What is a Hot Subdwarf?


This page is still under construction


The following text was part of an introduction to hot subdwarfs from a funding proposal I wrote, it was aimed at astronomers who may not know what a hot subdwarf is. When I have more time I'll work on a more general description of what Hot Subdwarfs are.


Hot subdwarfs stars are a class of stars defined as having effective temperatures exceeding 20,000 K, with surface gravities higher and luminosities lower than main sequence stars of the same temperature. The surface gravities of hot subdwarf stars are intermediate between main sequence and white dwarf stars, generally with log g>5 (Saffer et al. 1994). So, like Population II main sequence stars which have smaller radii and lower luminosity than their Population I counterparts, these stars are referred to as ``subdwarfs''. However, since there is evidence that hot subdwarfs belong to an old population, they are much too hot to be extensions of the Pop II main sequence and are thus in a post-main sequence evolutionary state. There are two major spectral types of hot subdwarfs: subdwarf B (sdB) and subdwarf O (sdO). I will be focusing on the more numerous sdB stars, since it is believed that they comprise a homogeneous population of objects with similar formation histories.

In the Hertzsprung-Russell (HR) Diagram, hot subdwarfs fall at fainter visual magnitudes and bluer colors than the horizontal branches of most globular clusters. However, some globular clusters have extended horizontal branches (EHB), with endpoints that reach the regime of the field hot subdwarfs in color-magnitude diagrams. In particular field sdB stars are consistent with belonging to the EHB star population from globular clusters (Humason & Zwicky 1947) --- thus the field sdB stars are understood as being core helium burning objects with very low hydrogen envelope mass (Menv<~0.05 Msun) and total masses of M~0.5-0.55 Msun (Saffer et al. 1994). Thus, understanding the origins and properties of galactic field sdBs can lead to a better understanding of the ``second parameter'' (dictating the structure of globular cluster horizontal branches) and of stellar evolution theory (describing post-main sequence evolution and mass loss).

Additionally, hot subdwarfs are believed to be the primary contributor to the ultraviolet excess (UVX) seen in ultraviolet (UV) observations (such as those from the Hubble Space Telescope, the Astro-1 and 2 missions, and other space-based UV missions) of ``normal'' elliptical galaxies, spiral galaxy bulges, and other old stellar populations --- ``normal'' in this sense meaning those galaxies without a central active galactic nuclei (AGN), or evidence for recent merger or starburst activity (O'Connell 1999; Brown et al. 2000). In some cases hot horizontal branch stars have been imaged directly in elliptical galaxies (such as the observations of Brown et al., 2000, using the Space Telescope Imaging Spectrograph, STIS, on the HST) --- these observations demonstrate that hot subdwarfs can be the primary contributors to the integrated UVX from elliptical galaxies. It is believed from both models and observations that the lifetime UV output of hot subdwarf stars is sensitive to their physical properties. For instance, it depends strongly on their helium abundance and changes of only a few 0.01 Msun in the mean envelope mass of an EHB population can significantly alter the UV spectrum of an elliptical galaxy (O'Connell 1999). Thus existing and future UV observations would be a very delicate probe of the star formation and chemical enrichment histories of galaxies provided we understood the basic astrophysics of hot subdwarfs and their production by their parent populations. So, to understand the mechanisms of UVX in galaxies we must combine the integrated light observations of these distant galaxies with the stellar astrophysics derived from observations of globular clusters and field hot subdwarfs in our own galaxy. Additionally, UVX stars are important contributors to the interstellar ionizing radiation field of old populations. Characterization of the UV light of nearby elliptical galaxies and its predicted evolution are basic to the development of realistic ``K-corrections'' for cosmological applications to high redshift galaxies and the interpretation of the cosmic background light (O'Connell 1999). Thus understanding field hot subdwarfs in our own galaxy can lead to a better understanding of many other areas including: cosmology, chemical enrichment histories of galaxies, stellar mass loss, and UV background radiation.


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