Tony Lynas-Gray

Abstract:

© 2019 The Author(s). 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 (sdBV r ) 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 sdBV r stars have T eff 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 T eff = 52 000 ± 3000 K and log g = 5.6 ± 0.2, and with a mass ∼0.1 M o could be a progenitor to an extremely low-mass white dwarf.

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.

Abstract:

Among the competing evolution theories for subdwarf-B (sdB) stars is the binary evolution scenario. EC 20117-4014 (=V4640 Sgr) is a spectroscopic binary system consisting of a pulsating sdB star and a late F main-sequence companion (O’Donoghue et al. 1997), however the period and the orbit semi-major axes have not been precisely determined. This paper presents orbital characteristics of the EC 20117-4014 binary system using 20 years of photometric data. Periodic Observed minus Calculated (O-C) variations were detected in the two highest amplitude pulsations identified in the EC 20117-4014 power spectrum, indicating the binary system’s precise orbital period (P = 792.3 days) and the light-travel time amplitude (A = 468.9 s). This binary shows no significant orbital eccentricity and the upper limit of the eccentricity is 0.025 (using 3σ as an upper limit). This upper limit of the eccentricity is the lowest among all wide sdB binaries with known orbital parameters. This analysis indicated that the sdB is likely to have lost its hydrogen envelope through stable Roche lobe overflow, thus supporting hypotheses for the origin of sdB stars. In addition to those results, the underlying pulsation period change obtained from the photometric data was P˙ = 5.4 (±0.7) × 10−14 d d−1 , which shows that the sdB is just before the end of the core helium-burning phase.