Testing ultralow amplitude Cepheid candidates in the Galactic disk by TESS and Gaia^⋆

¹ CSFK, MTA Centre of Excellence, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
e-mail: tarczaynehez.dora@csfk.org
² Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd Research Network (ELKH), Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
³ MTA CSFK Lendület Near-Field Cosmology Research Group, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
⁴ Eötvös Loránd University, Institute of Physics, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary

Received: 7 February 2023
Accepted: 12 June 2023

Abstract

Ultralow amplitude (ULA) and strange mode Cepheids are thought to be pulsating variable stars that are near to or are at the edges of the classical instability strip. Until now, a few dozen such variable star candidates have been found both in the Large Magellanic Cloud and the Milky Way. For the present work, we studied six ULA Cepheid candidates in the Milky Way, identified by using CoRoT and 2MASS data. In order to identify their positions in the period–luminosity and color–magnitude diagrams, we used the Gaia DR3 parallax and brightness data of each star to calculate their reddening-free absolute magnitudes and distances. Furthermore, we calculated the Fourier parameters (e.g., period and amplitude) of the light variations based on CoRoT and TESS measurements, and established the long-term phase shifts for four out of six stars. Based on the results, we conclude that none of the six ULA Cepheid candidates are pulsating variable stars, but rather rotation-induced variable stars (rotational spotted and α² Canum Venaticorum variables) that are either bluer or fainter than Cepheids would be.