3,798 research outputs found

    Asteroseismology of Solar-Type and Red-Giant Stars

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    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

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    (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

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    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

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    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

    Ab initio Study of Luminescence in Ce-doped Lu2_2SiO5_5: The Role of Oxygen Vacancies on Emission Color and Thermal Quenching Behavior

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    We study from first principles the luminescence of Lu2_2SiO5_5:Ce3+^{3+} (LSO:Ce), a scintillator widely used in medical imaging applications, and establish the crucial role of oxygen vacancies (VO_O) 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 Δ{\Delta}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-VO_O complexes together with the other Ce-based luminescent center. Further, the luminescence thermal quenching behavior is analyzed. The 4f−5d4f-5d crossover mechanism is found to be very unlikely, with a large crossing energy barrier (Efd_{fd}) 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 Ce3+,∗^{3+,*} and in the ionized electronic state Ce4+^{4+}. 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

    Discriminating between overshooting and rotational mixing in massive stars: any help from asteroseismology?

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    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
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