(astronomy) A member of a class of very hot stars (100,000-35,000 K) which characteristically show broad bright emission lines in their spectra; luminosities are high, probably in the range 104-105 times that of the sun; these stars are probably very young and represent an early short-lived stage in stellar evolution.
Wolf-Rayet star
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(¦völf rī¦ā ′stär)
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A type of hot, luminous star that is distinguished by its extremely dense and fast wind. The spectacularly bright, discrete bands of atomic emission from these winds greatly facilitated their discovery with the aid of a visual spectroscope by the French astronomers Charles Wolf and Georges Rayet at the Paris Observatory in 1867. See also Astronomical spectroscopy.
The Wolf-Rayet phenomenon is a typical phase in the advanced evolution of a massive star (about 20–100 times the Sun's mass), or sometimes among lower-mass stars in the planetary nebula stage. The first fusion process in the core of a massive star involves the conversion of hydrogen into helium via the carbon-nitrogen-oxygen (CNO) cycle, in which helium and nitrogen are enhanced at the expense of the initially abundant hydrogen and traces of carbon and oxygen. When such fusion products are visible in the winds, a WN-type Wolf-Rayet star is seen, whose spectrum is dominated by Doppler-broadened atomic lines of helium and nitrogen in various stages of ionization. Later, when the second fusion process occurs, helium is converted mainly into carbon and oxygen, with nitrogen being virtually destroyed. A WC-type Wolf-Rayet star is then seen with lines mainly of carbon and helium. A brief oxygen-rich phase may occur (WO), after which all other phases are so rapid that the chances of seeing them are negligible. At that point, it is believed that the Wolf-Rayet star will explode as a supernova, resulting in the collapse to a black hole in most cases. See also Black hole; Doppler effect; Nucleosynthesis; Planetary nebula; Supernova.
Beneath the dense winds that often hide the stellar surface, massive Wolf-Rayet stars have surface temperatures ranging from 30,000 to 150,000 (54,000 to 270,000°F), radii of 1 to 15 solar units, and luminosities of 105 to 106 times that of the Sun. They are losing matter at a rate that is typically 109 times that of the Sun's wind, at speeds ranging from 1000 to 3000 km/s (600 to 1800 mi/s). Although massive Wolf-Rayet stars appear to be rare (only 200 are known so far in the Milky Way Galaxy, out of an estimated total population of 1000–2000), all massive stars likely pass through a Wolf-Rayet stage toward the end of their relatively short lives. See also Solar wind; Star;
| Wikipedia: Wolf-Rayet star |
Wolf-Rayet stars (often referred to as WR stars) are evolved, massive stars (over 20 solar masses), which are losing mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s. While our own Sun loses approximately 10−14 solar masses every year, Wolf-Rayet stars typically lose 10−5 solar masses a year.
Wolf-Rayet stars are very hot, with surface temperatures in the range of 25,000 K to 50,000 K. It is believed that the star in the galaxy NGC 2770 that exploded into a supernova on January 9, 2008 — SN 2008D, the first supernova ever observed in the act of exploding — was a Wolf-Rayet star.
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Observation history
In 1867, astronomers using the 40 cm Foucault telescope at the Paris Observatory, discovered three stars in the constellation Cygnus (now designated HD191765, HD192103 and HD192641), that displayed broad emission bands on an otherwise continuous spectrum.[1] The astronomers' names were Charles Wolf and Georges Rayet, and thus this category of stars became named Wolf-Rayet (WR) stars.[2] Most stars display absorption bands in the spectrum, as a result of overlaying elements absorbing light energy at specific frequencies. The number of stars with emission lines is quite low, so these were clearly unusual objects.
The nature of the emission bands in the spectra of a Wolf-Rayet star remained a mystery for several decades. Edward C. Pickering theorized that the lines were caused by an unusual state of hydrogen, and it was found that this "Pickering series" of lines followed a pattern similar to the Balmer series, when half-integral quantum numbers were substituted. It was later shown that the lines resulted from the presence of helium; a gas that was discovered in 1868.[3]
By 1929, the width of the emission bands was being attributed to the Doppler effect, and hence that the gas surrounding these stars must be moving with velocities of 300–2400 km/s along the line of sight. The conclusion was that a Wolf-Rayet star is continually ejecting gas into space, producing an expanding envelope of nebulous gas. The force ejecting the gas at the high velocities observed is radiation pressure.[4]
In addition to helium, emission lines of carbon, oxygen and nitrogen were identified in the spectra of Wolf-Rayet stars.[5] In 1938, the International Astronomical Union classified the spectra of Wolf-Rayet stars into types WN and WC, depending on whether the spectrum was dominated by lines of nitrogen or carbon-oxygen respectively.[6]
Description
Wolf-Rayet stars are a normal stage in the evolution of very massive stars, in which strong, broad emission lines of helium and nitrogen ("WN" sequence) or helium, carbon, and oxygen ("WC" sequence) are visible. Due to their strong emission lines they can be identified in nearby galaxies. About 230 Wolf-Rayets are known in our own Milky Way Galaxy[7], about 100 are known in the Large Magellanic Cloud, while only 12 have been identified in the Small Magellanic Cloud.
