Dipolar modes in luminous red giants

TL;DR

This study investigates dipolar oscillations in luminous red giants, revealing mode trapping mechanisms in the convective envelope and showing that these modes are largely unaffected by the deep interior structure.

Contribution

It provides a detailed analysis of mode trapping in luminous red giants and clarifies the damping mechanisms affecting dipolar oscillations.

Findings

Dipolar modes are effectively trapped in the outer convective envelope at high luminosities.

Gravity wave emission at the envelope base is a secondary damping mechanism.

Frequencies of dipolar modes are insensitive to the deep interior structure.

Abstract

Lots of information on solar-like oscillations in red giants has been obtained thanks to observations with CoRoT and Kepler space telescopes. Data on dipolar modes appear most interesting. We study properties of dipolar oscillations in luminous red giants to explain mechanism of mode trapping in the convective envelope and to assess what may be learned from the new data. Equations for adiabatic oscillations are solved by numerical integration down to the bottom of convective envelope, where the boundary condition is applied. The condition is based on asymptotic decomposition of the fourth order system into components describing a running wave and a uniform shift of radiative core. If the luminosity of a red giant is sufficiently high, for instance at M = 2 Msun greater than about 100 Lsun, the dipolar modes become effectively trapped in the acoustic cavity, which covers the outer part of convective envelope. Energy loss caused by gravity wave emission at the envelope base is a secondary or negligible source of damping. Frequencies are insensitive to structure of the deep interior.