SB 796: a high-velocity RRc star

TL;DR

This paper presents a detailed study of SB 796, a high-velocity RRc star discovered in WASP data, revealing its pulsation, radial velocity variations, and complex Galactic orbit over billions of years.

Contribution

It provides the first detailed characterization of SB 796, including its pulsation, velocity, and orbital dynamics, highlighting its status as a high-velocity, high-altitude RRc star.

Findings

SB 796 is an RRc variable with a 0.26585-day period.

Radial velocity semi-amplitude is 5.6 km/s, with a high stellar velocity of ~250 km/s.

SB 796's orbit oscillates between 0.5 and 20 kpc from the Galactic center.

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 $\sim$ 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 $5.6\pm0.2$ 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 $\sim 250$ km/s, turning SB 796 to be a high-velocity star, while its present position, as derived from the Gaia astrometry, is at $\sim 3.5$ 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 $\sim 10$ kpc. During its $\sim10$ Gyrs estimated life time, SB 796 therefore passed $\sim 30$ times near the Galactic center.