A generalized quadratic balance relation for nonradial nonadiabatic pulsations

Abstract

We derive a generalized quadratic balance relation for linear nonadiabatic, nonradial stellar pulsations, starting from a sesquilinear amplitude analogue of the pressure-volume-rate work. The derivation requires neither weak nonadiabaticity nor averaging over a pulsation cycle. The relation is expressed in terms of work, generalized norm and boundary contributions and is subsequently recast into kinetic-energy-power form $\mathcal{P}+ΔB=2\Re(σ)E_{\rm kin}$. The volume power is decomposed into a thermodynamic exchange term and response terms associated with compression, horizontal-area deformation and gravitational stratification effects. The equivalent forms of the relation provide diagnostics for checking the computed eigenfrequencies and estimating mode excitation rates. The properties of the balance relation for various types of modes in an envelope of a model AGB star are examined. We analyze the terms entering the power $\mathcal{P}$ for radial and nonradial p-modes, strange modes as well as examples of low frequency outer-envelope gravity modes and thermal modes. The results show that the magnitude of mode driving is not determined solely by the thermodynamic work term.