Double-mode radial-non-radial RR Lyrae stars in the OGLE photometry of the Galactic bulge

TL;DR

This study identifies a new group of 147 Galactic bulge RRc stars exhibiting double-mode pulsations involving radial and non-radial modes, revealing a tight sequence in the Petersen diagram and suggesting widespread occurrence of such pulsations.

Contribution

First detection of a large, distinct group of double-mode radial-non-radial RRc pulsators in OGLE data, expanding understanding of pulsation modes in RR Lyrae stars.

Findings

147 newly identified double-mode RRc stars

Clear sequence in Petersen diagram with period ratios around 0.613 and 0.631

Indication that non-radial pulsations are common among RRc stars

Abstract

Non-radial modes are excited in classical pulsators, both in Cepheids and in RR Lyrae stars. Firm evidence come from the first overtone pulsators, in which additional shorter period mode is detected with characteristic period ratio falling in between 0.60 and 0.65. In the case of first overtone Cepheids three separate sequences populated by nearly 200 stars are formed in the Petersen diagram, i.e. the diagram of period ratio versus longer period. In the case of first overtone RR Lyrae stars (RRc stars) situation is less clear. A dozen or so such stars are known which form a clump in the Petersen diagram without any obvious structure. Interestingly, all first overtone RR Lyrae stars for which precise space-borne photometry is available show the additional mode, which suggests that its excitation is common. Motivated by these results we searched for non-radial modes in the OGLE-III photometry of RRc stars from the Galactic bulge. We report the discovery of 147 stars, members of a new group of double-mode, radial-non-radial mode pulsators. They form a clear and tight sequence in the Petersen diagram, with period ratios clustering around 0.613 with a signature of possible second sequence with higher period ratio (0.631). The scatter in period ratios of the already known stars is explained as due to population effects. Judging from the results of space observations this still mysterious form of pulsation must be common among RRc stars and with our analysis of the OGLE data we just touch the tip of the iceberg.