Reaching for the (invisible) stars

“There are far more hydrogen-poor supernovae than our current models can explain. Either we can’t detect the stars that evolve this way, or we have to revise all our models,” says ISTA Assistant Professor Ylva Götberg. She and Maria Drout, an Associate Assistant Professor at the Dunlap Institute for Astronomy & Astrophysics at the University of Toronto, Canada, pioneered this research. “Single stars usually explode as hydrogen-rich supernovae. The fact that they are hydrogen-poor means that the progenitor star must have lost its thick, hydrogen-rich envelope. This happens naturally in about a third of all massive stars through the stripping of the envelope by a binary star,” says Götberg. Now, Götberg and Drout have combined their expertise in theoretical modeling and observation to track down the missing stars. Their search is successful: They have documented a novel stellar population that finally fills a major gap in our knowledge and sheds light on the origin of hydrogen-poor supernovae.

The stars Götberg and Drout are searching for occur in pairs: each star is interlocked with a companion star in a binary system. Some binary systems are well known to us Earthlings: these include the brightest star in our night sky, Sirius A, and its fainter companion, Sirius B. The Sirius binary system is only 8.6 light-years from Earth—a stone's throw in cosmic terms. This explains the observed brightness of Sirius A in our night sky.

Astrophysicists assume that the missing stars originally formed from massive binary systems. In a binary system, the stars orbit each other until the thick, hydrogen-rich envelope of the more massive star expands. Eventually, the expanding shell is attracted more strongly to the companion star than to its own core. This triggers a mass transfer that ultimately leads to the shedding of the entire hydrogen-rich shell, exposing the hot and compact helium core—more than ten times hotter than the Sun's surface. This is precisely the kind of star Götberg and Drout are looking for. "Scientists already suspected that intermediate-mass helium stars formed through binary interactions play an important role in astrophysics. Yet such stars have not been observed until now," says Götberg. Indeed, there is a large mass gap between the known classes of helium stars: the more massive Wolf-Rayet (WR) stars have more than ten times the mass of the Sun, while the low-mass subdwarf stars might have about half the Sun's mass. According to models, however, the progenitors of hydrogen-poor supernovae after mass transfer range between 2 and 8 solar masses.