3,600 research outputs found
Asteroseismology of Solar-Type and Red-Giant Stars
We are entering a golden era for stellar physics driven by satellite and
telescope observations of unprecedented quality and scope. New insights on
stellar evolution and stellar interiors physics are being made possible by
asteroseismology, the study of stars by the observation of natural, resonant
oscillations. Asteroseismology is proving to be particularly significant for
the study of solar-type and red-giant stars. These stars show rich spectra of
solar-like oscillations, which are excited and intrinsically damped by
turbulence in the outermost layers of the convective envelopes. In this review
we discuss the current state of the field, with a particular emphasis on recent
advances provided by the Kepler and CoRoT space missions and the wider
significance to astronomy of the results from asteroseismology, such as stellar
populations studies and exoplanet studies.Comment: The following paper will appear in the 2013 volume of Annual Reviews
of Astronomy and Astrophysics (88 pages, 7 figures; references updated;
further corrections to typos during galley-proof review
Infrared excess around nearby RGB stars and Reimers law
(Abridged) The spectral energy distributions of a well-defined sample of 54
RGB stars are constructed, and fitted with the dust radiative transfer model
DUSTY. The central stars are modeled by MARCS model atmospheres. In a first
step, the best-fit MARCS model is derived, determining the effective
temperature. In a second step, models with a finite dust optical depth are
fitted and it is determined whether the reduction in chi2 in such models with
one additional free parameter is statistically significant.
23 stars are found to have a significant infrared excess, which is
interpreted as mass loss. The dust optical depths are translated into mass-loss
rates assuming a typical expansion velocity of 10 km/s and a dust-to-gas ratio
of 0.005.
The mass-loss rates are compared to those derived for luminous stars in
globular clusters, by fitting both the infrared excess, as in the present
paper, and the chromospheric lines. There is excellent agreement between these
values and the mass-loss rates derived from the chromospheric activity. There
is a systematic difference with the literature mass-loss rates derived from
modeling the infrared excess, and this has been traced to technical details on
how the DUSTY radiative transfer model is run.
If the present results are combined with those from modeling the
chromospheric emission lines, we obtain the fits Log Mdot = (1.0 +- 0.3) Log L
+ (-12.0 +- 0.9) and Log Mdot = (0.6 +- 0.2) Log (LR/M) + (-11.9 +- 0.9).
The predictions of these mass-loss rate formula are tested against the RGB
mass loss determination in NGC 6791. Using a scaling factor of (8 +- 5), both
relations can fit this value. That the scaling factor is larger than unity
suggests that the expansion velocity and/or dust-to-gas ratio, or even the dust
opacities, are different from the values adopted.Comment: It was pointed out that the mass used for NGC 6719 is incorrect (its
1.2 and not 1.6 Msol). The numbers in table 6 are correct, but the inference
drawn from it not. The result is that the scaling factors eta_1 and eta_2
become slightly smaller. The conclusions of the paper remain unchanged. This
version has the updated Table 6 and eta's. These changes will appear as an
erratum to the A&A pape
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
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
Ab initio Study of Luminescence in Ce-doped LuSiO: The Role of Oxygen Vacancies on Emission Color and Thermal Quenching Behavior
We study from first principles the luminescence of LuSiO:Ce
(LSO:Ce), a scintillator widely used in medical imaging applications, and
establish the crucial role of oxygen vacancies (V) in the generated
spectrum. The excitation energy, emission energy and Stokes shift of its
luminescent centers are simulated through a constrained density-functional
theory method coupled with a SCF analysis of total energies, and
compared with experimental spectra. We show that the high-energy emission band
comes from a single Ce-based luminescent center, while the large experimental
spread of the low-energy emission band originates from a whole set of different
Ce-V complexes together with the other Ce-based luminescent center.
Further, the luminescence thermal quenching behavior is analyzed. The
crossover mechanism is found to be very unlikely, with a large crossing energy
barrier (E) in the one-dimensional model. The alternative mechanism
usually considered, namely the electron auto-ionization, is also shown to be
unlikely. In this respect, we introduce a new methodology in which the
time-consuming accurate computation of the band gap for such models is
bypassed. We emphasize the usually overlooked role of the differing geometry
relaxation in the excited neutral electronic state Ce and in the
ionized electronic state Ce. The results indicate that such electron
auto-ionization cannot explain the thermal stability difference between the
high- and low-energy emission bands. Finally, a hole auto-ionization process is
proposed as a plausible alternative. With the already well-established excited
state characterization methodology, the approach to color center identification
and thermal quenching analysis proposed here can be applied to other
luminescent materials in the presence of intrinsic defects.Comment: 13 pages, 8 figures, accepted by Phys. Rev. Material
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