New hot subdwarf variables from the EC survey

Abstract

We present new results for hot subdwarf stars from the Edinburgh-Cape (EC) blue object survey. EC 03089–6421, an sdO star recently discovered to be a very rapid pulsator with periods near 31.1 and 34.2 s (29.2 and 32.1 mHz), is shown to have an even faster pulsation near 26.6 s (37.6 mHz) although all pulsations are variable in amplitude and effectively disappear in one of our runs. Five stars are discovered to be of the rapidly-pulsating (sdBVr) type: EC 01441–6605, EC 10834–1301, EC 11275–2504, EC 11545–1459, and EC 21281– 5010 each exhibit between one and three variations in the range 2–3 min (∼8.2–6.4 mHz) and are therefore p-mode pulsators. EC 15061–1442 shows a large-amplitude variation with a period ∼0.075 d if the star is a reflection effect binary and 0.15 d if an ellipsoidal variable. No eclipses are detected. Low-resolution EC survey classification spectroscopy is combined with published photometry from GALEX, WISE, and 2MASS and parallaxes from GAIA to derive, where possible, fundamental stellar parameters. The sdBVr stars have Teff in the range 34 000–45 000 K and log g between 5.0 and 5.6 consistent with p-mode pulsators. Derived stellar masses and radii are in the ranges 0.15–0.4 M and 0.15–0.2 R. The sdOV star, EC 03089–6421 is found to have Teff = 52 000 ± 3000 K and log g = 5.6 ± 0.2, and with a mass ∼0.1 M could be a progenitor to an extremely low-mass white dwarf.