Wolf–Rayet star

HD 184738, also known as Campbell’s Star. This is actually a planetary nebula and the central star is an old low mass star unlike the main class of young massive Wolf-Rayet stars.^[2]

Wolf–Rayet stars (often referred to as WR stars) are evolved, massive stars (over 20 solar masses when they were on the main sequence) which are losing mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s. They typically lose 10⁻⁵ solar masses a year, a billion times higher than the sun.^[3] Wolf–Rayet stars are extremely hot, with surface temperatures in the range of 30,000 K to around 200,000 K.^[1] 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.

The Gamma Velorum system contains the brightest Wolf Rayet star in the sky, while Theta Muscae is the second brightest example. The extreme massive luminous star R136a1 in 30 Doradus is also a Wolf Rayet star.

Observation history

M1-67 is the youngest wind-nebula around a Wolf-Rayet star, called WR124, in our Galaxy.
Credit: ESO

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 (HD 191765, HD 192103 and HD 192641, now designated as WR 134, WR135, and WR137 respectively) that displayed broad emission bands on an otherwise continuous spectrum.^[5] Most stars only display absorption lines or bands in their spectra, as a result of overlying elements absorbing light energy at specific frequencies, 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] Pickering noted similarities between Wolf-Rayet spectra and nebular spectra, and this similarity led to the conclusion that some or all Wolf Rayet stars were the central stars of planetary nebulae.^[7]

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.^[8] It was well known that many stars with Wolf Rayet type spectra were the central stars of planetary nebulae, but also that many were not associated with an obvious planetary nebula or any visible nebulousity at all.^[9]

In addition to helium, emission lines of carbon, oxygen and nitrogen were identified in the spectra of Wolf–Rayet stars.^[10] 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.^[11]

Classification

Spectrum of WR137, a WC7 star^[1] and one of the three original WR stars

Wolf–Rayet stars were named on the basis of the strong broad emission lines in their spectra, identified with Helium, Nitrogen, Carbon, Silicon, and Oxygen, but with Hydrogen lines usually weak or absent. The first system of classification split these into stars with dominant lines of ionised Nitrogen (N_III, N_IV, and N_V) and those with dominant lines of ionised carbon (C_III and C_IV) and sometimes Oxygen (O_III - O_VI), referred to as WN and WC respectively.^[9] The two classes WN and WC were further split into temperature sequences WN5-WN8 and WC6-WC8 based on the relative strengths of the 541.1nm He_II and 587.5 nm He_I lines. Wolf Rayet emission lines frequently have a broadened absorption wing (P Cygni profile) suggesting circumstellar material.

The WN spectral sequence has been expanded to include WN2 - WN9, and the definitions refined based on the relative strengths of the N_III lines at 463.4-464.1 nm and 531.4 nm, the N_IV lines at 347.9-348.4 nm and 405.8 nm, and the N_V lines at 460.3 nm, 461.9 nm, and 493.3-494.4 nm.^[12] These lines are well separated from areas of strong and variable He emission and the line strengths are well correlated with temperature. Stars with spectra intermediate between WN and Ofpe have been classified as WN10 and WN11 although this nomenclature is not universally accepted.

The WC spectral sequence has been expanded to include WC4 - WC9, although some older papers have also used WC1 - WC3. The WO types WO1 - WO4 have also been added for even hotter stars where emission of ionised Oxygen dominates that of ionised Carbon, although the actual chemical abundances in the stars are likely to be comparable. The primary emissions lines used to distinguish the WC sub-types are C_II 426.7 nm, C_III at 569.6 nm, C_III/IV465.0 nm, C_IV at 580.1-581.2 nm, and O_V at 557.2-559.8 nm. For WO stars the main lines used are C_IV at 580.1 nm, O_IV at 340.0 nm, O_V at 557.2-559.8 nm, O_VI at 381.1-383.4 nm, O_VII at 567.0 nm, and O_VIII at 606.8 nm.^[13] The division between WC and WO spectra is easily made based on the presence or absence of C_III emission.

Detailed modern studies of Wolf Rayet stars can identify additional spectral features, indicated by suffixes to the main spectral classification:

• h for hydrogen emission;
• ha for hydrogen emission and absorption;
• w for wide lines;
• s for narrow (sharp) lines;
• d for dust (occasionally vd, pd, or ed for variable, periodic, or episodic dust).

The classification of Wolf Rayet spectra is complicated by the frequent association of the stars with dense nebulosity, dust clouds, or binary companions. A suffix of "+ abs" is often used to indicate the presence of absorption lines in the spectrum, likely to be associated with a more normal companion star.

The hotter WR spectral sub-classes are described as early and the cooler ones as late, consistent with other spectral types. WNE and WCE refer to early type spectra while WNL and WCL refer to late type spectra, with the dividing line approximately at sub-class six or seven. There is no such thing as a late WO star. There is a strong tendency for WNE stars to be hydrogen-poor while the spectra of WNL stars frequently include hydrogen lines.^[13][14]

Physical properties

AB7 is one of the highest excitation nebulae in the Magellanic Clouds, two satellite galaxies of our own Milky Way

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.^[15][16] 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.^[17] Additionally, about 100 are known in the Large Magellanic Cloud,^[18] while only 12 have been identified in the Small Magellanic Cloud.^[19] Many more are known in the galaxies in the Local Group, such as M33, where 206 Wolf–Rayet stars are known,^[20] and M31, where 154 Wolf–Rayet stars are known.^[21]

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).^[22]

It can be seen that the WNh stars are completely different objects from the WN stars without hydrogen. Despite the similar spectra, they are much more massive, much larger, and some of the most luminous stars known. They have been detected as early as WN5h in the Magellanic clouds.

