teff) diagrams resulting from
different choices of opacity tables, and for three different metallicities.Comment: 9 pages, 4 figures. Accepted for publication in Communications in
Asteroseismolog
On the proper use of the Schwarzschild and Ledoux criteria in stellar evolution computations
The era of detailed asteroseismic analyses opened by space missions such as
CoRoT and has highlighted the need for stellar models devoid
of numerical inaccuracies, in order to be able to diagnose which physical
aspects are being ignored or poorly treated in standard stellar modeling. We
tackle here the important problem of fixing convective zones boundaries in the
frame of the local mixing length theory. First we show that the only correct
way to locate a convective zone boundary is to find, at each iteration step,
through interpolations or extrapolations from points , the mass where the radiative luminosity is equal to the
total one. We then discuss two misuses of the boundary condition and the way
they affect stellar modeling and stellar evolution. The first one consists in
applying the neutrality condition for convective instability on the
side of the convective boundary. The second way of
misusing the boundary condition comes from the process of fixing the convective
boundary through the search for a change of sign of a possibly
\textit{discontinuous} function. We show that these misuses can lead to
completely wrong estimates of convective core sizes with important consequences
for the following evolutionary phases. We point out the advantages of using a
double mesh point at each convective zone boundaries. The specific problem of a
convective shell is discussed and some remarks concerning overshooting are
given.Comment: 14 pages, 10 figures, to appear in A&
Inference from adiabatic analysis of solar-like oscillations in Red giants
The clear detection with CoRoT and KEPLER of radial and non-radial solar-like
oscillations in many red giants paves the way to seismic inferences on the
structure of such stars. We present an overview of the properties of the
adiabatic frequencies and frequency separations of radial and non-radial
oscillation modes, highlighting how their detection allows a deeper insight
into the properties of the internal structure of red giants. In our study we
consider models of red giants in different evolutionary stages, as well as of
different masses and chemical composition. We describe how the large and small
separations computed with radial modes and with non-radial modes mostly trapped
in the envelope depend on the stellar global parameters and evolutionary state,
and we compare our theoretical predictions and first KEPLER data.Finally, we
find that the properties of dipole modes constitute a promising seismic
diagnostic of the evolutionary state of red-giant stars.Comment: 6 pages, 5 figures. Proceedings of IV Helas International Conference:
"Seismological Challenges for Stellar Structure", Lanzarote (Canary Islands,
Spain), 1-5 February 201
He abundances in disc galaxies. I. Predictions from cosmological chemodynamical simulations
Accepted for publication in A&AWe investigate how the stellar and gas-phase He abundances evolve as a function of time within simulated star-forming disc galaxies with different star formation histories. We make use of a cosmological chemodynamical simulation for galaxy formation and evolution, which includes star formation as well as energy and chemical enrichment feedback from asymptotic giant branch stars, core-collapse supernovae, and Type Ia supernovae. The predicted relations between the He mass fraction, Y, and the metallicity, Z, in the interstellar medium of our simulated disc galaxies depend on the galaxy star formation history. In particular, dY/dZ is not constant and evolves as a function of time, depending on the specific chemical element that we choose to trace Z; in particular, dY/dX O and dY/dX C increase as a function of time, whereas dY/dX N decreases. In the gas-phase, we find negative radial gradients of Y, due to the inside-out growth of our simulated galaxy discs as a function of time; this gives rise to longer chemical enrichment timescales in the outer galaxy regions, where we find lower average values for Y and Z. Finally, by means of chemical-evolution models, in the galactic bulge and inner disc, we predict steeper Y vs. age relations at high Z than in the outer galaxy regions. We conclude that for calibrating the assumed Y-Z relation in stellar models, C, N, and C+N are better proxies for the metallicity than O because they show steeper and less scattered relations.Peer reviewedFinal Published versio
Ledoux's convection criterion in evolution and asteroseismology of massive stars
Saio et al. (2006) have shown that the presence of an intermediate convective
zone (ICZ) in post-main sequence models could prevent the propagation of
g-modes in the radiative interior and hence avoid the corresponding radiative
damping. The development of such a convective region highly depends on the
structure of the star in the mu-gradient region surrounding the convective core
during the main sequence phase. In particular,the development of this ICZ
depends on physical processes such as mass loss, overshooting (Chiosi & Maeder
1986, Chiosi et al. 1992, see also Godart et al., these proceedings) and
convective instability criterion (Schwarzschild's or Ledoux's criteria). In
this paper we study the consequences of adopting the Ledoux's criterion on the
evolution of the convective regions in massive stars (15 and 20 Msun), and on
the pulsation spectrum of these new B-type variables (also called SPBsg).Comment: Contribution to the Proceedings of the 38th LIAC/HELAS-ESTA/BAG, 2008
Accepted for publication in CoAs
The red giants in NGC 6633 as seen with CoRoT, HARPS and SOPHIE
The open cluster NGC 6633 was observed with CoRoT in 2011 and simultaneous
high-resolution spectroscopy was obtained with the SOPHIE and HARPS
spectrographs. One of the four targets was not found to be a cluster member.
For all stars we provide estimates of the seismic and spectroscopic parameters.Comment: Proc. of the workshop "Asteroseismology of stellar populations in the
Milky Way" (Sesto, 22-26 July 2013), Astrophysics and Space Science
Proceedings, (eds. A. Miglio, L. Girardi, P. Eggenberger, J. Montalban
How Much do we Trust Stellar Models? Foreword
peer reviewe
Probing the properties of convective cores through g modes: high-order g modes in SPB and gamma Doradus stars
In main sequence stars the periods of high-order gravity modes are sensitive
probes of stellar cores and, in particular, of the chemical composition
gradient that develops near the outer edge of the convective core. We present
an analytical approximation of high-order g modes that takes into account the
effect of the mu gradient near the core. We show that in main-sequence models,
similarly to the case of white dwarfs, the periods of high-order gravity modes
are accurately described by a uniform period spacing superposed to an
oscillatory component. The periodicity and amplitude of such component are
related, respectively, to the location and sharpness of the mu gradient.
We investigate the properties of high-order gravity modes for stellar models
in a mass domain between 1 and 10 Msun, and the effects of the stellar mass,
evolutionary state, and extra-mixing processes on period spacing features. In
particular, we show that for models of a typical SPB star, a chemical mixing
that could likely be induced by the slow rotation observed in these stars, is
able to significantly change the g-mode spectra of the equilibrium model.
Prospects and challenges for the asteroseismology of gamma Doradus and SPB
stars are also discussed.Comment: 18 pages, 29 figures, accepted for publication in MNRA