New results on long secondary periods in red giants

Abstract

Approximately 30% of variable red giants have light curves which show a Long Secondary Period (LSP) in addition to their primary oscillation. No model has been proposed that can satisfactorily explain the LSPs. Here we present velocity curves obtained from VLT spectra for a large sample of LMC red giants exhibiting LSPs. We use the velocity data in addition to both MACHO and OGLE light curves to examine the properties of the stars, and to evaluate models for the cause of LSPs. Extant models generally involve either binarity or pulsation. We show that both of these have severe problems explaining the light and velocity variations. However, new mid-infrared observations of stars with LSPs indicate the presence of a significant amount of circumstellar dust, most likely in a disk-like configuration. This observation favours the presence of a binary companion.