Superluminous supernovae are a new and exotic class of stellar explosions, radiating up to 100 times more energy than normal supernovae. The origin of this luminosity and the properties of the progenitor stars have been shrouded in mystery. They may be powered by rapidly spinning magnetic neutron stars, accreting black holes, or huge amounts of radioactivity. However, what type of stars give rise to these, and why they occur exclusively in unusual dwarf galaxies is currently unexplained.
In a new study led by Dr Anders Jerkstrand published this month in the Astrophysical Journal ( " Long-duration Superluminous Supernovae at Late Times") several important new advances are presented.
Dr Jerkstrand is an expert in calculating models of supernovae years after they have exploded. When they expand and cool their spectra begin to reveal signatures of the elements inside the star that exploded. By taking deep spectra one to two years after the supernovae occurred we can get an insight into the inner layers of the progenitor. This requires sophisticated models of how radiation passes through the expanding gas and requires the latest atomic physics to be included in the detailed models. These computer models were run on the UK's DIRAC supercomputer. What made this study unique was the combination of state-of-the-art new models applied to the highest-quality data ever collected on these supernovae at such late times by the PESSTO survey with the European Southern Observatory's facilities.
The study reveals the first clear picture of the chemical composition of these explosions, by adding new high signal-to-noise observations to recalibrations of old ones, bringing the total number of observed supernovae to 4. The new spectra are demonstrated to have strong similarities with the class of gamma-ray burst powered supernovae, the first time this link has been established. Gamma-ray burst supernovae are though to arise by the formation of a black hole that punches a relativistic jet through the infalling star, or possibly by the formation of highly magnetic neutron stars.
In a second important discovery, the spectral synthesis models revealed that these superluminous supernovae contain the highest oxygen masses inferred for any supernova so far. The spectra show incredibly strong emission lines requiring more than about 10 solar masses of oxygen to reproduce. These explosions must therefore come from extremely massive stars, with over 40 solar masses on the main sequence. Masses in this range are indeed the most massive ones that have been inferred by any method for any supernova.
With these important developments, detailed multi-dimensional models involving the collapse, explosion, and late-time energy input of massive stellar core are currently being pursued by other groups throughout the world. This promises to expand our knowledge of stellar evolution and supernova explosions into new and unexplored regimes.
This figure from the paper shows the spectral appearance of superluminous supernovae at late times. Strong lines of oxygen, magnesium, an calcium are seen.