Clues to origin of luminous supernovae may lie in ultraviolet
17 March 2015
The widespread use of type Ia supernovae (SNe Ia) in cosmology, as one of the farthest rungs in the extragalactic distance ladder and as tools to study dark energy, depends on the accuracy with which their luminosity can be measured. The classic luminosity calibration relations used in cosmological studies apply only to SNe Ia with "normal" spectra. However, wide-field supernova searches (including CAASTRO's SkyMapper survey) are now revealing the true observational diversity of SNe Ia, uncovering a rare, ultraluminous subclass of SNe Ia which do not obey the calibration relations.
ANU-based CAASTRO Associate Investigator Dr Richard Scalzo's previous work on this subclass provides strong evidence that their ejected masses exceed the Chandrasekhar limiting mass for white dwarfs, justifying the commonly used label "super-Chandra". Since all type Ia supernovae are believed to be explosions of white dwarfs, super-Chandra SNe Ia provide challenges to our understanding of white dwarf physics and stellar evolution. Super-Chandra SNe Ia are not only very luminous, but very blue – suggesting strong ultraviolet (UV) emission, which could arise from a shock driven by the supernova ejecta into a cloud of material surrounding the progenitor. Such clouds are also predicted by models of white dwarf mergers, and could explain the high luminosities of super-Chandra SNe Ia.
With a spectrum resembling other super-Chandra SNe Ia, LSQ12gdj was discovered just a few days after explosion – making it an excellent test case to search for UV emission from shocks. Dr Scalzo, the ANU group, and their European and American collaborators observed LSQ12gdj with the Swift space telescope as well as ground-based optical telescopes. Early in its evolution, over a quarter of LSQ12gdj's luminosity was emitted at UV wavelengths visible only to Swift (compared with 5-10% for normal SNe Ia). However, no more than 10% of LSQ12gdj's peak luminosity is likely to come from shocks, so any material surrounding the progenitor must be very compact. When all this is taken into account, LSQ12gdj's appearance is consistent with a Chandrasekhar-mass progenitor – showing that UV observations are crucial to understand these events fully.
R. A. Scalzo, M. Childress, B. Tucker, F. Yuan, B. Schmidt, P. J. Brown, C. Contreras, N. Morrell, E. Hsiao, C. Burns, M. M. Phillips, A. Campillay, C. Gonzalez, K. Krisciunas, M. Stritzinger, M. L. Graham, J. Parrent, S. Valenti, C. Lidman, B. Schaefer, N. Scott, M. Fraser, A. Gal-Yam, C. Inserra, K. Maguire, S. J. Smartt, J. Sollerman, M. Sullivan, F. Taddia, O. Yaron, D. R. Young, S. Taubenberger, C. Baltay, N. Ellman, U. Feindt, E. Hadjiyska, R. McKinnon, P. E. Nugent, D. Rabinowitz, E. S. Walker in MNRAS 445 (2014) "Early ultraviolet emission in the Type Ia supernova LSQ12gdj: No evidence for ongoing shock interaction"