Hot Subdwarfs

Hot subdwarfs are hots stars with masses somewhat less than that of the Sun. They are so-called because on a Hertzsprung-Russell diagram they would be to the left - or below - the upper main-sequence, but they would not be as faint as the sequence of white dwarfs. As astronomical surveys have extended to fainter and bluer limits in the search for quasars, ever increasing numbers of faint hot stars in our own Galaxy have been discovered.They appear to come in two main flavours, representing the principal links between red giants, asymptotic-branch giantsand white dwarfs.

sdB stars

Traditional subdwarf B stars are about 0.5 solar masses, and are thought to be almost pure helium stars converting helium to carbon in their cores. They are known as sdB stars because their spectra are dominated by hydrogen lines. They are analogous to extremely blue stars in globular clusters known as horizontal branch stars. As such, they represent the next stage after a low mass star has completed its evolution as a red giant. The main problem they pose is how the outer layers of hydrogen have been expelled. Normally, a red giant will switch on core helium burning without losingthe layers of hydrogen around the helium core. sdOB stars are related to sdB stars, possibly being the sdB stars which have completed core helium burning and are evolving to become white dwarfs. Research in Armagh focuses on several issues:

Spectral classification

Classification schemes for hot subdwarfs are confusing.
What types of subdwarf are there?
sdB stars have very thin hydrogen atmospheres. Where have
the hydrogen-layers from the progenitor red-giant gone ?
How is this question related to the fraction of sdB stars which
are binaries ?

Detection of binaries

What is this fraction and how can we detect the companions
to binary sdB stars ?
Most sdB stars are extremely hydrogen-rich. Why are some
sdB stars extremely hydrogen-poor ?

Pulsations in sdB stars

Can we detect the surface motions due to pulsations of the
stellar surface?
What do these motions tell us about the mode and origin
of the pulsations?

sdO stars

In contrast, the sdO stars tend to be more luminous and hotter, showing principally lines of ionized helium in their spectra. The majority represent the final stage in the evolution of a star before it becomes a white dwarfs. The central stars of most planetary nebulae are, in fact, sdO stars. The majority have electron-degenerate cores consisting of carbon and oxygen, surrounded by oniion layer shells of helium and hydrogen. Both of these may very thin, or even absent leading to some quite unusual surface abundance characteristics amongst the hot subdwarfs.

PG1159 stars, [WC] stars and DO white dwarfs.

As the sdO star contracts it may experience a number of events which produce unusual phenomena in its atmosphere.

In particular, if its luminoisty becomes high enough it may excite an ionized stellar wind, giving rise to a Wolf-Rayet-like spectrum. Being hydrogen-deficient and carbon-rich, such stars are sometimes known as [WC] stars.

As it becomes increasingly compact, but before its surface starts too cool, the contracting star becomes so hot that both hydrogen and helium are fully ionized and the stellar spectrum only shows highly ionized lines due to carbon and other light elements. Such stars are known as PG1159 stars and DO white dwarfs. These include the hottest stellar surfaces measured (only the surfaces of neutron stars may be hotter). In one case the surface temperature is a staggering 150 000 K, and the surface consists almost entirely of carbon and oxygen. This is the naked core of a post-AGB star from which ll of the original hydrogen and all the helium into which it has been converted has been removed.

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Last modified: 08/06/00

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