Asteroseismology and evolution of EHB stars

Let me first clarify the basic terminology with respect to EHB stars, which can sometimes be confusing as the terms EHB and sdB are often used to label the same stars. The terms Extended Horizontal Branch or Extreme Horizontal Branch have been used interchangeably to describe the sequence of stars observed to lie bluewards of the normal Horizontal Branch stars in globular clusters, and also in temperature/gravity plots of hot field stars. The EHB feature was first described and associated with field sdB and sdO stars by Greenstein & Sargent (1974). Now, EHB stars are taken to mean core helium burning stars with an envelope too thin to sustain hydrogen burning. It is also understood that not all sdB stars are EHB stars. In particular, if a star loses its envelope without the core reaching the mass required for the helium flash, its cooling track can take it through the sdB domain on its way to become a helium core WD. The sdB/sdO terms are used to describe the spectroscopic appearance and do not presume any particular evolutionary stage. Several subclassification schemes have been used, but most common nowadays is the one introduced by Moehler et al. (1990). This scheme names as sdB stars those of the hot subdwarfs showing He I absorption lines, as sdO stars those showing He II, and as sdOB stars those showing features of both. Additionally the terms He-sdB and He-sdO are used to describe stars in which the helium lines dominate over the Balmer lines. The EHB forms a sequence of stars from the coolest sdBs to the sdOB domain, and it is clear that most stars given this classification are in fact EHB stars. For the He-rich objects a coherent picture has yet to emerge.

The current canonical picture of the EHB stars was mostly established by Heber (1986), in which the EHB stars are helium core burning stars with masses close to the core helium flash mass of ∼0.47 M ⊙ , and an extremely thin hydrogen envelope, too thin to sustain hydrogen burning (no more than 1% by mass). It is understood that they are post red giant branch (RGB) stars that have started core helium burning in a helium flash before or after the envelope was removed by any of several possible mechanisms. The lifetime of EHB stars from the zero-age EHB (ZAEHB) to the terminal age EHB (TAEHB), when core helium runs out, takes between 100 and 150 Myrs. The post-EHB evolution will take them through the sdO domain directly to the white dwarf (WD) cooling curve without ever passing through a second giant stage. The time they spend shell helium burning before leaving the sdO domain can be up to 20 Myrs.

Although the future evolution of EHB stars after core He-exhaustion has always been presumed quite simple, the paths that lead to the EHB have always been somewhat mysterious. New hope that the evolutionary paths leading to the formation of EHB stars can be resolved has been kindled by the discovery that many of them pulsate, which has opened up the possibility of probing their interiors using asteroseismological methods. These pulsators are known as sdBV stars, and several distinct subclasses are now recognised (see the Asteroseismology section below). But in order to understand what questions asteroseismology can ask and answer, it is essential to understand the different paths that produce EHB stars. Only by understanding the evolutionary history of these stars is it possible to construct realistic models of their interiors which are needed for asteroseismology to be able to distinguish between the different formation scenarios. For this reason we will review the essential points of the Formation and Evolution first, after starting with a look at the observed properties of the hot subdwarf population in The Observed EHB below.

Besides the spectacularly rapid pulsations in the EHB stars, another factor that has contributed to the recent burst in interest in EHB stars is the realisation that these stars are the main contributor to the UV-upturn phenomenon observed in elliptical galaxies. An excellent review of the UV upturn and the binary population synthesis models required to model this phenomenon, can be found in Podsiadlowski et al. (2008). For a more in-depth review of the properties of all hot subdwarf stars, the exhaustive review by Heber (2009) is recommended.

Hot subdwarf stars were found in the galactic caps already by Humason & Zwicky (1947). By the time Greenstein & Sargent (1974) wrote their seminal paper, the number of such faint blue stars had grown to 189, permitting a systemic study of the population. The PG survey (Green et al. 1986), which covered more than 10 000 square degrees at high galactic latitudes, found that of 1874 UV-excess objects detected more than 1000 were hot subdwarfs, so these stars dominate the population of faint blue stars down to the PG survey limit (B = 16.5). Together with the large sample of subdwarfs detected in the HS survey and analysed by Edelmann et al. (2003), these have provided a rich source of hot

…(Full text truncated)…

This content is AI-processed based on ArXiv data.