Hot Subluminous Stars

Abstract

Hot subluminous stars of spectral type B and O are core helium-burning stars at the blue end of the horizontal branch or have evolved even beyond that stage. Most hot subdwarf stars are chemically highly peculiar and provide a laboratory to study diffusion processes that cause these anomalies. The most obvious anomaly lies with helium, which may be a trace element in the atmosphere of some stars (sdB, sdO) while it may be the dominant species in others (He-sdB, He-sdO). Strikingly, the distribution in the Hertzsprung-Russell diagram of He-rich versus He-poor hot subdwarf stars of the globular clusters ω Cen and NGC2808 differ from that of their field counterparts. The metal-abundance patterns of hot subdwarfs are typically characterized by strong deficiencies of some lighter elements as well as large enrichments of heavy elements. A large fraction of sdB stars are found in close binaries with white dwarf or very low-mass main sequence companions, which must have gone through a common-envelope (CE) phase of evolution. Because the binaries are detached they provide a clean-cut laboratory to study this important but yet poorly understood phase of stellar evolution. Hot subdwarf binaries with sufficiently massive white dwarf companions are viable candidate progenitors of type Ia supernovae both in the double degenerate as well as in the single degenerate scenario as helium donors for double detonation supernovae. The hyper-velocity He-sdO star US708 may be the surviving donor of such a double detonation supernova. Substellar companions to sdB stars have also been found. For HWVir systems the companion mass distribution extends from the stellar into the brown dwarf regime. A giant planet to the acoustic-mode pulsator V391 Peg was the first discovery of a planet that survived the red giant evolution of its host star. Evidence for Earth-size planets to two pulsating sdB stars have been reported and circumbinary giant planets or brown dwarfs have been found around HWVir systems from eclipse timings. The high incidence of circumbinary substellar objects suggests that most of the planets are formed from the remaining CE material (second generation planets). Several types of pulsating star have been discovered among hot subdwarf stars, the most common are the gravity-mode sdB pulsators (V1093 Her) and their hotter siblings, the p-mode pulsating V361 Hya stars. Another class of multi-periodic pulsating hot subdwarfs has been found in the globular cluster ω Cen that is unmatched by any field star. Asteroseismology has advanced enormously thanks to the high-precision Kepler photometry and allowed stellar rotation rates to be determined, the interior structure of gravity-mode pulsators to be probed and stellar ages to be estimated. Rotation rates turned out to be unexpectedly slow calling for very efficient angular momentum loss on the red giant branch or during the helium core flash. The convective cores were found to be larger than predicted by standard stellar evolution models requiring very efficient angular momentum tr...

Three new pulsating sdB stars discovered with SuperWASP

“…Following from that discovery, several mechanisms have been proposed for the resulting low-mass H envelope of the EHB stars (Han et al 2002(Han et al , 2003. For a thorough overview of the sdB stars as a whole, see Heber ( , 2016.…”

Pulsating white dwarfs

“…Pulsations are an important tool to study the stellar interior, and [33] - [37] report on theoretical models and pulsations of ELMs. [38] and [39] show there are thousands of stars, photometrically classified as blue horizontal branch stars by [41][42][43], that have spectroscopic estimated surface gravities much higher than main sequence stars (log g ≥ 4.75) and therefore must have radii smaller than the Sun, classifying them as sdAs, in line with the hot subdwarfs reviewed by [44]. [14] discuss they are possibly ELM white dwarf stars.…”

News on the X-ray emission from hot subdwarf stars

“…Although they are characterized by comparable temperatures and spectral types, their masses and luminosities are significantly smaller than those of massive stars. Therefore, in the HerzprungRussell (HR) diagram they are well below the main sequence (Heber 2016). Historically, they have been deeply investigated in the optical-UV domain, while they remained undetected at X-rays.…”