Hot subdwarfs: Small stars marking important events in stellar evolution: Ludwig Biermann Award Lecture 2014

Abstract

Hot subdwarfs are considered to be the compact helium cores of red giants which lost almost their entire hydrogen envelope. What causes this enormous mass loss is still unclear. Binary interactions are invoked, and a significant fraction of the hot subdwarf population is indeed found in close binaries. In a large project we search for close binary sdBs with the most and the least massive companions. Significantly enhancing the known sample of close binary sdBs we performed the first comprehensive study of this population. Triggered by the discovery of two sdB binaries with close brown dwarf companions in the course of this project, we were able to show that the interaction of stars with substellar companions is an important channel to form sdB stars. Finally, we discovered a unique and very compact binary system consisting of an sdB and a massive white dwarf which qualifies as a progenitor candidate for a supernova of type Ia. In addition to that, we could connect those explosions to the class of hypervelocity hot subdwarf stars which we consider as the surviving companions of such events. Being the stripped cores of red giants, hot subdwarfs turned out to be important markers of peculiar events in stellar evolution ranging all the way from star-planet interactions to the progenitors of stellar explosions used to measure the expansion of our Universe.

Shell helium-burning hot subdwarf B stars as candidates for blue large-amplitude pulsators

“…(e.g., Heber 2009Heber , 2016Geier 2015). The shell helium-burning (SHeB) follows the CHeB phase and we show in this section that some SHeB sdBs may aptly reproduce the properties of BLAPs.…”

The population of hot subdwarf stars studied withGaia

“…Hot subdwarf binaries with massive white dwarf companions are candidates for the progenitors of type Ia supernovae. They are possibly ejected by such supernovae as hypervelocity stars (see Geier 2015). Hot subdwarfs dominate old stellar populations in blue and ultraviolet bands.…”

Follow-up observations of X-ray emitting hot subdwarf stars: the He-rich sdO BD +37° 1977

“…They form a rather heterogeneous class of stars, which includes both He-poor and He-rich stars (Heber & Jeffery 1992;Heber et al 2006;Hirsch et al 2008), and luminous and compact stars, according to their low or high surface gravity, respectively (Napiwotzki 2008). This variety of properties is probably the consequence of different evolutionary histories (Heber 2009;Geier 2015): in the case of the compact stars, the He-poor ones are post-EHB stars, while the origin of the He-rich ones might be either the merging of two He-core or C/O-core white dwarfs (Iben 1990;Saio & Jeffery 2000, 2002 or the so-called late hot-flasher scenario (Brown et al 2001); instead, the luminous sdO stars are post-AGB stars. Evolutionary models suggest that most sdO stars are the outcome of the evolution of single stars, but some of them could descend from binary systems that underwent a commonenvelope phase; in this case it is possible that the sdO stars have a compact companion, typically a white dwarf (WD).…”