Exploring Low-Amplitude Variability in First Overtone Cepheids with TESS

TL;DR

The paper tackles the problem of characterizing low-amplitude periodicities in Milky Way overtone Cepheids, focusing on non-radial mode signatures. It leverages TESS full-frame-image photometry and a Fourier-based prewhitening pipeline to detect additional frequencies, categorizing them by period ratios to the first overtone: the 0.61 and 0.68 groups, plus other periodicities. The authors report 127 stars with extra signals, including 17 with the second overtone, 83 in the 0.61 group, 15 in the 0.68 group, and 22 other frequencies, with significant temporal variability challenging existing pulsation models. The findings substantially expand the Galactic census of these phenomena, reveal complex amplitude/frequency variability on monthly scales, and underscore the need for longer, higher-quality time-series data to constrain non-radial pulsation theories in Cepheids.

Abstract

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 their period ratio with 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 may be 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, which were produced with the MIT Quick Look Pipeline. We applied standard Fourier analysis and classified the additional signals according to their period ratio to the overtone pulsation period. We found 127 stars in total to exhibit additional periodicities. In 17 stars, these can be identified as a second radial overtone. A further 83 stars were observed to display periodic signals with a ratio of $P_{\mathrm{x}}/P_{1\mathrm{O}}$ in the range 0.60$-$0.65. In 15 stars, the $P_{1\mathrm{O}}/P_{\mathrm{x}}$ is found to be near $\sim$0.68, of which six are also found to be in the previous group. Furthermore, we observed the presence of low-amplitude signals in 22 stars outside the aforementioned period ratios. It is possible that some of these may be direct detections of non-radial modes, with no harmonic frequency peak in the 0.60$-$0.65 period range. The TESS measurements revealed that the amplitudes and frequencies of these signals often vary within a TESS sector, a phenomenon that challenges theoretical models.

Figures (14)

• Figure 1: Example TESS QLP light curves and residual spectra for Cepheids with additional signals. Note that the frequency ranges on the right panels are different. Red dashed lines mark the positions of the subtracted frequencies of the first overtone mode ($f_{\rm{1O}}$, 2$f_{\rm{1O}}$, 3$f_{\rm{1O}}$). The low-amplitude peaks identified are labelled. $f_{\rm{sh}}$ denotes the subharmonic frequency of the $f_{0.61}$ peak.
• Figure 2: Petersen diagram of radial multimode classical Cepheids in the Milky Way. Colored dots are drawn in from the catalog of pietr and from the the candidates of Rathour2021. The black crosses show the double mode stars found in this study.
• Figure 3: Light curves segments are presented for three 1O2O stars that exhibit visible change in the amplitude ratios. The three columns represent the primary and the two extended missions. The sectors from which the data is adopted are indicated in the brackets.
• Figure 4: Petersen diagram of the 0.68 stars found in this work (blue squares) plotted onto the results of smolec2023 found in the same range of period ratio (green diamonds for LMC and purple dots for SMC). The values derived for the same stars, but from different sectors are connected with black lines.
• Figure 5: Petersen diagram of the 0.61 group. The previously discovered Galactic members of the group pietr2013Rathour2021 are marked with orange asterisk and triangles. Black lines connect the values for the same star. Red symbols denote stars for which we found multiple peaks for the 0.61 frequency group in the same light curve. The stars which are also member of the 0.68 group are marked with circles.