Excitation of solar-like oscillations across the HR diagram

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 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 P max , the 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 χ k , the eddy time-correlation; using a Lorentzian, χ L k , results in s = 2.6, whereas a Gaussian, χ G k , gives s = 3.1. Finally, values of V max , the maximum in the mode velocity, are estimated from the computed power laws for P 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 χ 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.

Stochastic Excitation of Acoustic Modes in Stars

“…71) for a set of stars located near the main sequence from K to F and a subgiant K IV star. The comparison between these calculations and those based on SG's formalism has been undertaken by Samadi et al (2007). The maximum in P 3D was found systematically lower than those from calculations based on SG's formalism (Eqs.…”

“…(40)- (44)). These systematic differences were attributed by Samadi et al (2007) to the low spatial resolution of the hydrodynamical 3D simulations computed by Stein et al (2004).…”

“…They also found that the entropy fluctuation is small compared to the Reynolds stress contribution. However, as shown by Samadi et al (2007), the relative contribution of the entropy to the total excitation rate increases rapidly with the effective temperature, T eff . For instance, the solar-like pulsator HD 49333 has a significantly higher T eff than the Sun.…”

“…Moreover, Jacoutot et al (2008b) found that computed mode excitation rates depend significantly on the adopted sub-grid model. Furthermore, Samadi et al (2007) have shown that, at a given length scale, χ k tends toward a Gaussian when the spatial resolution is decreased. This is illustrated in Fig.…”

“…For instance, the solar-like pulsator HD 49333 has a significantly higher T eff than the Sun. Samadi et al (2009c) found that for this star the entropy fluctuations contributes up to ∼ 30 % while it is only about 15 % in the case of the Sun (see Samadi et al 2007) and in the case of α Cen A (see Samadi et al 2008).…”