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Wolf-Rayet stars represent an evolutionary phase in the lives of massive stars during which they undergo heavy mass loss. They are characterised by an extraordinary spectrum which is dominated by emission lines of highly ionised elements.
Keywords: Wolf-Rayet, W-R, WR stars
Wolf-Rayet stars are evolved, massive, extremely hot (up to ~50,000 K) and very luminous (105 to 106 L¤). They are extremely rare, reflecting their short lifespan.
Their surface composition is extremely exotic, being dominated by helium rather than hydrogen, and typically showing broad wind emission lines of elements like carbon (WC type), nitrogen (WN type), or oxygen: the products of core nucleosynthesis. The presence or absence of hydrogen, respectively, is used to distinguish the so-called ‘late’ type WN stars (WNL) from the ‘early’ (WNE) types.
"The dense stellar winds completely veil the underlying atmosphere so that an atmospheric analysis can only be done with dynamical, spherically extended model atmospheres, such as those developed by Hillier (1991), Hamann (1994), and Schmutz (1991). Significant progress has been achieved in that respect so that W-R stars can be placed on the HRD with some confidence (see e.g., Hamann, Koesterke, & Wessolowski 1993). Almost all of the Galactic WNL stars have an observable amount of hydrogen at their surface; some have more than 50% hydrogen (Hamann et al. 1991; Crowther et al. 1995). This property of WNL stars is opposed to other W-R subtypes. The ionising spectrum of WNL stars is very similar to what is observed in luminous O stars (Esteban et al. 1993). Despite their similar luminosities and effective temperatures, W-R and O stars differ drastically in their masses. The most luminous O stars have current masses around 80 M¤ (see, e.g., Kudritzki et al. 1991), whereas WNL stars in binary systems have an average mass of 20 M¤ (Cherepashchuk 1991). Standard evolutionary models assume that heavy mass loss reduces the mass of O stars, so that W-R stars are the low mass descendants of previously massive O stars (Maeder 1990). The close relationship between WNL and extremely luminous O supergiants is also suggested by the similar spectral morphology of the least extreme WNL stars and the most extreme O stars (Walborn 1974; Walborn et al. 1985, 1992)" (Nota et al. 1996, p. 384).
|The first spectral studies of Wolf-Rayet objects were undertaken by the French astronomers, C. Wolf and G. Rayet, after whom the class is named. Instead of dark absorption lines, the stellar winds give rise to strong emission bands of highly ionised elements (Conti & Massey 1989). Lines are broadened in the hottest subtypes, which is believed to be correlated with v¥.||
|Their surface composition is highly anomalous, being dominated by helium rather than hydrogen. The latter is apparently absent in many cases. Three sequences and a transitional phase are recognised by Langer 1990:|
The spectral lines show high levels of ionisation including the presence of ionised helium, He+, but also C+3 or N+3, which suggests temperatures equal to or exceeding those of O-type stars.
|"As for ordinary stars, they are binned into subclasses, in which higher values mean later (i.e. cooler) types. Calibration of W-R stars in Galactic open clusters and in the Large Magellanic Cloud, yields a very tight correlation between Mv and subclass [see van der Hucht et al. 1988]. ... With mean bolometric correction||
|Mbol - Mv » -4.5 ± 0.2||(1)|
|for most W-R subclasses (Smith and Maeder 1988), this implies that the total luminosity also increases systematically from earlier to later WN or WC subtypes (e.g. Mbol » -11 for WN7 to -8 for WN4 or Mbol » -9 for WC8 to -8 for WC5)" (Moffat et al. 1989, p. 230).|
|Thus the hottest W-Rs are the least brilliant; on
the H-R diagram, the W-R zone slopes downward to the left and narrows like a funnel as
shown in fig 2. (After Moffat et al. 1989, p. 233.)
Note that the positions plotted in the figure refer to the cores of the stars [® sidebar].
There are some curious occurrence trends associated with W-Rs. Few or no WN8/9 stars are known to occur in WR+O binary systems, as opposed to nearly 60% of WN6/7 stars which do. Known orbital periods range from 1.6 days to 2886 days (Cherepashchuk 1992, p. 124). Shorter period binaries emit more X-radiation, probably from shock waves formed as the O-star ploughs through the W-R wind, or, more particularly, by blobs of matter ejected by the W-R object entering the photosphere of the O-star. "About half of the well-investigated W-R+O binaries are eclipsing with the amplitude of photometric variability DV = 0m.025 to 0m.5" (Cherepashchuk 1992, p. 124).
WN8/9 stars rarely occur in clusters; again as opposed to WN6/7 stars which do. Noting that particularly massive LBVs such as h Carinae occur in young clusters together with WN7 types, Moffat et al. 1989 speculates (p.234) that WN8/9 stars may be derived from lower mass LBV progenitors.
