Oxygen sequence Wolf-Rayet (WO) stars are some of the rarest stars in the universe. To date, only nine WO stars are known, two of which are in binary systems. WO stars are remarkably hot, with surface temperatures ranging from 150,000 K to 210,000 K. For comparison, the Sun’s surface temperature is only 5,778 K. Core-helium burning is the process where helium in a star’s core undergoes nuclear fusion, leading to the production of oxygen and carbon. WO stars are massive stars that have passed the end of core-helium burning and have shed their outer envelopes through powerful stellar winds to reveal hotter underlying layers. Also, after a massive star exhausts the helium in its core, it naturally contracts and heats up. This is consistent with the high surface temperatures observed for WO stars.
WO stars are extremely luminous objects with hundreds of thousands of times the Sun’s luminosity. Despite their extreme luminosities, WO stars are generally smaller in size than the Sun. For example, the WO star WR102 is almost 300,000 times more luminous than the Sun, but its diameter is only 40 percent of the Sun’s. The intensely hot surfaces of WO stars are observed to be enriched with the products from core-helium burning, particularly carbon and oxygen. The helium surface mass fraction on WO stars is usually between 20 to 30 percent, but ranges from 44 percent for the least hot WO star to as low as 14 percent for the hottest known WO star. Observations have shown that the oxygen and carbon surface mass fractions can be as high as 24 and 62 percent, respectively, as in the case for the WO star WR102.
WO stars represent a very brief stage in the evolution of massive stars, predicted to be the final evolutionary stage of massive stars with initial masses between 40 to 60 times the Sun’s mass. These remarkable stars are likely to explode as type Ic supernovae in ~1,000 to 10,000 years. Type Ic supernovae are a class of stellar explosions caused by the core collapse of massive stars that have shed their outer envelopes of hydrogen and helium. As a result, type Ic supernovae do not contain hydrogen and helium. For comparison, type Ib supernovae are another class of stellar explosions involving massive stars that have only shed their outer envelope of hydrogen.
Locations of several WO stars on the Hertzsprung-Russell diagram. Also indicated are several WC stars (i.e. carbon sequence Wolf-Rayet stars). Tramper et al. (2015).
Model showing the evolution of the surface mass fractions of the WO star WR102 since the onset of core-helium burning. Tramper et al. (2015).
Tramper et al. (2015), “Massive stars on the verge of exploding: the properties of oxygen sequence Wolf-Rayet stars”, arXiv:1507.00839 [astro-ph.SR]