Supernovae over the limit

Type Ia supernovae—the powerful explosions of compact white dwarfs—look remarkably uniform. That uniformity, which makes them useful to cosmologists as standard candles for gauging distances across the universe, has been attributed to a common mechanism: When a white dwarf gaining mass from its binary companion nears the Chandrasekhar limit of 1.4 solar masses, it must contract and then explode. Several years ago researchers with the Supernova Legacy Survey spotted a supernova, SN 2003fg, that bore all the marks of being type Ia but appeared far too luminous to have had a Chandrasekhar-mass progenitor. Now three research teams—from the Nearby Supernova Factory (SNfactory), the University of California, Berkeley, and the universities of Tokyo and Hiroshima in Japan—have gathered more detailed data on two more type Ia supernovae, SN 2007if and SN 2009dc, that were at least as luminous as SN 2003fg and shared some of its other unusual features. Comparing their observations with published theoretical models, the Berkeley and SNfactory researchers found that each of their supernovae was likely the result of a merging pair of white dwarfs whose combined mass exceeded the Chandrasekhar limit. The too-bright supernovae are rare enough that they themselves probably won’t have much of an effect on cosmological measurements. But they do suggest that better understanding the physics of type Ia supernovae—and how to categorize them by their observed properties and explosion mechanisms—could yield a better standard candle. (R. A. Scalzo et al., Astrophys. J. 713, 1073, 2010 ; J. M. Silverman et al., http://arxiv.org/abs/1003.2417 ; M. Yamanaka et al., Astrophys. J. 707, L118, 2009.) —Johanna Miller