Kepler and the Long Period Variables

TL;DR

This study uses Kepler photometry to analyze AGB star light curves, revealing multiple pulsation modes, long secondary periods, and the stability of short-term variations, with implications for understanding stellar pulsation mechanisms.

Contribution

It provides detailed analysis of AGB star variability using Kepler data, highlighting multiple modes and long secondary periods, and discusses the nature of these variations in relation to stellar pulsation.

Findings

AGB stars exhibit multiple pulsation modes, often including the first overtone and fundamental.

Long secondary periods are common and likely intrinsic, linked to multiple mode pulsation.

Short-term variations are stable and well modeled by sinusoids, with no rapid flares detected.

Abstract

High precision Kepler photometry is used to explore the details of AGB light curves. Since AGB variability has a typical time scale on order of a year we discuss at length the removal of long term trends and quarterly changes in Kepler data. Photometry for a small sample of nine SR AGB stars are examined using a 30 minute cadence over a period of 45 months. While undergoing long period variations of many magnitudes, the light curves are shown to be smooth at the millimagnitude level over much shorter time intervals. No flares or other rapid events were detected on the sub-day time scale. The shortest AGB period detected is on the order of 100 days. All the SR variables in our sample are shown to have multiple modes. This is always the first overtone typically combined with the fundamental. A second common characteristic of SR variables is shown to be the simultaneous excitation of multiple closely separated periods for the same overtone mode. Approximately half the sample had a much longer variation in the light curve, likely a long secondary period. The light curves were all well represented by a combination of sinusoids. However, the properties of the sinusoids are time variable with irregular variations present at low level. No non-radial pulsations were detected. It is argued that the long secondary period variation seen in many SR variables is intrinsic to the star and linked to multiple mode pulsation.