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Register a new userSolar-like oscillations in massive main-sequence stars. I. Asteroseismic signatures of the driving and damping regions
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Motivated by the recent detection of stochastically excited modes in the massive star V1449 Aql (Belkacem et al., 2009b), already known to be a $beta$ Cephei, we theoretically investigate the driving by turbulent convection. By using a full non-adiabatic computation of the damping rates, together with a computation of the energy injection rates, we provide an estimate of the amplitudes of modes excited by both the convective region induced by the iron opacity bump and the convective core. Despite uncertainties in the dynamical properties of such convective regions, we demonstrate that both are able to efficiently excite $p$ modes above the CoRoT observational threshold and the solar amplitudes. In addition, we emphasise the potential asteroseismic diagnostics provided by each convective region, which we hope will help to identify the one responsible for solar-like oscillations, and to give constraints on this convective zone. A forthcoming work will be dedicated to an extended investigation of the likelihood of solar-like oscillations across the Hertzsprung-Russell diagram.
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Context. The advent of space-borne missions such as CoRoT or Kepler providing photometric data has brought new possibilities for asteroseismology across the H-R diagram. Solar-like oscillations are now observed in many stars, including red giants and main- sequence stars. Aims. Based on several hundred identified pulsating red giants, we aim to characterize their oscillation amplitudes and widths. These observables are compared with those of main-sequence stars in order to test trends and scaling laws for these parameters for both main-sequence stars and red giants. Methods. An automated fitting procedure is used to analyze several hundred Fourier spectra. For each star, a modeled spectrum is fitted to the observed oscillation spectrum, and mode parameters are derived. Results. Amplitudes and widths of red-giant solar-like oscillations are estimated for several hundred modes of oscillation. Amplitudes are relatively high (several hundred ppm) and widths relatively small (very few tenths of a {mu}Hz). Conclusions. Widths measured in main-sequence stars show a different variation with the effective temperature than red giants. A single scaling law is derived for mode amplitudes of both red giants and main-sequence stars versus their luminosity to mass ratio. However, our results suggest that two regimes may also be compatible with the observations.
The last decade has seen a rapid development in asteroseismology thanks to the CoRoT and Kepler missions. With more detailed asteroseismic observations available, it is becoming possible to infer exactly how oscillations are driven and dissipated in solar-type stars. We have carried out three-dimensional (3D) stellar atmosphere simulations together with one-dimensional (1D) stellar structural models of key benchmark turn-off and subgiant stars to study this problem from a theoretical perspective. Mode excitation and damping rates are extracted from 3D and 1D stellar models based on analytical expressions. Mode velocity amplitudes are determined by the balance between stochastic excitation and linear damping, which then allows the estimation of the frequency of maximum oscillation power, $ u_{max}$, for the first time based on ab initio and parameter-free modelling. We have made detailed comparisons between our numerical results and observational data and achieved very encouraging agreement for all of our target stars. This opens the exciting prospect of using such realistic 3D hydrodynamical stellar models to predict solar-like oscillations across the HR-diagram, thereby enabling accurate estimates of stellar properties such as mass, radius and age.
Chemical element transport processes are among the crucial physical processes needed for precise stellar modelling. Atomic diffusion by gravitational settling nowadays is usually taken into account, and is essential for helioseismic studies. On the other hand, radiative accelerations are rarely accounted for, act differently on the various chemical elements, and can strongly counteract gravity in some stellar mass domains. In this study we aim at determining whether radiative accelerations impact the structure of solar-like oscillating main-sequence stars observed by asteroseismic space missions. We implemented the calculation of radiative accelerations in the CESTAM code using the Single-Valued Parameter method. We built and compared several grids of stellar models including gravitational settling, but some with and others without radiative accelerations. We found that radiative accelerations may not be neglected for stellar masses larger than 1.1~M$_{odot}$ at solar metallicity. The difference in age due to their inclusion in models can reach 9% for the more massive stars of our grids. We estimated that the percentage of the PLATO core program stars whose modelling would require radiative accelerations ranges between 33 and 58% depending on the precision of the seismic data. We conclude that, in the context of Kepler, TESS, and PLATO missions, which provide (or will provide) high quality seismic data, radiative accelerations can have a significant effect when inferring the properties of solar-like oscillators properly. This is particularly important for age inferences. However, the net effect for each individual star results from the competition between atomic diffusion including radiative accelerations and other internal transport processes. This will be investigated in a forthcoming companion paper.
We present a dense grid of evolutionary tracks and isochrones of rotating massive main-sequence stars. We provide three grids with different initial compositions tailored to compare with early OB stars in the Small and Large Magellanic Clouds and in the Galaxy. Each grid covers masses ranging from 5 to 60 Msun and initial rotation rates between 0 and about 600 km/s. To calibrate our models we used the results of the VLT-FLAMES Survey of Massive Stars. We determine the amount of convective overshooting by using the observed drop in rotation rates for stars with surface gravities log g < 3.2 to determine the width of the main sequence. We calibrate the efficiency of rotationally induced mixing using the nitrogen abundance determinations for B stars in the Large Magellanic cloud. We describe and provide evolutionary tracks and the evolution of the central and surface abundances. In particular, we discuss the occurrence of quasi-chemically homogeneous evolution, i.e. the severe effects of efficient mixing of the stellar interior found for the most massive fast rotators. We provide a detailed set of isochrones for rotating stars. Rotation as an initial parameter leads to a degeneracy between the age and the mass of massive main sequence stars if determined from its observed location in the Hertzsprung-Russell diagram. We show that the consideration of surface abundances can resolve this degeneracy.
We present a brief overview of the history of attempts to obtain a clear detection of solar-like oscillations in cluster stars, and discuss the results on the first clear detection, which was made by the Kepler Asteroseismic Science Consortium (KASC) Working Group 2.
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