A search for SX Phe stars amongKeplerδ Scuti stars

Abstract

The SX Phe stars are Population II pulsating variables lying below the horizontal branch. Their pulsation characteristics resemble those of δ Sct stars. Since old stars in the Kepler field situated in the thick disc or halo are likely to have kinematics different from δ Sct stars, we use proper motions and tangential velocities to search for possible SX Phe variables among 1424 Kepler δ Scuti stars. Tangential velocities derived from distances obtained from the photometric calibration in the Kepler input catalogue appear to be an effective method of separation between thin-disc and thick-disc/halo stellar populations. Using this method, 34 candidate SX Phe stars are identified, eight of which are located more than 0.5 kpc above the galactic plane. We find that the light curves of these stars cannot be distinguished from those of normal δ Scuti stars. It is generally thought that field SX Phe stars have high amplitudes and only one or two frequencies, but such light curves are rare among the candidate SX Phe stars in the Kepler field. We suggest that the relatively high amplitudes, which until now have been a defining characteristic of field SX Phe stars, are merely a selection effect. This supports the recent finding of several low-amplitude multiperiodic SX Phe stars in some globular clusters.