Double-mode Cepheids in M31

TL;DR

This paper reports the discovery of four double mode Cepheids in M31, including the first candidates for fundamental and first overtone pulsators in this galaxy, identified through a systematic archival data search.

Contribution

It presents the first identification of double mode Cepheids in M31, expanding understanding of pulsation modes in extragalactic Cepheids.

Findings

Discovered four double mode Cepheids in M31.

Identified one as first and second overtone pulsator.

Two pulsate in fundamental mode and first overtone with specific periods.

Abstract

Until now, double mode Cepheids (or beat Cepheids) were known in the Galaxy, Magellanic Clouds and M33. Curiously, none of more than 2000 Cepheids in M31 was claimed to show two pulsation modes. We conducted a systematic search for double mode Cepheids in the archival data and discovered four such objects. We identify one of the stars as a first and second overtone pulsator even though its secondary period is subject to strong aliasing. Two stars turn out to pulsate in the fundamental mode and the first overtone. Their fundamental periods are 9.392 d and 9.163 d. This makes them the first candidates for the fundamental mode and the first overtone Cepheids, which double mode pulsations are caused by the 1:2 resonance of the fundamental mode and the second overtone.