Wolf-Rayet Star

Wolf-Rayet Star


Wolf–Rayet stars (often referred to as WR stars) are evolved, massive stars (over 20 solar masses initially) 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.[2]

Wolf–Rayet stars are extremely hot, with surface temperatures in the range of 30,000 K to around 200,000 K.[3] They are also highly luminous, from tens of thousands to several million times the bolometric luminosity of the Sun, although not exceptionally bright visually since most of their output is in far ultraviolet and even soft X-rays.

Observation history


In 1867, using the 40 cm Foucault telescope at the Paris Observatory, astronomers Charles Wolf and Georges Rayet[4] discovered three stars in the constellation Cygnus (now designated HD191765, HD192103 and HD192641) that displayed broad emission bands on an otherwise continuous spectrum.[5] Most stars display absorption bands in the spectrum, as a result of overlying 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.[6]

By 1929, the width of the emission bands was being attributed to Doppler broadening, 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.[7]

In addition to helium, emission lines of carbon, oxygen and nitrogen were identified in the spectra of Wolf–Rayet stars.[8] 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.[9]

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 500 Wolf–Rayets are catalogued in our own Milky Way Galaxy.[10][11] This number has changed dramatically during the last few years as the result of photometric and spectroscopic surveys in the near-infrared dedicated to discovering this kind of object in the Galactic plane.[12] Additionally, about 100 are known in the Large Magellanic Cloud,[13] while only 12 have been identified in the Small Magellanic Cloud.[14] Many more are known in the galaxies in the Local Group, such as M33, where 206 Wolf–Rayet stars are known,[15] and M31, where 154 Wolf–Rayet stars are known.[16]

Several astronomers, among them Rublev (1965)[17] and Conti (1976)[18] 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 and WO stars). WO stars too, according to some authors, could be more advanced on its evolution showing oxygen-rich layers produced during carbon burning[19] . Most of these stars are believed finally to progress to become supernovae of Type Ib or Type Ic. There is however one group of WR stars that have strong hydrogen lines in their spectra indicating the existence of a hydrogen atmosphere. These are the WNh (and also WNha) stars and they have not yet shed their hydrogen shells.

Some Wolf–Rayet stars of the carbon sequence ("WC"), especially those belonging to the latest types, are noticeable due to their production of dust. Usually this takes places on those belonging to binary systems as a product of the collision of the stellar winds forming the pair,[10] as is the case of the famous binary WR 104; however this process occurs on single ones too.[3]

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.[20]

Evolution models

The majority of WR stars are now understood as being at a natural state in the evolution of massive stars (not counting the less common planetary nebula central stars). WR stars are not thought to form in low metallicity stars because they do not lose enough mass, instead proceeding directly to pair-instability or photodisintegration supernovae. Here are the likely sequences in the evolution of single stars of different masses without high rotation rates. Stars with very high rotation rates and stars in binary systems may skip some steps due to accelerated mass loss. Low-mass stars explode as supernovae before they lose enough mass to become Wolf–Rayet stars.[20][21]

Initial Mass (M) Evolutionary Sequence Supernova Type
90+ O → Of → WNLh (→ WNE) → WC Ib (or IIn?)
60–90 O → Of/WNLh ↔ LBV → WNL → WC Ib (or IIn?)
40–60 O → BSG → LBV ↔ WNL (→ WNE) → WC Ib
(rarely) O → BSG → LBV ↔ WNL (→ WNE) → WC → WO Ic
30–40 O → BSG → RSG (↔ LBV)→ WNE → WC Ib
20–30 O (→ BSG) → RSG ↔ BSG (blue loops) → RSG II-L (or IIb)
10–20 O → RSG II-P

Key:

O O-type main-sequence star
Of evolved O-type showing N and He emission
Of/WNLh "slash star", spectrum between Of and WNLh (quiescent LBV?)
BSG blue supergiant
RSG red supergiant
LBV luminous blue variable
WNL "late" WN-class Wolf–Rayet star (about WN6 to WN9)
WNLh WNL plus hydrogen lines
WNE "early" WN-class Wolf–Rayet star (about WN2 to WN6)
WC WC-class Wolf–Rayet star
WO WO-class Wolf–Rayet star