Examples of Wolf-Rayet stars occurring in clusters include two in the ~3 million year old NGC 6231 in Scorpius, one in NGC 2359 in Canis Major, one (HD 148937) associated with NGC 6164-65 in Norma, and another (HD 192163) associated with NGC 6888 in Cygnus.
Mass and Luminosity
Wolf-Rayets are massive stars. Masses can generally only be estimated from binary systems, which are commonly W-R+O systems. For such systems, the mass ratio MW-R/MO, denoted q, increases for cooler W-R subtypes. Thus, hotter W-Rs tend to be less massive with respect to their O-type companions. Typical values for q range from ~0.2 for the hottest (WC5, see below) types to ~1 for cooler (WN7) stars. The spectral types of the O companions show no correlation with W-R subtype.
Typical masses are around 16 to 18 M¤ but the range is very great: from 5 M¤ to 48 M¤ or, in one case (WR 22, HD 92740), 77 M¤. Masses of the O star in W-R+O binary systems range from 14 to 57 M¤, with a mean of 33 M¤ (Cherepashchuk 1992, p. 123).
All subtypes of W-Rs show a correlation between variability and luminosity similar to other supergiants, that is increasing variability with higher luminosity which, in W-Rs, corresponds to cooler stars. Thus, as they become hotter, they become more stable. The microvariability time scale is in the order of one day.
Stellar Winds and Mass Loss
Not until the 1980s did it became clear that WR stars represent an evolutionary phase in the lives of massive stars during which they undergo heavy mass loss (Willis 1991). Their spectra indicate that the stars are embedded in luminous and turbulent shells of ejecta flowing outwards at speeds comparable to the expansion velocities of novae (Cherepashchuk 1992, p. 129, quotes values of n0 = 200 km s-1 and n¥ = 2200 km s-1), although, in the case of Wolf-Rayet stars, the "explosion" is on-going; the expanding shell is being constantly fed with material from the main body of the star, at rates of 10-5 to 10-4 M¤ per year.
|"Overall, observations indicate that WR winds are especially strong, and even optically thick to continuum scattering by electrons. Notably, inferred WR wind momenta M-dot.v¥ are generally substantially higher than for OB stars of comparable luminosity, placing them well above the OB-star line in the WML relation. In fact, the inferred ratio of the wind momentum to that of stellar luminosity, h º M-dot.v¥/(L/c), is typically much above unity, sometimes as high as h = 10 to 50" (Owocki 2000).|
|"Among the W-R stars, the luminous WNL subtypes (especially WN8) are the most variable, probably as a consequence of blob ejection in [their very strong stellar winds.] The underlying mechanism which triggers this ejection is possibly related to wind instabilities and may thus be quite different from the source of variability in luminous supergiants or LBVs in quiescence, where photospheric effects dominate" (Moffat et al. 1989, p. 229).|
|Some of the mass loss appears to occur as blobs of matter being ejected in all directions; if there is a preference for the equatorial plane it is not obvious from current observations. The presently favoured triggering mechanism for blob ejection in W-R stars is simply stochastic [random] instabilities in the stellar wind although there remains the problem that the fast winds appear to be more stable than slower winds, contrary to theory.|
|Cherepashchuk 1992 estimates that up to 80% of the mass lost by W-R stars may be in the form of blobs ("dense and compact clouds of matter" p. 127).|
The Wolf-Rayet progenitor stars and formation processes are not yet clearly understood. The presence of ‘core material’ in their surface layers suggests two formation scenarios: extensive mixing of the inner and outer layers by some means, or uncovering of the convective core of massive stars through deep erosion of their outer layers.
Various mixing mechanisms have been suggested, including differential rotation in fast spinning massive stars and large increases of the convective core size ("convective overshooting," see Langer 1990 for more detailed discussion and references).
In the second scenario, Wolf-Rayet stars are essentially the naked cores of massive stars which have sloughed off their outer layers. The progenitor stars might be red supergiants, luminous blue variables or possibly binary stars which have lost their outer envelope to a close companion through Roche-lobe overflow. While the last possibility may be relevant in some cases (many WR stars are binaries or higher multiples; see table 1) it cannot be considered a general solution since many Wolf-Rayet stars are believed to be single.
It appears that W-Rs can evolve from any sufficiently massive star. It is feasible that the most massive stars, ~120 M¤, may lose their original envelope during H burning, evolving directly from Of to W-R stages (Maeder 1989, p.16), or perhaps via a short-lived intermediate Ofpe/WN9 phase.
Theoretical models for main sequence stars – even those as massive as 100 M¤ – suggest that they can evolve to cool surface temperatures after core hydrogen is exhausted, provided mixing is restricted (Langer 1990 proposes that molecular weight gradients might impede convection).
