A new class of pulsating hot subdwarfs

Abstract

Using high-cadence observations from the Zwicky Transient Facility at low Galactic latitudes, we have discovered a new class of pulsating, hot, compact stars. We have found four candidates, exhibiting blue colors ($g-r\le -0.1$ mag), pulsation amplitudes of $>5\%$, and pulsation periods of $200-475$ sec. Fourier transforms of the lightcurves show only one dominant frequency. Phase-resolved spectroscopy for three objects reveals significant radial velocity, T${}_{\mathrm{eff}}$ and log(g) variations over the pulsation cycle, consistent with large amplitude radial oscillations. The mean T${}_{\mathrm{eff}}$ and log(g) for these stars are consistent with hot subdwarf B (sdB) effective temperatures and surface gravities. We calculate evolutionary tracks using MESA and adiabatic pulsations using GYRE for low-mass helium-core pre-white dwarfs and low mass helium-burning stars. Comparison of low-order radial oscillation mode periods with the observed pulsation periods show better agreement with the pre-white dwarf models. Therefore, we suggest that these new pulsators and Blue Large-Amplitude Pulsators (BLAPs) could be members of the same class of pulsators, composed of young $\approx 0.25-0.35$ M${}_{\odot }$ helium-core pre-white dwarfs.

accepted for publication in ApJL, 9 pages, 3 figures and 2 tables

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