SB 796: a high-velocity RRc star

Abstract

We report here on a detailed study of a c-type RR Lyrae variable (RRc variable), SB 796, serendipitously discovered in a search of the WASP public data for stars that display large photometric periodic modulation. SB 796 displays a period of P = 0.26585 d and semi-amplitude of ~ 0.1 mag. Comparison of the modulation shape and period with the detailed analysis of LMC variables indicates that SB 796 is an RRc variable. Gaia DR2 classification corroborated our result. Radial-velocity (RV) follow-up observations revealed a periodic variation consistent with a sine modulation, with a semi-amplitude of 5.6 ± 0.2 km/s, and a minimum at phase of maximum brightness. Similar amplitude and phase were previously seen in other RRc variables. The stellar averaged RV is ~250 km/s, turning SB 796 to be a high-velocity star, while its present position, as derived from the Gaia astrometry, is at ~3.5 kpc below the Galactic plane. Integration of the stellar Galactic motion shows that SB 796 oscillates at a range of 0.5–20 kpc Galacto-centric distance, passing near the Galactic center about three times in 1 Gyr. The Galactic radial motion takes SB 796 up and down the plane to a scale height of ~10 kpc. During its ~10 Gyrs estimated life time, SB 796 therefore passed ~30 times near the Galactic center.