Double-mode radial-non-radial RR Lyrae stars. OGLE-IV photometry of two high cadence fields in the Galactic bulge

TL;DR

This study analyzes high-cadence OGLE-IV photometry of RR Lyrae stars in the Galactic bulge, revealing common non-radial pulsation modes, complex frequency patterns, and their coexistence with Blazhko modulation, expanding the understanding of stellar pulsations.

Contribution

It presents the largest sample of RRc and RRd stars with non-radial modes, demonstrating their prevalence and complex behavior in the Galactic bulge.

Findings

27% of RRc stars show additional non-radial modes

Multiple secondary modes often appear simultaneously and are non-stationary

Subharmonics are detected in 20% of stars

Abstract

We analyse the OGLE-IV photometry of the first overtone and double-mode RR Lyrae stars (RRc/RRd) in the two fields towards the Galactic bulge observed with high cadence. In 27 per cent of RRc stars we find additional non-radial mode, with characteristic period ratio, P x /P 1O \in (0.6, 0.64). It strongly corroborates the conclusion arising from the analysis of space photometry of RRc stars, that this form of pulsation must be common. In the Petersen diagram the stars form three sequences. In 20 stars we find two or three close secondary modes simultaneously. The additional modes are clearly non-stationary. Their amplitude and/or phase vary in time. As a result, the patterns observed in the frequency spectra of these stars may be very complex. In some stars the additional modes split into doublets, triplets or appear as a more complex bands of increased power. Subharmonics of additional modes are detected in 20 per cent of stars. They also display a complex structure. Including our previous study of the OGLE-III Galactic bulge data, we have discovered 260 RRc and 2 RRd stars with the additional non-radial mode, which is the largest sample of these stars so far. The additional mode is also detected in two Blazhko RRc stars, which shows that the modulation and additional non-radial mode are not exclusive.