At least a quarter of supernovae eject sub-Chandrasekhar masses
Despite their ongoing use as tools for cosmology, type Ia supernovae (SNe Ia) are still not fully understood physically. In the classic scenario, a white dwarf and a larger star orbit each other, with the white dwarf gradually accreting material from its companion until reaching the Chandrasekhar limiting mass (1.4 times the mass of our Sun) when it is expected to become unstable and explode. This scenario is now being actively challenged, due to tight observational limits on signatures of the accretion process (such as X-ray emission from host galaxies or hydrogen lines in SN Ia spectra) and to the low predicted rate of Chandrasekhar-mass explosions. Alternative models, such as white dwarf mergers or collisions, may eject different amounts of material.
Previous observational work by CAASTRO Associate Investigator Dr Richard Scalzo (ANU) has shown that for “normal” SNe Ia used to study the cosmological dark energy, ejected mass correlates strongly with rate of luminosity decline after peak. It also confirms that the mass of radioactive nickel-56 synthesised in the explosion correlates with the peak luminosity. The relation between decline rate and luminosity, used to calibrate SNe Ia as “standard candles” in cosmology, may thus reflect an underlying relation between ejected mass and nickel mass, giving a clue to the explosion mechanism.
In a recent paper with CAASTRO co-authors Dr Ashley Ruiter (ANU) and Dr Stuart Sim (Queens University Belfast), Dr Scalzo has exploited these relations to measure the detailed distribution of ejected masses for a sample of over 300 type Ia supernovae used in cosmological analyses. While the distribution has a sharp peak at the Chandrasekhar mass, it also shows a long tail towards lower masses and very few (< 2%) high-mass events. This picture shows for the first time that both Chandrasekhar-mass and sub-Chandrasekhar-mass explosions occur at significant rates. While white dwarf mergers or detonations of low-mass white dwarfs could explain some of the distribution’s tail, some events with inferred sub-Chandrasekhar ejected masses are fainter than predicted in these scenarios – suggesting that these models need revision to explain the SN Ia calibration relation.
R. A. Scalzo, A. J. Ruiter, S. A. Sim in MNRAS 445 (2014) “The ejected mass distribution of Type Ia supernovae: a significant rate of non-Chandrasekhar-mass progenitors”