Observations of Cepheids with the MOST satellite: Contrast between Pulsation Modes

TL;DR

This study uses high-precision satellite photometry to compare pulsation stability in Cepheids, revealing that overtone pulsators exhibit more cycle-to-cycle variability and instability than fundamental mode pulsators.

Contribution

It provides new observational evidence of pulsation instability in overtone Cepheids using continuous, high-quality space-based data.

Findings

Overtone pulsator SZ Tau shows cycle-to-cycle variation.

Fundamental pulsator RT Aur exhibits highly stable light curves.

Overtone pulsation instability is linked to variations in Fourier parameters.

Abstract

The quantity and quality of satellite photometric data strings is revealing details in Cepheid variation at very low levels. Specifically, we observed a Cepheid pulsating in the fundamental mode and one pulsating in the first overtone with the Canadian MOST satellite. The 3.7-d period fundamental mode pulsator (RT Aur) has a light curve that repeats precisely, and can be modeled by a Fourier series very accurately. The overtone pulsator (SZ Tau, 3.1 d period) on the other hand shows light curve variation from cycle to cycle which we characterize by the variations in the Fourier parameters. We present arguments that we are seeing instability in the pulsation cycle of the overtone pulsator, and that this is also a characteristic of the O-C curves of overtone pulsators. On the other hand, deviations from cycle to cycle as a function of pulsation phase follow a similar pattern in both stars, increasing after minimum radius. In summary, pulsation in the overtone pulsator is less stable than that of the fundamental mode pulsator at both long and short timescales.