Spectroscopic follow-up of hot subdwarf variables found in ZTF -- Atmospheric and fundamental properties of radial-mode sdB pulsators

Abstract

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, twelve more have been discovered in the Zwicky Transient Facility (ZTF) survey that display similar characteristics. However, due to 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. Here we aim to spectroscopically define these peculiar sdBVs as a population. We collected low-resolution spectroscopy on a sample of sdBVs discovered in the ZTF survey, including time-series observations. We fitted the spectra to a grid of theoretical models to determine their mean effective temperature, surface gravity and helium abundance and any corresponding variability. We then use these properties to estimate the mass, radius and luminosity using a spectral energy distribution fitting method. 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 $T_{\textrm{eff}}$ -- $\log\,g$ 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.

Figures (17)

• Figure 1: Color magnitude diagram constructed for objects in the ZTF-sdBV sample which have reasonable parallax measurements, using Gaia photometric $G$, $G_{BP}$ and $G_{RP}$ color-corrected magnitudes. Each object is color-mapped with its corresponding pulsation period, which is scaled to the observed range of pulsation periods for the entire sample.
• Figure 2: Left panel: SED fitting for a single sdB (ZTF-sdBV1) where the gray line represents the best fitting sdB model spectrum. Right panel: composite SED for a sdB + cool companion (ZTF-sdBV6) where the blue line denotes the sdB model and the red line shows the cool companion model from the PHOENIX grid, while the gray line is the composite spectrum. Colored points on both panels represent photometric flux measurements from a variety of sources, which are listed in Appendix \ref{['sec:apped:sed']}.
• Figure 3: Photometric results for a typical rm-sdBV (ZTF-sdBV1). Top panel: resulting DFT frequency spectrum, after removing aliasing effects, plotted along with the 5$\sigma$ detection level (red). Bottom panel: ZTF light curve (black), phase-folded on the detected frequency, overlaid with binned values (red), and plotted over two pulsation cycles for visualization.
• Figure 4: Top panel: period distribution for the ZTF-sdBVs (green). For comparison, a population of BLAPs (blue) from the OGLE, ZTF and Omega White surveys are also shown. Bottom panel: amplitude distribution for the same objects, converted from $r$ and $I$ band magnitudes to parts-per-thousand.
• Figure 5: Phase-resolved, spectroscopic results for a typical rm-sdBV (ZTF-sdBV4). Top panel: relative RV variations. Middle panel: $T_{\textrm{eff}}$ measurements. Bottom panel: $\log\,g$ measurements. The measured values (black) are shown along with their single harmonic fitting (red), around the mean value (blue). All results are plotted over two pulsation cycles for visualization.