Oxygen Wolf-Rayet (WO) stars represent a very rare late stage in the evolution of massive stars. Their spectra show strong emission lines of helium-burning products, in particular highly ionized carbon and oxygen. They can teach us about the final evolutionary stages of massive stars as they are on the verge of exploding as supernovae.
We use the X-Shooter instrument on the Very Large Telescope in Chili investigate several WO stars. We find that the stars are extremely hot, 150 kK to 210 kK, more than 100,000 times brighter than the sun, with surfaces rich in oxygen and carbon (helium mass fractions less than 44% -14%). They lose mass at a rate that is 2-3 times higher than predicted.
We conclude that WO stars indeed represent the final evolutionary stage of very massive star that start their live with masses of about 40−60M⊙. They are in their final evolutionary stages (post core-helium burning) and predicted to explode as “type Ic supernovae” within just a few thousand years.
Tramper et al. (2015), Accepted for publication in Astronomy & Astrophysics.