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,^[15] as is the case of the famous binary WR 104; however this process occurs on single ones too.^[1]

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. These are now generally excluded from the class denoted as Wolf–Rayet stars, or referred to as Wolf-Rayet type stars.^[25]

Evolution

Theories about how WR stars form, develop, and die have been slow to form compared to the explanation of less extreme stellar evolution. They are rare, distant, and often obscured, and even into the 21st century many aspects of their lives are unclear.

Early ideas

WR 136 is WN6 star where the atmosphere shed during the red supergiant has been shocked by the hot fast WR winds to form a visible bubble nebula

Several astronomers, among them Rublev (1965)^[26] and Conti (1976)^[27] 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. This has proved to be essentially correct, but with much complexity between a main sequence O star and the final WR star.

Early modelling of the evolution of massive stars showed that they evolve away from the main sequence, not towards hotter temperature and a WR state, but by expanding and cooling to become blue and then red supergiants. These supergiants are only modestly more luminous than the main sequence stars they originate from, but are progressively more unstable as their cores become hotter and their atmospheres more extended. Simple models of nuclear fusion showed that these red supergiants burned heavier elements in their cores until exploding as a supernova, but not becoming WR stars.

Further models showed that there was an upper limit to the stability of luminous stars. Sufficiently massive stars do not become red supergiants, instead shedding their atmospheres so quickly that they remain as blue supergiants, eventually shedding their atmospheres completely and entering the "Wolf Rayet funnel", an area of the HR diagram where WR stars become progressively smaller and hotter as they shed more and more of their outer layers.^[28][29] The suggestion was that earlier and hotter stars were the later stages of evolution from the later and cooler WR stars, but the results of this evolutionary sequence didn't match observations very well.

Current models

Most WR stars are now understood as being at a natural state in the evolution of the most massive stars (not counting the less common planetary nebula central stars), either after a period as a red supergiant, after a period as a blue supergiant, or directly from the most massive main sequence stars.^[30] Only the lower mass red supergiants are expected to explode as a supernova at that stage, while more massive red supergiants progress back to hotter temperatures as they expel their atmospheres. Some explode while at the yellow hypergiant or LBV stage, but many become Wolf Rayet stars.

Massive main sequence stars create a very hot core which fuses hydrogen via the CNO process and results in strong convection throughout the whole star. This causes mixing of fused elements to the surface, a process that is enhanced by rotation, possibly by differential rotation where the core is spun up to a faster rotation than the surface. Such stars show nitrogen at their surface at a very young age, combined with strong stellar winds. These stars develop an Of spectrum, Of* if they are sufficiently hot, which develops into a WNh spectrum as the levels of nitrogen at the surface increase. This explains the high mass and luminosity of the WNh stars, which are still burning hydrogen at the core and have lost little of their initial mass. These will eventually expand into blue supergiants (LBVs?) as hydrogen at the core becomes depleted, or if mixing is efficient enough (e.g. through rapid rotation) they may progress directly to WN stars without hydrogen.

Observations of supernova revealed that around a quarter of core collapse supernovae are of Type Ib, which originates from a progenitor with almost no hydrogen, and Type Ic, which originates from a progenitor with almost no hydrogen and very little helium. This corresponded rather well to WC and WO stars and as this was investigated it appeared that WR stars were likely to end their lives violently rather than fade away to a neutron star. Thus every star with an initial mass more than about 9 times the sun would inevitably result in a supernova explosion, many of them from the WR stage.^[25][30][31]

The simple progression of WR stars from low to hot temperatures, resulting finally in WO stars, is not supported by observation. WO stars are extremely rare and all the known examples are more luminous and more massive than the relatively common WC stars. Alternative theories suggest either that the WO stars are only formed from the most massive main sequence stars,^[1] or that they form an extremely short-lived end stage of just a few thousand years before exploding.^[30][32][33]

Key:

• O: O-type main-sequence star
• Of: evolved O-type showing N and He emission
• BSG: blue supergiant
• RSG: red supergiant
• YHG: yellow hypergiant
• 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

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.^[34] 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, and explains why type Ibc supernovae are less common than type II. WNh stars, spectroscopically similar but actually a much less evolved star which has only just started to expel its atmosphere, are an exception and still retain much of their initial mass. 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. An alternative explanation is that these stars are so massive that they could not form as normal main sequence stars, instead being the result of mergers of less extreme stars.

Supernovae

Although it is widely accepted that most or all type Ibc supernovae progenitors were WR stars, no conclusive identification has been made of such a progenitor.^[33] Although WR stars are luminous they are not visually bright, especially the hottest of them that are expected to be supernova progenitors and theory suggests that the progenitors of type Ibc supernovae observed to date would not be bright enough to be detected, although they place constraints on the properties of those progenitors. One candidate is under observation as pre-outburst observations show a likely WR star at the site of iPTF13bvn.^[35]

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.

Examples

Hubble Space Telescope image of nebula M1-67 around Wolf–Rayet star WR 124.

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".^[36] The second brightest is Theta Muscae.^[37]

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

• Gamma-ray burst
• Hypernova
• Starburst galaxy
• Wolf–Rayet nebula
• WR 104

References

External links

Wikimedia Commons has media related to Wolf-Rayet stars.

• physics.usyd.edu.au 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.
• 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)

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