Exploring low-amplitude variability in first-overtone Cepheids with TESS

Classical Cepheid stars that pulsate in the first overtone radial mode often exhibit additional periodicities at the millimagnitude level. Extensive studies of the OGLE data of the Magellanic Clouds have revealed distinct groups based on the ratio of the star's additional period to the period of the first overtone mode. These groups are similar to those found in overtone RR Lyrae stars. Theoretical calculations suggest that some of the observed periodicities are consistent with non-radial modes, while others remain unexplained. Currently, we only know of a handful of examples from the Galactic Cepheid sample that exhibit low-amplitude periodicities. The purpose of this study is to undertake a systematic search for low-amplitude variability in overtone Cepheids of the Milky Way in the photometric data of the full-frame images of the Transiting Exoplanet Survey Satellite (TESS), which were produced with the MIT Quick Look Pipeline. We applied standard Fourier analysis and classified the additional signals according to the ratio of their period to the overtone pulsation period. We find 127 stars in total that exhibit additional periodicities. In 17 of these stars, the periodicities can be identified as a second radial overtone. A further 83 of the stars were observed to display periodic signals with a P_ x /P_ O ratio in the range. In 15 stars, the P_ O /P_ x is found to be ∼0. 68; six of these stars are also found to be in the previous group. Furthermore, we observed the presence of low-amplitude signals in 22 stars outside the aforementioned period ratio ranges. It is possible that some are direct detections of non-radial modes, with no harmonic frequency peak in the period range. Most of the properties of the additional signals detected in this study are consistent with previous findings regarding Cepheids in the Magellanic Clouds. However, TESS measurements reveal that the amplitudes and frequencies of these signals often vary within a TESS sector, a phenomenon that challenges theoretical models. We also find that reliable analysis of these signals with TESS is possible for a moderate fraction of overtone Cepheids. The primary constraints are the limited extent of the data and the relatively poor photometric quality in some Galactic fields. Further careful corrections to the light curves may improve detectability.