RRab variables with identical light-curve shapes at different pulsation periods

TL;DR

This study identifies RRab stars in the Galactic bulge with nearly identical light-curve shapes but different pulsation periods, revealing complex relationships between their physical properties and challenging existing pulsation models.

Contribution

It reports the discovery of hundreds of RRab star pairs with similar light curves but different periods and analyzes their physical differences, providing new insights into RR Lyrae variability.

Findings

149 pairs of similar light-curve RRab stars identified across multiple bands.

Short-period stars are less metal-poor, fainter, smaller, and hotter than long-period counterparts.

Physical property differences include approximately -0.5 dex in [Fe/H] and -0.13 mag in M_V.

Abstract

In this paper, we report on the detection of RRab stars with quasi-identical-shape light curves but period differences as large as $0.05-0.21$ d using the Galactic bulge data of the OGLE-IV survey. We have examined stars with shorter periods than the Oosterhoff~I ridge of the bulge. These stars generally have smaller amplitudes and larger Fourier phase-differences than the typical bulge RRab stars have at the same period. Many of these "anomalous" stars have good-quality light curves without any sign of the Blazhko modulation. Examining their Fourier parameters revealed that several of these stars show very similar light-curve to the typical bulge RR Lyrae. We found hundreds of quasi-identical-shape light-curve pairs with different periods between the "anomalous"- and the "normal"-position RRab stars based on the OGLE $I$-band data. The OGLE $V$-band, and the archive VVV and MACHO surveys $K_s$-, $b$- and $r$-band data of these stars were also checked for light-curve-shape similarity. Finally, 149 pairs with identical-shape light curves in each available photometric band were identified. Calculating the physical properties of the variables using empirical formulae, on average, $-0.5$~dex, $-0.13$~mag, 0.67, and 165~K differences between the [Fe/H], $M_V$, $R/R_\odot$, and $T_{\mathrm{eff}}$ values of the members of the pairs are derived, being the short-period stars less metal-poor, fainter, smaller and hotter than the long-period variables. To explain the existence of variables with different physical properties and pulsation periods but with identical-shape light curves is a challenging task for modelling.