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

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 $\beta$ Cephei stars in low metallicity environments, as well as the difficulty in theoretically explaining the excitation of the pulsation modes observed in some $\beta$ Cephei and hybrid SPB-$\beta$ 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 $\rm M_\odot$ computed with OPAL and OP opacity tables and two different metal mixtures. We find that in a typical $\beta$ 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-$\beta$ Cephei pulsators. Moreover, higher overtone p-modes are excited in $\beta$ Cephei models and unstable modes are found in a larger number of models for lower metallicities, in particular $\beta$ 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

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

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&