Spectroscopic Observations of Four Candidates for Blue Large-Amplitude Pulsators. No BLAPs at High Galactic Latitudes

Abstract

We have obtained spectroscopic observations for four short-period variable objects detected in ZTF, Gaia, and Pan-STARRS (ZGP) data and classified as Blue Large-Amplitude Pulsators (BLAPs) in McWhirter and Lam (2022): ZGP-BLAP-03, ZGP-BLAP-04, ZGP-BLAP-10, and ZGP-BLAP-15. The variables have periods between 46 and 56 min, full amplitudes of 0.13-0.22 mag in the $r$ band, and light curve shapes typical for radially pulsating stars. Three of them were found at high galactic latitudes (|$b$|>30 deg). We have identified object ZGP-BLAP-03 as an early F-type star, while objects ZGP-BLAP-04 and ZGP-BLAP-15 as low-metallicity late A-type stars. These are the three objects found at high galactic latitudes and located several kiloparsecs from the Sun. Thus, they are SX Phoenicis-type variable stars residing in the Galactic halo. In the case of low-latitude object ZGP-BLAP-10, we report the presence of helium lines in its spectrum and atmospheric parameters in agreement with known BLAPs. This and other results indicate that BLAPs are absent in metal-poor environments.