Supernovae are extremely bright stellar explosions – superluminous
supernovae are even brighter. However, the nature of these most luminous
explosions has remained a mystery. In a new study, MPA researchers now
present their simulations of superluminous supernova spectra months and
even years after the outbreak and show that they are very similar to
gamma-ray bursts, another type of highly energetic explosions. In
addition, the results point to very high masses of oxygen and magnesium,
suggesting very massive progenitor stars that will use an exotic
explosion mechanism rather than the standard neutrino-driven explosion
believed to power most supernovae.
Superluminous supernovae are a new and exotic class of stellar
explosions, radiating up to 100 times more energy than normal
supernovae. Despite being so bright, they were discovered only about 10
years ago, as they occur at large distances and are quite rare (one per
every thousand normal supernovae).
The origin of the enormous luminosity and the properties of the
progenitor stars have been shrouded in mystery. They may be powered by
rapidly spinning and highly magnetized neutron stars (so called
magnetars), accretion onto a newly formed black hole, huge amounts of
radioactivity, or violent collisions with dense circumstellar matter.
What type of progenitor stars give rise to them? Why do they occur
exclusively in unusual dwarf galaxies?
In a new study led by Dr Anders Jerkstrand, a Marie Curie Fellow at MPA,
several important new advances are presented, which are based on
calculating spectral models of supernovae. “Several months and years
after the supernova has exploded, when the ejected material expands and
cools, the spectra reveal signatures of the elements that have been
produced inside the star,” Jerkstrand explains. “By comparing observed
to modelled spectra in this phase, we can get an insight into the inner
layers of the progenitor, which in turn provides strong constraints on
the origin and nature of these explosions.”
Interpretation of the spectra requires sophisticated models of how radiation passes through the expanding gas and requires the latest atomic physics to be included in the detailed models. 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. The new spectra are demonstrated to have strong
similarities with gamma-ray burst supernovae, the first time this link
has been established. Gamma-ray burst supernovae are thought to arise by
the formation of a black hole that punches a relativistic jet through
the infalling star, or by the formation of a highly magnetic neutron
star. Gamma ray bursts are similarly rare as superluminous supernovae,
and also occur in irregular dwarf galaxies at low metallicity. Some of
them are actually accompanied with supernovae, but until now always at
much lower luminosities, and not lasting as long as superluminous
supernovae.
Interpretation of the spectra requires sophisticated models of how
radiation passes through the expanding gas and requires the latest
atomic physics to be included in the detailed models. 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. The new spectra are demonstrated to have strong
similarities with gamma-ray burst supernovae, the first time this link
has been established. Gamma-ray burst supernovae are thought to arise by
the formation of a black hole that punches a relativistic jet through
the infalling star, or by the formation of a highly magnetic neutron
star. Gamma ray bursts are similarly rare as superluminous supernovae,
and also occur in irregular dwarf galaxies at low metallicity. Some of
them are actually accompanied with supernovae, but until now always at
much lower luminosities, and not lasting as long as superluminous
supernovae.
In a second important discovery, the spectral synthesis models
revealed that these superluminous supernovae contain among the highest
oxygen masses inferred for any supernova so far. The spectra show very
strong emission lines requiring more than about 10 solar masses of
oxygen and 1 solar mass of magnesium. These explosions must therefore
come from extremely massive stars, with over 40 solar masses on the main
sequence. Stars in this mass range are unlikely to explode with the
large inferred kinetic energies by the standard neutrino-driven
mechanism, and a more exotic mechanism such as a magneto-rotational
driven jets or black hole accretion is needed.
Detailed multi-dimensional models involving the collapse, explosion,
and late-time energy input of the massive stellar core are currently
being pursued by several groups around the world. Together with the new
constraints derived in this study, this promises to expand our knowledge
of stellar evolution and supernova explosions into new and unexplored
regimes.
Contact:
Jerkstrand, Anders
Contact:
Postdoc
Phone:
2282
Email: anders@mpa-garching.mpg.de
