Hot Subluminous Stars U. Heber1 Dr. Remeis-Sternwarte & ECAP, Astronomical Institute, University of Erlangen-N¨urnberg, Germany 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 labora- tory 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 vs. He-poor hot subdwarf stars of the globular clusters ! 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 ele- ments 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 phase of evolution. Because the binaries are detached they provide a clean-cut laboratory to study this important but yet purely 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 sys- arXiv:1604.07749v1 [astro-ph.SR] 26 Apr 2016 tems 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 ob- jects suggests that most of the planets are formed from the remaining common- envelope material (second generation planets). { 2 { 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 ! 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 transport on the red giant branch. The masses of hot subdwarf stars, both single or in binaries, are the key to understand the stars' evolution. A few pulsating sdB stars in eclipsing binaries have been found that allow both techniques to be applied for mass determination. The results, though few, are in good agreement with predictions from binary population synthesis calculations. New classes of binaries, hosting so-called extremely low mass (ELM) white dwarfs (M<0.3 M ), have recently been discovered, filling a gap in the mosaic of binary stellar evolution. Like most sdB stars the ELM white dwarfs are the stripped cores of red giants, the known companions are either white dwarfs, neutron stars (pulsars) or F- or A-type main sequence stars ("EL CVn" stars). In the near future, the Gaia mission will provide high-precision astrometry for a large sample of subdwarf stars to disentangle the different stellar populations in the field and to compare the field subdwarf population with the globular clusters' hot subdwarfs. New fast-moving subdwarfs will allow the mass of the Galactic dark matter halo to be constrained and additional unbound hyper-velocity stars may be discovered. Subject headings: stars: low mass; stars: subdwarfs; stars: horizontal branch; stars: chemically peculiar; stars: white dwarfs; stars: white dwarfs: helium; stars: white dwarfs: extremely low mass; stars: UV-bright; stars: brown dwarfs; stars: planetary systems; stars: atmospheres; stars: abundances; stars: rotation; stars: magnetic field, stars: oscillations, stars: population II; stars: evolution; stars: kinematics and dynamics; stars: variable: HW Vir, XY Sex, EL CVn, V1093 Her, V499 Ser, V361 Hya, V0366 Aqr; binaries: close; binaries: spectroscopic; binaries: eclipsing; globular clusters: individual (! Cen, NGC 2808, NGC 6752, NGC 5986, M80), open clusters: individual (NGC 188, NGC 6791) { 3 { 1. Introduction The hot subluminous stars of B and O-type (sdB, sdO) represent several late stages in the evolution of low-mass stars. In the Hertzsprung-Russell diagram they can be found between the main sequence and the white-dwarf sequence (see Fig. 1). The discovery of subluminous blue stars at high Galactic latitudes dates back to the 1950s after exploitation of the Humason & Zwicky (1947) photometric survey of the North Galactic Pole and Hyades regions (e.g. Luyten 1953; Greenstein 1956; M¨unch 1958). The number of known objects remained small until the 1980s when the Palomar-Green survey (PG, Green et al. 1986) of the northern Galactic hemisphere was published (see Greenstein 1987; Lynas-Gray 2004, for overviews of early developments). Several other photometric surveys followed, e.g. the Kitt Peak-Downes survey of the Galactic plane (KPD, Downes 1986) and the Edinburgh-Cape Survey (EC, Stobie et al. 1997b) for the southern sky, as well as objective prism surveys, e.g. Byurakan surveys (FBS, SBS, Mickaelian et al. 2007), the Hamburg Quasar Survey (HS, Hagen et al. 1995) for the northern and the Hamburg ESO survey (HE, Wisotzki et al. 1996) for the southern sky. The Sloan Digital Sky survey has doubled the number of known hot subdwarfs within a few years and the Galaxy Evolution Explorer (GALEX) all-sky survey extended the list further. Most of the B-type subdwarfs were identified as helium burning stars of about half a solar mass at the blue end of the horizontal branch, the so-called Extreme Horizontal Branch (EHB, Heber 1986). In contrast to the normal horizontal branch (HB) stars, the hydrogen envelope is too thin (Menv < 0:01 M ) to sustain hydrogen shell burning. Hence, subluminous B stars are the stripped cores of red giants, which managed to ignite helium while retaining just a little bit of hydrogen as their envelope. Maxted et al. (2001) found two-thirds of all sdBs to be in detached binary systems with short periods from hours to days and their companion stars to be mostly white dwarfs. More recent investigations indicate that half of the sdBs reside in such close binaries (Napiwotzki et al. 2004; Copperwheat et al. 2011), still a very high fraction. Their orbital separations are only a few solar radii. Since the sdB stars have evolved from red giants, much larger in size than the present binary orbit, the progenitor system must have undergone a common- envelope phase, during which the orbit shrank when the companion was engulfed in the red giant's envelope. Finally, the common envelope is ejected and a tight, detached binary system remains. The evolution through common-envelope ejection is crucial to an understanding of various types of binaries, including X-ray binaries, double neutron stars, double white dwarfs and sdB binaries. However, common-envelope formation and evolution is poorly understood because of the complexity of the physics involved (for a review see Ivanova et al. 2013). Amongst the various types of post-common-envelope systems, the sdB binaries provide an { 4 { COOL SUBDWARFS WHITE DWARFS Fig. 1.| Hertzsprung-Russell diagram highlighting the position of the extreme horizon- tal branch (EHB) populated by subluminous B stars and located between he upper main sequence and the white dwarf sequence. The EHB is separated from the blue horizontal branch (BHB). The location of subluminous O stars as well as stars having evolved from the asymptotic giant branch is shown for comparison. From Heber (2009); copyright Annual Review of Astronomy & Astrophysics; reproduced with permission. { 5 { important laboratory for studying common-envelope evolution. Because the systems are non-interacting, the properties of the components can be derived in more detail, with higher precision, and selection effects are less severe than for interacting systems such as cataclysmic variables (Pretorius et al. 2007). Hence, sdB stars form an important piece in the puzzle of common-envelope evolution. Many sdB binaries host unseen white dwarf companions and sdBs with neutron star or black hole partners are predicted to exist. Such systems join the zoo of compact binary systems and play a role as progenitors of the cosmologically-important type Ia supernovae and other transient phenomena via mergers driven by gravitational wave radiation. Because of their short periods the orbital gravitational wave emission may be strong enough to be detectable by future space-based missions. An extensive review of the formation and evolu- tion of compact binary star systems has been published recently by Postnov & Yungelson (2014). However, besides degenerate companions, main-sequence stars and brown dwarfs have been discovered to orbit sdB stars. Binary population synthesis has identified several possible channels for the formation of hot subdwarfs, both in binaries as well as single stars that in- volve Roche lobe overflow, common-envelope ejection and gravitational wave-driven mergers of helium white dwarfs.