September 29, 2012
Evidence Shows Supernova Was Caused By A Yellow Supergiant
redOrbit Staff & Wire Reports - Your Universe Online
A team of Japanese researchers has discovered evidence that a yellow supergiant (YSG) was, in fact, the progenitor for a recently discovered supernova -- a discovery which they say raises serious questions regarding our understanding of the evolution of massive stars.
Melina Bersten of the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) and colleagues analyzed evidence and discovered that supernova SN2011dh, which is located in the nearby galaxy M51, was created by the explosion of a yellow supergiant star, not a red supergiant or a blue compact star as is usually the case.
The supernova, which was "one of the brightest and best studied supernovae of 2011" due to its proximity, "showed hydrogen lines in the spectrum at early times, and then evolved into a helium-dominated spectrum," the researchers said. Those are signs which tend to indicate that the progenitor star had lost the majority of its hydrogen-rich envelope before it exploded, they pointed out.
"By searching through archival images obtained with the Hubble Space Telescope before the supernova explosion, two groups of astronomers independently detected a source at a location closely matching that of the supernova," they said. "Photometry of this pre-supernova source is compatible with a YSG star. If it is indeed the progenitor of the supernova, the question arises as to how such a star would undergo an explosion."
Current evolution models suggest that stars that are massive enough to cause a supernova due to a core collapse should end their lives as either red supergiants (if they are in the low mass range) or as compact blue stars (if they are in the high mass range). The yellow supergiant phase, however, is a short-lived, intermediate stage during which supernova explosions are not expected to occur.
"Based on early optical emission and radio observations of SN2011dh, some authors claimed that the actual progenitor must have been a compact object. Therefore, the detected YSG star could have been a companion of the exploded star, or even an unrelated object that matched the projected supernova location by chance," the researchers said.
However, in the September 20 edition of the Astrophysical Journal, the authors state that they presented evidence that suggested that it was "an extended object, of radius compatible with that of a YSG star" that was responsible for the supernova, regardless of what previous theoretical models have claimed. During their research, they modeled early-time optical emission using hydrodynamical calculations, showing a light curve that could only have been produced by an exploding yellow supergiant.
Those findings led to two additional questions: how did the star lose the bulk of its hydrogen envelope, and how did a yellow supergiant such as this one explode in the first place? In order to solve those questions, Bersten and colleagues "carried out stellar evolution calculations for a system of two massive stars in a close orbit, in which phases of mass transfer occur."
"By assuming a system of stars with initial masses of 16 and 10 times the mass of the Sun, and an initial period of 125 days, they were able to obtain a configuration for the mass-donor star -- the one that eventually explodes -- that matches closely the observations of the YSG object in the pre-supernova images and has a core mass that is consistent with their hydrodynamic modeling," they explained. "Moreover, the amount of hydrogen left in the envelope of the exploding star was in the correct range to classify the supernova as type IIb. This is in contrast to a single star that evolves to a red supergiant."