Photometric Oscillations of Low Luminosity Red Giant Stars

TL;DR

This study analyzes photometric oscillations in hundreds of low luminosity red giant stars using Hubble data, revealing how variability depends on stellar position in the color-magnitude diagram and matches pulsation models.

Contribution

It provides detailed observational evidence linking red giant variability to stellar properties and supports pulsation theories with high-precision HST data.

Findings

Variability amplitude increases with stellar radius.

Oscillation time scales correlate with position in the CMD.

Most red giants exhibit detectable oscillations.

Abstract

I present details of the variations of several hundred red giant stars on time scales of a few hours to a few days from Hubble Space Telescope (HST) observations of a low-extinction galactic bulge sample from an intensive seven day campaign. Variations in the red giants are shown to be a strong function of position within the color-magnitude diagram (CMD) in accord with general expectations from theory. Amplitudes are greater for stars with larger radii, whether this results from higher luminosity at the same effective temperature or lower temperature at a fixed apparent magnitude. Likewise, characteristic time scales for the variations increase to the upper right in a CMD as does the ratio of amplitudes measured at 606 nm compared to 814 nm. Characteristic variation time scales are well matched by low-order radial pulsation modes. The effective sample discussed here extends from about two magnitudes above the bulge turnoff at which red giant radii are ~7 R/R_{\odot} at 5,000 K with typical amplitudes of ~0.5 mmag to ~40 R/R_{\odot} at 4,000 K with amplitudes of ~3.5 mmag. Variability characteristics are quite similar at any given position in the CMD, and at levels in the CMD where oscillations are easily detected nearly all red giants show such. If these variations represent oscillations with sufficient lifetimes to derive accurate mode frequencies more extensive observations, e.g. as should soon be provided by the Kepler Mission}, would provide a rich asteroseismic return.