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.
Hydrogen-deficient stars had been noted prior to the discovery of their hydrogen deficiency. In 1797, Edward Pigott noted the profound variation in stellar magnitude of R Coronae Borealis (R CrB). [2] [3] In 1867, Charles Wolf and Georges Rayet discovered unusual emission line structure in Wolf-Rayet stars.
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]
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]
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. [2] Helium-rich B stars, such as σ Orionis E, are chemically unusual spectral B or OB main sequence stars that show strong neutral helium lines. Hydrogen-deficient binaries, such as υ Sgr, have helium lines on a metallic spectrum and show large radial velocities that are thought to result from Population I stars orbiting the Galactic Center. Type Ib and Ic supernovae show no hydrogen absorption lines and are associated with stars that have lost their hydrogen envelope through supernova core collapse.
This type of hydrogen-deficient star occurs at late stages of stellar evolution. R CrB stars are hydrogen-deficient, carbon-rich stars that are notable for their light variation; they may dim by five stellar magnitudes over a period of days, then recover. [2] These dimming events likely arise from stellar surface dynamics, rather than their exceptional chemical composition. Extreme helium stars have absent hydrogen emission or absorption lines, but have strong neutral helium lines and strong CII and NII lines. Born-again stars are stars that evolve over a period of years to migrate between the post-AGB and AGB regions of the Hertzsprung–Russell diagram. [1] For example, Sakurai’s Object (V4334 Sgr) evolved from a faint blue star in 1994 to a yellow supergiant in 1996. [2] One proposed mechanism for this migration is the final helium flash scenario. [6]
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 are typically hot and compact. WC stars are massive Population I stars with broad emission lines for HeI, HeII, CII - CIV, NII, and NIII ions. [2] They have surface temperatures from 14,000K to 270,000K. Of-WR(C) stars have strong carbon emission lines and also show hydrogen deficiency in the inner part of their nebulae. O(He) stars are characterized by HeII absorption while having CIV, NV and OVI emission lines. PG1159 stars, also termed O(C) stars, are dominated by carbon absorption line spectra. They are notable for complex pulsations and being among the hottest known stars. [2]
The first hydrogen-deficient white dwarfs were discovered by Milton Humason and Fritz Zwicky in 1947 and Willem Luyten in 1952. [2] These stars had no hydrogen lines, but very strong HeI absorption lines. HZ 43 is such a star; early ultraviolet observations showed a temperature greater than 100,000K, but more recent measurements in far UV show an effective temperature of 50,400K. [8] AM CVn stars are binary pairs of hydrogen-deficient white dwarfs with orbital sizes of only tens of Earth radii. [2]
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]
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines. Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary mainly due to the temperature of the photosphere, although in some cases there are true abundance differences. The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature.
Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon. The spectra indicate very high surface enhancement of heavy elements, depletion of hydrogen, and strong stellar winds. The surface temperatures of known Wolf–Rayet stars range from 20,000 K to around 210,000 K, hotter than almost all other kinds of stars. They were previously called W-type stars referring to their spectral classification.
A carbon star is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes most of the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere and a strikingly ruby red appearance. There are also some dwarf and supergiant carbon stars, with the more common giant stars sometimes being called classical carbon stars to distinguish them.
R Coronae Borealis is a low-mass yellow supergiant star in the constellation of Corona Borealis. It is the prototype of the R Cor Bor class of variable stars, which fade by several magnitudes at irregular intervals. R Coronae Borealis itself normally shines at approximately magnitude 6, just about visible to the naked eye, but at intervals of several months to many years fades to as faint as 15th magnitude. Over successive months it then gradually returns to its normal brightness, giving it the nickname "reverse nova", after the more common type of star which rapidly increases in brightness before fading.
A B-type main-sequence star is a main-sequence (hydrogen-burning) star of spectral type B and luminosity class V. These stars have from 2 to 16 times the mass of the Sun and surface temperatures between 10,000 and 30,000 K. B-type stars are extremely luminous and blue. Their spectra have strong neutral helium absorption lines, which are most prominent at the B2 subclass, and moderately strong hydrogen lines. Examples include Regulus and Algol A.
Upsilon Sagittarii is a spectroscopic binary star system in the constellation Sagittarius. Upsilon Sagittarii is the prototypical hydrogen-deficient binary (HdB), and one of only four such systems known. The unusual spectrum of hydrogen-deficient binaries has made stellar classification of Upsilon Sagittarii difficult.