Higher-mass stars are much rarer, both because they form less often and because they only exist for a short time. This means that Wolf–Rayet stars themselves are very rare because they only form from the most massive main sequence stars, but also that type II-P supernovae are the most common amongst massive stars. Although Wolf–Rayet stars form from exceptionally massive stars, most of them are only moderately massive because they only form after losing the bulk of their outer layers. For example, γ2 Velorum A currently has a mass around 9 times the sun, but began with a mass at least 40 times the sun.[22] An exception are the WNh stars which are spectroscopically similar but actually a much less evolved star which has only just started to expel its atmosphere. The most massive stars currently known are all WNh stars rather than O-type main sequence stars, an expected situation because such stars start to move away from the main sequence only a few thousand years after they form.

It is possible for a Wolf–Rayet star to progress to a "collapsar" stage in its death throes if it doesn't lose sufficient mass. This is when the core of the star collapses to form a black hole, either directly or by pulling in the surrounding ejected material. This is thought to be the precursor of a long gamma-ray burst.

Observations of supernova progenitors and the known WC stars do not currently support the idea that WC stars evolve from the highest mass stars. An alternate proposal is that they evolve from the highest mass red supergiants, above about 25 M, which have not been observed to be supernova progenitors.[3] Most higher mass stars would then explode as supernovae either in a blue supergiant, LBV, or WN phase, before reaching the WC stage. WO stars are observed to be luminous enough to be the end point for stars of 60 M or more, but no intermediate WC equivalents have been observed. It isn't clear if this is simply because of the very low numbers of this type of star, or because WO stars develop by a different mechanism.

WNh stars

Wolf Rayet stars are united as a group by the prominent emission lines of helium, carbon, nitrogen, and oxygen, and by the comparative lack of any signs of hydrogen in their spectra. However, one group of WR stars do show significant hydrogen. These are the WNh, or WNLh, stars. They show little carbon or oxygen, so there are no "WCh" stars known, and they are generally relatively cool, hence the "late" designation WNLh. Types such as WN9h are the most common, although they have been identified as early as WN5h.

In contrast to the other WR stars, these are not highly evolved stars that have exhausted hydrogen in their cores. They show helium and nitrogen fusion products at their surface because of vigorous convection which dredges up these elements from the core even while the core is still burning hydrogen. This occurs only in the most massive stars, and most likely only in combination with rapid rotation. WNh stars are both initially more massive and have lost relatively little mass compared to other WR stars, hence they are amongst the most luminous stars known. They have similar spectra to the slash stars which also show helium and nitrogen lines but are more obviously regular supergiant star. There are also intermediate cases, and most likely a continuum as massive stars rapidly evolve away from the main sequence.[23]

Examples

The most visible example of a Wolf–Rayet star is Gamma 2 Velorum (γ² Vel), which is a naked eye star for those located south of 40 degrees northern latitude. Due to the exotic nature of its spectrum (bright emission lines in lieu of dark absorption lines) it is dubbed the "Spectral Gem of the Southern Skies".[24] There are no other naked eye Wolf–Rayet stars.

The most massive star and probably most luminous star currently known, R136a1, is also a Wolf–Rayet star of the WNh type indicating it has only just started to evolve away from the main sequence. This type of star, which includes many of the most luminous and most massive stars, is very young and usually found only in the centre of the densest star clusters. Occasionally a runaway Wolf–Rayet star such as VFTS 682 is found outside such clusters, probably having been ejected from a multiple system or by interaction with other stars.

See also

References

External links

  • Aperture Masking Interferometry observations.
  • harvard.edu Wolf–Rayet Stars: Spectral Classifications
  • astro.lsa.umich.edu ApJ 525:L97-L100 Nov. 10, 1999. Monnier, Tuthill & Danchi: Pinwheel Nebula Around WR98a (PDF)
  • uk.arxiv.org ApJ Jan. 3,2005. Dougherty, et al.: High Resolution Radio Observations of the Colliding Wind Binary WR140 (PDF)
  • harvard.edu A catalog of northern Wolf–Rayet Stars and the Central Stars of Planetary Nebulae (Harvard)
  • nytimes.com Scientists See Supernova in Action
  • nasa.gov Big Old Stars Don't Die Alone (NASA)