Giant supernovae raise new questions for astronomers
An international research team has made a breakthrough discovery in astronomical research: the mass of a particular type of supernova exceeds what was previously believed to be its limit. Their findings, soon to be published in The Astrophysical Journal, could change the way that cosmologists measure how the universe is expanding and may provide a new structural model to better understand supernovae. The research was supported in part by a Marie Curie grant from the EU's Seventh Framework Programme (FP7). Supernovae are stellar explosions that send a burst of radiation and luminosity throughout a galaxy and can last for several weeks or months before fading. Type 1a supernovae are usually used as a cosmic ruler to measure distances to supernovae's host galaxies. Cosmologists also use them to try to understand the way the universe expanded in the past and how it is likely to do so in the future, and to explore the nature of dark energy. Type 1a supernovae, called white dwarfs, are caused by the explosion of dead cores of stars. Scientists initially thought that white dwarfs were not able to exceed the Chandrasekhar limit, a critical mass that equals about 1.4 times that of the Sun, before it explodes in a supernova. The Chandrasekhar limit is a vital tool for cosmologists to measure distances to supernovae. However, since 2003 four particularly bright supernovae have been observed. Dubbed the 'super Chandrasekhar supernovae', their appearance has triggered speculation on whether they exceeded the Chandrasekhar limit. To investigate this, the Yale University-led research team of US and French physicists called the Nearby Supernova Factory measured the mass of the white dwarf star responsible for one of the extra bright supernovae (called SN 2007if). Telescopes in Chile and the US states of Hawaii and California confirmed that SN 2007if had a mass 2.1 times greater than that of the Sun which means it exceeded the Chandrasekhar limit. The star was also found to have had a central mass, a shell of material that was expelled during the explosion and a surrounding mass of pre-existing material. The team's work on measuring masses for the entire star system will help cosmologists develop a better understanding of how the super supernovae develop. 'We don't really know much about the stars that lead to these supernovae,' commented team leader Professor Richard Scalzo from Yale University. 'We want to know more about what kind of stars they were and how they formed and evolved over time.' Professor Scalzo believes that SN 2007if may have been created by the merging of two white dwarfs rather than a single explosion of one. The researchers now intend to study the other super Chandrasekhar supernovae to try to ascertain if they too were created by more than one white dwarf. Cosmologists are currently exploring how stars with masses above the Chandrasekhar limit could exist without collapsing under their own weight. If it is found that these immense supernovae have different rules of physics this could be groundbreaking for future cosmological research. 'Supernovae are being used to make statements about the fate of the universe and our theory of gravity,' said Professor Scalzo. 'If our understanding of supernovae changes it could significantly impact our theories and predictions.'
Countries
Chile, France, United States