Spectroscopic follow-up of hot subdwarf variables found in ZTF

Context. Hot subdwarf variables (sdBVs) that display large-amplitude (> 1%) short-period variability as a result of radial-mode pulsations have recently become objects of interest, as they show unique properties among the sdBV classes. Since the discovery of objects such as Balloon 090100001 and CS 1246, 12 more have been discovered in the Zwicky Transient Facility (ZTF) survey that display similar characteristics. However, due to a lack of broad spectroscopic investigations, it remains unclear whether these objects constitute a distinct class of radial-mode dominant sdBVs that share common atmospheric and fundamental properties. Aims. Here we aim to spectroscopically define these peculiar sdBVs as a population and determine if they constitute a unique class of pulsating hot subdwarfs or share similar properties with preexisting classes. Methods. We collected low-resolution spectroscopy on a sample of sdBVs discovered in the ZTF survey, including time-series observations. We fit the spectra to a grid of theoretical models to determine their mean effective temperature, surface gravity, helium abundance, and any corresponding variability. We then used these properties to estimate the mass, radius, and luminosity using a spectral energy distribution fitting method. Results. We show that the resulting properties are similar to the radial-mode dominant sdBVs, Balloon 090100001 and CS 1246, and that they are distinguishable from other similar radial-mode pulsators, such as blue large-amplitude pulsators. We find that these stars, on average, have mean effective temperatures of 28 300 K and surface gravity measurements of log g = 5.56, with changes in these parameters on the order of 1000 K and 0.10 dex, respectively. The location of these stars on the T eff – log g plane places them on the boundary region between the low-amplitude, multi-periodic V361 Hya and V1093 Her stars, where the hybrid DW Lyn pulsators lie. The masses and radii of the majority of the sdBVs in our sample align with canonical-mass sdB properties.