Wolf-Rayet Stars

Massive stars: Those stars that start their lives on the main sequence above 8 solar masses are called massive stars. They end their relatively short lives in catastrophic implosions/explosions as core-collapse supernovae. They represent the most massive and luminous stellar component in the Universe and contain the internal furnaces in which the lion's share of the heavy chemical elements are fused and ejected. Evolving rapidly, they drive the chemistry, structure and evolution of galaxies, dominating the ecology of the Universe - not only as supernovae, but also during their entire lifetimes, with far-reaching consequences. All this despite their relatively small numbers compared to other stars.

Wolf-Rayet Stars: At the end of their evolution and before the final supernova, the most massive of massive stars (let's call them "very massive stars"), initially above about 25 solar masses, pass through an advanced phase of nuclear burning when the central fuel is He, i.e. the ashes of the original H-burning in the stellar core. Such stars are generally more compact and hotter than their progenitors, due partly to the contraction of the original stellar core leading to higher temperatures and densities needed to drive He-burning, and partly to their very strong winds that removed their outer originally less compact layers. These are the classical Wolf-Rayet stars, the last phase in the evolution of a very massive star before the final SN explosion.

The observational point of view: WR stars distinguish themselves by their strong, broad emission lines, as opposed to the narrow photospheric absorption lines seen in most normal stars like the Sun. These emission lines, which can reach 100 Angstroms in width, are formed by the Doppler-shifted sum of emission from various species of ionized atoms in the expanding winds around a stellar core, which is normally hidden from view by the dense wind. Atmospheric and internal models of WR stars suggest that the star beneath the wind is very hot and compact, with surface temperatures in the range of 30,000 to 150,000 K in the hottest cases, while the surrounding winds are always cooler than this.

WN & WC: The optical spectra of WR stars show two basic "flavours": (1) WN stars, which exhibit emission lines with enhanced He and N abundances, believed to arise via CNO-cycle fusion of H into He having taken place in the original stellar core (now lacking H) and brought to the surface, and (2) WC/WO stars, with enhanced He, C and O abundances (but no N), from triple-alpha He-fusion. Most of these are He-burning stars, with WN occurring when the envelope has removed enough outer unprocessed material via the strong wind to reveal H-burning core products, followed later by He-burning products as WC/WO. As with normal stars, each WR sequence is characterized by a temperature index from low values (hot) to higher values (less hot), e.g. WN2 to WN9, WC4 to WC9, WO1 to WO4.

There are at least three circumstances where the WR phenomenon arises:

  1. classical WR stars at the end of the evolution of stars with initial masses above about 25 solar masses (the actual limit depends on the initial atomic abundances, here assumed to be ~solar),

  2. WNH stars at the beginning of the life of a very luminous star on or near the H-burning main sequence for initial masses above about 80 solar masses,

  3. and the central stars of about 10% of Planetary Nebulae (WC only, denoted [WC]), where the initial masses were in the range 1-8 solar masses.

We will concentrate on 1., classical WR stars at the end of the evolution of stars which are also the most frequent.