Hydrogen-deficient star

A hydrogen-deficient star is a type of star that has little or no hydrogen in its atmosphere.^[2] Hydrogen deficiency is unusual in a star, as hydrogen is typically the most common element in a stellar atmosphere. Despite being rare, there are a variety of star types that display a hydrogen deficiency.

Observational history

Hydrogen-deficient stars had been noted prior to the discovery of their hydrogen deficiency. In 1797,

Hydrogen deficiency in a star was first discovered in 1891 by Williamina Fleming,^[2] where she stated “the spectrum of υ Sgr is remarkable since the hydrogen lines are very faint and of the same intensity as the additional dark lines”.^[4] In 1906, Hans Ludendorff found that Hγ Balmer spectral lines were absent in R CrB.^[2][5]

It was widely believed at the time that all stellar atmospheres contain hydrogen, so these observations were discounted. Not until quantitative spectral measurements became available in 1935-1940 did astronomers begin to accept that stars such as R CrB and υ Sgr were hydrogen deficient.^[2] As of 1970, relatively few of these stars were known. Large-scale stellar surveys since then have greatly increased the number and variety of known hydrogen-deficient stars. As of 2008, about 2,000 hydrogen-deficient stars were known.^[2]

Classification

Despite being relatively rare, there are many different types of hydrogen-deficient stars. They can be grouped into five general classes: massive or upper-main-sequence stars, low-mass supergiants, hot subdwarf stars, central stars of planetary nebulae, and white dwarfs.^[2] There have been other classification schemes, such as one based on carbon content.^[6]

Massive stars

Wolf-Rayet stars show bright bands in continuous spectra that come from ionized atoms such as helium. Although there was some controversy, these were accepted as hydrogen-deficient stars in the 1980s.

This type of hydrogen-deficient star occurs at late stages of stellar evolution.

He-sdB are subdwarfs with class B spectra with broader than usual H, HeI, and HeII lines. JL 87 in 1991 was the first He-sdB star to be reported.^[2][7] Since then this class of stars has been shown to have a wide range of hydrogen-to-helium ratios. Compact He-sdO stars have class O spectra, are typically nitrogen-rich, and may or may not be carbon-rich. Low-gravity He-sdO stars overlap with their compact cousins, but have lower surface gravity. It is hypothesized that R CrB and extreme Helium stars, if they evolve to become white dwarfs, would become similar to low-gravity He-sdO stars.^[2]

Central stars of planetary nebulae

Central stars of

The first hydrogen-deficient white dwarfs were discovered by

Hydrogen deficiency results from stellar evolution.[2] Over the course of a star's evolution, both the consumption of hydrogen in nuclear fusion and the removal of hydrogen layers by explosive processes can lead to a deficiency of hydrogen in its atmosphere.

Detailed theoretical models are still in their infancy. Modeling of hydrogen-deficient star evolution involves either a single-star approach or a binary-star approach.^[6]

For example, there have been two theories put forward to explain the formation of extreme helium stars.^[9] The helium final flash scenario is a single-star approach in which a helium flash serves to consume the hydrogen from the outer layer of the star. The double degenerate scenario is a binary-star approach in which a smaller degenerate helium white dwarf and a larger carbon-oxygen white dwarf orbit each other so closely that they eventually inspiral due to gravitational wave losses. At the Roche limit, mass transfer takes place from the helium to the carbon-oxygen star. The latter undergoes helium shell burning to form a supergiant and evolve to a hydrogen-deficient star. The double degenerate scenario provides a better fit to the observational data.^[9]

References