Excitation of solar-like oscillations across the HR diagram

Free access article

Issue		A&A Volume 463, Number 1, February III 2007


Page(s)		297 - 308
Section		Stellar structure and evolution
DOI		http://dx.doi.org/10.1051/0004-6361:20041953

A&A 463, 297-308 (2007)
DOI: 10.1051/0004-6361:20041953

Excitation of solar-like oscillations across the HR diagram

R. Samadi^{1, 2}, D. Georgobiani³, R. Trampedach⁴, M. J. Goupil², R. F. Stein⁵, and Å. Nordlund⁶

¹ Observatório Astronómico UC, Coimbra, Portugal
e-mail: Reza.Samadi@obspm.fr
² Observatoire de Paris, LESIA, CNRS UMR 8109, 92195 Meudon, France
³ Center for Turbulence Research, Stanford University NASA Ames Research Center, Moffett Field, USA
⁴ Research School of Astronomy and Astrophysics, Mt. Stromlo Observatory, Cotter Road, Weston ACT 2611, Australia
⁵ Department of Physics and Astronomy, Michigan State University, Lansing, USA
⁶ Niels Bohr Institute for Astronomy Physics and Geophysics, Copenhagen, Denmark

(Received 6 September 2006 / Accepted 14 November 2006 )

Abstract
Aims.We extend semi-analytical computations of excitation rates for solar oscillation modes to those of other solar-like oscillating stars to compare them with recent observations
Methods.Numerical 3D simulations of surface convective zones of several solar-type oscillating stars are used to characterize the turbulent spectra as well as to constrain the convective velocities and turbulent entropy fluctuations in the uppermost part of the convective zone of such stars. These constraints, coupled with a theoretical model for stochastic excitation, provide the rate ${\cal P}$ at which energy is injected into the p-modes by turbulent convection. These energy rates are compared with those derived directly from the 3D simulations.
Results.The excitation rates obtained from the 3D simulations are systematically lower than those computed from the semi-analytical excitation model.

We find that ${\cal P}_{\max}$ , the ${\cal P}$ maximum, scales as (L/M)^s where s is the slope of the power law and L and M are the mass and luminosity of the 1D stellar model built consistently with the associated 3D simulation. The slope is found to depend significantly on the adopted form of $\chi_k$ , the eddy time-correlation; using a Lorentzian, $\chi_k^{\rm L}$ , results in s=2.6, whereas a Gaussian, $\chi_k^{\rm G}$ , gives s=3.1. Finally, values of $V_{\rm max}$ , the maximum in the mode velocity, are estimated from the computed power laws for ${\cal P}_{\rm max}$ and we find that $V_{\max}$ increases as (L/M)^sv. Comparisons with the currently available ground-based observations show that the computations assuming a Lorentzian $\chi_k$ yield a slope, sv, closer to the observed one than the slope obtained when assuming a Gaussian. We show that the spatial resolution of the 3D simulations must be high enough to obtain accurate computed energy rates.

Key words: convection -- turbulence -- Sun: oscillations -- Hertzsprung-Russell (HR) and C-M -- stars: variables: general -- methods: numerical

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