1205.4846 (R. Samadi et al.)

Amplitudes of solar-like oscillations in red-giant stars: Evidences for non-adiabatic effects using CoRoT observations    [PDF]

R. Samadi, K. Belkacem, M. -A. Dupret, H. -G. Ludwig, F. Baudin, E. Caffau, M. -J. Goupil, C. Barban

A growing number of solar-like oscillations has been detected in red-giant stars thanks to CoRoT and Kepler space-crafts. The seismic data gathered by CoRoT on red-giant stars allow us to test mode driving theory in different physical conditions than main-sequence stars. Using a set of 3D hydrodynamical models representative of the upper layers of sub- and red-giant stars, we compute the acoustic modes energy supply rate (Pmax). Assuming adiabatic pulsations and using global stellar models where the surface stratification comes from the 3D hydrodynamical models, mode amplitude is computed in terms of surface velocity. The latter is then converted into intensity fluctuations using either a simplified adiabatic scaling relation or a non-adiabatic one. Given L and M (the luminosity and mass respectively), the energy supply rate Pmax is found to scale as (L/M)^(2.6) for both main-sequence and red-giant stars, extending previous results by Samadi et al. (2007). The theoretical amplitudes in velocity under-estimate the Doppler velocity measurements obtained so far from the ground for red-giant stars by about 30%. In terms of intensity, the theoretical scaling law based on the adiabatic intensity-velocity scaling relation results in an under-estimation by a factor of about 2.5 with respect to the CoRoT seismic measurements. On the other hand, using the non-adiabatic intensity-velocity relation significantly reduces the discrepancy with the CoRoT data. The theoretical amplitudes remain however 40% below the CoRoT measurements. Our results show that scaling relations of mode amplitude cannot be simply extended from main-sequence to red-giant stars in terms of intensity on the basis of adiabatic relations since non-adiabatic effects for red-giant stars are important and cannot be neglected. Possible reasons explaining these remaining differences are discussed.

View original: http://arxiv.org/abs/1205.4846