Tuesday, October 22, 2013

A Very Rare Type of Wolf-Rayet Star

Massive stars with more than 20 times the Sun’s mass are exceedingly rare. These stars are estimated to be as rare as ~1 in 1,000,000, possibly rarer. Nature does not like to make massive stars. Those that are formed, burn fast, shine bright and have brief lives of just a few million years. Despite their rarity, massive stars come in a very diverse range of stellar types. Some of these stellar types are so uncommon that they are represented by only a few known individual stars in a galaxy containing many billions of stars.

The Large Magellanic Cloud (LMC) is an irregular galaxy and also a satellite galaxy of the Milky Way. It is estimated to contain about 10 billion times the mass of the Sun. A region of the LMC known as Lucke-Hodge 41 (LH41, also known as NGC 1910) has a rich population of massive stars. This region is home to a high concentration of very rare stars including a few luminous blue variables (LBV), a yellow supergiant, a few Wolf-Rayet (WR) stars, as well as a number of other stellar oddities.  These stars are all massive stars at various stages of evolution.

WR stars represent advanced evolutionary stages in the evolution of massive stars. These stars shed mass rapidly by means of a very powerful stellar wind. WR stars are extremely hot and can have surface temperatures as high as ~200,000 K. They are also extremely luminous, ranging from tens of thousands to a few million times the luminosity of the Sun. Due to the extreme mass loss, WR stars are basically stripped-down versions of highly evolved massive stars. These stars have lost the bulk of their outer envelopes to reveal the products of nucleosynthesis in their interiors.

All WR stars show helium emission lines in their spectra. Depending on their evolutionary stage, the spectra of WR stars are dominated by emission lines of nitrogen, carbon or oxygen. As such, WR stars are classified accordingly as WN, WC or WO types. WO stars are believed to be WR stars that have evolved past the WC stage. They represent the most advanced and short-lived evolutionary stage in the life of a massive star before it explodes as a supernova.

While surveying the population of massive stars in LH41, a team of astronomers discovered a new type of WR star with strong enough emission lines from high ionization stages of oxygen and carbon to be classified as a WO star. This newly discovered star is identified as LH41-1042 and further classified as a WO4 star. LH41-1042 is the second known WO star in the LMC and its first known WO4 star, the other being a WO3 star.

Although most of the WR stars in the LMC have already been discovered, a number of WR stars might still await discovery in the most crowded regions of the LMC. Since so few WO stars are currently known, the discovery of another would provide a good opportunity to better understand these short-lived and thus exceedingly rare stars.

Kathryn F. Neugent et al. (2012), “The Discovery of a Rare WO-type Wolf-Rayet Star in the Large Magellanic Cloud”, The Astronomical Journal 144: 162 (4pp)