Several astronomers, among which Rublev (1965) [8] and Conti (1976)[9] originally proposed that the WR stars as a class are descended from massive O-stars in which the strong stellar winds characteristic of extremely luminous stars have ejected the unprocessed outer H-rich layers. The characteristic emission lines are formed in the extended and dense high-velocity wind region enveloping the very hot stellar photosphere, which produces a flood of UV radiation that causes fluorescence in the line-forming wind region. This ejection process uncovers in succession, first the nitrogen-rich products of CNO cycle burning of hydrogen (WN stars), and later the carbon-rich layer due to He burning (WC & WO stars). Most of these stars are believed finally to progress to become supernovae of Type Ib or Type Ic. A few (roughly 10%) of the central stars of planetary nebulae are, despite their much lower (typically ~0.6 solar) masses, also observationally of the WR-type; i.e., they show emission line spectra with broad lines from helium, carbon and oxygen. Denoted [WR], they are much older objects descended from evolved low-mass stars and are closely related to white dwarfs, rather than to the very young, very massive stars that comprise the bulk of the WR class.[10]
Evolution models
Evolution models of WR stars.[11]
For stars of ~40-75MS
- O → LBV → WN(H-poor) → WC → SN Ic
For stars of 25-40MS
- O → LBV → WN(H-poor) → SN Ib
OR
- O → RSG → WN(H-poor) → SN Ib
It is possible for a Wolf-Rayet star to progress to a "collapsar" stage in its death throes: This is when the core of the star collapses to form a black hole, pulling in the surrounding material. This is thought to be the precursor of a long gamma-ray burst.
The best known (and most visible) example of a Wolf-Rayet star is Gamma 2 Velorum (γ² Vel), which is a bright star visible to those located south of 40 degrees northern latitude. One of the members of the star system (Gamma Velorum is actually at least six stars) is a Wolf-Rayet star. Due to the exotic nature of its spectrum (bright emission lines in lieu of dark absorption lines) it is dubbed the "Spectral Gem of Southern Skies".[12]
See also
References
- ^ Huggins, William (1890-1). "On Wolf and Rayet's Bright-Line Stars in Cygnus". Proceedings of the Royal Society of London 49: 33–46. doi:. http://adsabs.harvard.edu/abs/1890RSPS...49...33H. Retrieved 2007-09-06.
- ^ Murdin, P. (2001). Wolf, Charles J E (1827-1918). Bristol: Institute of Physics Publishing. doi:.
- ^ Fowler, A. (1912). "Hydrogen, Spectrum of, Observations of the principal and other series of lines in the". Monthly Notices of the Royal Astronomical Society 73: 62–105. http://adsabs.harvard.edu/abs/1912MNRAS..73...62F. Retrieved 2007-02-04.
- ^ Beals, C. S. (1929). "On the nature of Wolf-Rayet emission". Monthly Notices of the Royal Astronomical Society 90: 202–212. http://adsabs.harvard.edu/abs/1929MNRAS..90..202B. Retrieved 2007-09-10.
- ^ Beals, C. S. (1933). "Classification and temperatures of Wolf-Rayet stars". The Observatory 56: 196–197. http://adsabs.harvard.edu/abs/1933Obs....56..196B. Retrieved 2007-09-10.
- ^ Swings, P. (1942). "The Spectra of Wolf-Rayet Stars and Related Objects". Astrophysical Journal 95: 112–133. doi:. http://adsabs.harvard.edu/abs/1942ApJ....95..112S. Retrieved 2007-09-10.
- ^ van der Hucht, K.A. 2001, New Astron. Rev., 45:135
- ^ Rublev 1965, Soviet Astronomy, 8, 848 Template:Ads
- ^ Conti, P.S. 1976, in Proc. 20th Colloq Int. Astrophys. Liege
- ^ Crowther, P.A., 2007, Physical Properties of Wolf-Rayet Stars, Ann. Rev. A&A, 45:177-219.
- ^ Physical Properties of Wolf-Rayet Stars, Crowther, Paul A., 2007
- ^ Hoffleit. "The Bright Star Catalogue, 5th Revised Ed.". http://www.alcyone.de/SIT/mainstars/SIT000822.htm#Cat1. Retrieved August 08 2007.
External links
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Wikimedia Commons has media related to: Wolf-Rayet stars |
- [1] Some Wolf-Rayet stars in binaries are close enough that we can image a rotating "pinwheel nebula" showing the dust generated by colliding winds in the binary system, from Aperture Masking Interferometry observations.
- [2]Wolf-Rayet Stars: Spectral Classifications
- [3]ApJ 525:L97-L100 Nov. 10, 1999. Monnier, Tuthill & Danchi: Pinwheel Nebula Around WR98a (PDF)
- [4]ApJ Jan. 3,2005. Dougherty, et al.: High Resolution Radio Observations of the Colliding Wind Binary WR140 (PDF)
- [5]A catalog of northern Wolf-Rayet Stars and the Central Stars of Planetary Nebulae (Harvard)
- [6]Scientists See Supernova in Action
- [7]Big Old Stars Don't Die Alone (NASA)
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