A cool surface may initiate a high mass loss rate. -- Langer doesn’t actually say this
|Red supergiants have the requisite cool surface
and those more massive than about 50 M¤
may be Wolf-Rayet progenitors. Whether some of them evolve also through
a luminous blue variable (LBV) phase, and whether this would be before or
after the red supergiant phase, is unknown, although stellar evolution in
this part of the H-R diagram almost certainly encounters the Humphreys-Davidson
Limit. WN8 stars are close to LBVs on the H-R diagram and possess some
common features, namely a high degree of variability, narrow emission lines,
and high luminosity. On the other hand, the time scales of the variations
are quite different: in the order of one day for WN8 stars as opposed to
ten days or more in quiescent LBVs. In W-R stars the variability is a wind
phenomenon; in LBVs and red supergiants it is atmospheric.
WN7 stars are as luminous but hotter and more stable, perhaps on account of being farther from the H-D limit.
Stars around, or just below, 40 M¤ may become exotic red supergiants – perhaps OH/IR stars – and then W-Rs. Those well below ~40 M¤ probably do not encounter the H-D limit and probably do not become W-Rs either.
Wolf-Rayet stars which do evolve from LBVs must become Wolf-Rayet stars almost immediately at central helium ignition (since the LBV phase lasts only a very short time; in the order of 105 years).
Once Wolf-Rayet stars are formed, their subsequent evolution is dominated by their mass loss, since "the mass-loss time-scale is comparable to the [life time of the star] which means that the internal structure and surface properties of the star are neither those of the more massive star nor those of a main sequence star of lower mass.... In some cases most of the hydrogen-rich outer envelope of a star may be lost leaving a helium-burning star close to the helium main sequence" (Tayler 1994, p.176).
|"All of the above trends, together with
the high mass loss rates, favour subtype evolution from cooler to hotter
subtypes within each sequence. Our interpretation is as follows: all W-R
stars start as WNL, the most massive, luminous and least evolved of all
subtypes. As the strong wind peels off the outer layers, the surface abundance
gets more exotic with time and the core radius shrinks. Transfer from WN
to WC, basically a surface phenomenon, will occur at a given WN-subtype
which depends on the initial metallicity Z.... In short, extreme mass loss
forces W-R stars to evolve downwards (fainter) and to the left (hotter)
in the H-R diagram, more or less towards the He-ZAMS. This is no surprise
since W-R stars are helium burning, with hydrogen-rich surface impurities
that diminish with time" (Moffat et al. 1989, pp.232-233).
The WNL ® WNE ® WC regions on the evolutionary sequence seems to correspond to incomplete hydrogen burning, complete hydrogen burning, and incomplete helium burning, respectively.
The stars showing broadened emission lines are supposed to occupy the left side of the W-R ‘funnel’ (marked B in fig. 3) and thus are hotter for similar luminosity. It is presumed they may have evolved from more massive progenitors.
Once a star enters the funnel, it does not return red-ward (to the LBV region).
"With typical mass loss rates M » 4 ´ 10-5 M¤ yr-1 (e.g. Schmutz and Hamann 1986), the total mass lost in the W-R phase (mean lifetime t ~ 5 ´ 10-5 yr: Maeder and Meynet 1987) is D M = Mt ~ 20 M¤ (variations of ± a factor of 2 are possible), which is entirely compatible with the above interpretation. For example, a W-R star in a binary, going from q = 1.0 to 0.2, will lose D q ´ Mo » 24 M¤ for an average Mo = 30 M¤ O-type companion" (Moffat et al. 1989, pp.233).
"After the WNL stage, W-R evolution progresses downwards in the HR Diagram, as the stars lose more mass, shrink in size and head for the He-ZAMS" (Moffat et al. 1989, p.229).
"Just as massive supergiants show increasing variability as they approach the Humphreys-Davidson instability limit (horizontally in the HR Diagram), so the W-R stars show decreasing variability as they recede from the H-D limit (at first horizontally into the WNL domain, then, with their high mass loss rates, plunging irreversibly downwards as ever hotter, smaller and fainter, strong-line W-R stars)" (Moffat et al. 1989, p. 229).
Eventually the W-R star will run out of fusible material, ending its life as an early WC (WO) star in a type Ib supernova.
No WR star is so easily found as the bright naked eye (1m.7) Gamma2 (g2) Velorum, a famous visual multiple comprising a brilliant Wolf-Rayet (WC8) primary, possibly the nearest such star to us, and an unrelated magnitude 4, type B companion (g 1 Velorum). There are two wide companions, the more distant itself having a very close companion bringing the complement to five. Finally, the primary itself is a spectroscopic binary, the unseen component being a giant type O7 star.
Further examples of Wolf-Rayet stars include two occurring in the ~3 million year old NGC 6231 in Scorpius, one in NGC 2359 in Canis Major, HD 148937 (associated with NGC 6164-65 in Norma, about 4000 light years distant, see Malin 1993 p. 160) and HD 192163 (associated with NGC 6888 in Cygnus, also about 4000 light years distant, spectral type WN).
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