Type Ib and Type Ic supernovae are categories of supernovae that are caused by the stellar core collapse of massive stars. These stars have shed or been stripped of their outer envelope of hydrogen, and, when compared to the spectrum of Type Ia supernovae, they lack the absorption line of silicon. Compared to Type Ib, Type Ic supernovae are hypothesized to have lost more of their initial envelope, including most of their helium. The two types are usually referred to as stripped core-collapse supernovae.
An R Coronae Borealis variable is an eruptive variable star that varies in luminosity in two modes, one low amplitude pulsation, and one irregular, unpredictably-sudden fading by 1 to 9 magnitudes. The prototype star R Coronae Borealis was discovered by the English amateur astronomer Edward Pigott in 1795, who first observed the enigmatic fadings of the star. Only about 150 RCB stars are currently known in our Galaxy while up to 1000 were expected, making this class a very rare kind of star.
An AM Canum Venaticorum star, is a rare type of cataclysmic variable star named after their type star, AM Canum Venaticorum. In these hot blue binary variables, a white dwarf accretes hydrogen-poor matter from a compact companion star.
An extreme helium star is a low-mass supergiant that is almost devoid of hydrogen, the most common chemical element of the Universe. Since there are no known conditions where stars devoid of hydrogen can be formed from molecular clouds, it is theorized that they are the product of the mergers of helium-core and carbon-oxygen core white dwarfs.
PV Telescopii variable is a type of variable star that is established in the General Catalogue of Variable Stars with the acronym PVTEL. This class of variables are defined as "helium supergiant Bp stars with weak hydrogen lines and enhanced lines of He and C". That is, the hydrogen spectral lines of these stars are weaker than normal for a star of stellar class B, while the lines of helium and carbon are stronger. They are a type of extreme helium star.
PV Telescopii, also known as HD 168476, is a variable star in the southern constellation of Telescopium. It is too dim to be visible to the naked eye, having an apparent visual magnitude that has been measured varying from 9.24 down to 9.40. The star is the prototype of a class of objects called PV Telescopii variables. It is located at an estimated distance of approximately 23 kilolight-years from the Sun, but is drifting closer with a radial velocity of −169 km/s.
A subdwarf O star (sdO) is a type of hot, but low-mass star. O-type subdwarfs are much dimmer than regular O-type main-sequence stars, but with a brightness about 10 to 100 times that of the Sun, and have a mass approximately half that of the Sun. Their temperature ranges from 40,000 to 100,000 K. Ionized helium is prominent in their spectra. Gravity acceleration is expressed by log g between 4.0 and 6.5. Many sdO stars are moving at high velocity through the Milky Way and are found at high galactic latitudes.
WR 24 is a Wolf-Rayet star in the constellation Carina. It is one of the most luminous stars known. At the edge of naked eye visibility it is also one of the brightest Wolf Rayet stars in the sky.
An O-type star is a hot, blue-white star of spectral type O in the Yerkes classification system employed by astronomers. They have temperatures in excess of 30,000 kelvin (K). Stars of this type have strong absorption lines of ionised helium, strong lines of other ionised elements, and hydrogen and neutral helium lines weaker than spectral type B.
BX Circini is a star in the constellation Circinus. Its variability was discovered in 1995, with its apparent magnitude ranging from 12.57 to 12.62 over a period of 2 hours 33 minutes. It is currently classified as a PV Telescopii variable star, but has been put forward as the prototype of a new class of pulsating star—the BX Circini variables—along with the only other known example, V652 Herculis. This class of star is rare, possibly because this is a brief stage of stellar evolution. Its mass has been calculated to be around 40 percent that of the Sun, but the radius is a few times larger than that of the Sun. The average surface temperature is high, and has been measured at 23,390 ± 90 K using optical spectra, but 1750 K cooler if analysing it in both the visual and ultraviolet. The temperature appears to vary by 3450 K.
Melnick 34, also called BAT99-116, is a binary Wolf–Rayet star near R136 in the 30 Doradus complex in the Large Magellanic Cloud. Both components are amongst the most massive and most luminous stars known, and the system is the most massive known binary system.
WR 135 is a variable Wolf-Rayet star located around 6,000 light years away from Earth in the constellation of Cygnus, surrounded by a faint bubble nebula blown by the intense radiation and fast wind from the star. It is just over four times the radius of the sun, but due to a temperature of 63,000 K it is 250,000 times as luminous as the sun.
DY Centauri is a variable star in the constellation Centaurus. From its brightness, it is estimated to be 7000 parsecs (23000 light-years) away from Earth.
NGC 6822-WR 12 is a WN-type Wolf-Rayet star located in the galaxy NGC 6822, about 1.54 million light years away in the constellation of Sagittarius. NGC 6822-WR 12 was the first Wolf-Rayet star to be discovered in the galaxy, and is one of only four known in the galaxy.
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