Letters of Intent received in 2014

LoI 2016-239
Stellar Explosions and their impact on the Universe

Topics

Supernovae, stellar evolution, nucleosynthesis, radiation transport, cosmology

Rationale

The advent of untargeted surveys has led to the discovery of many classes of
explosions of or on stars that were previously unknown or only theoretically
predicted. At the same time, theory has been active in producing new avenues to
explosive stellar events. All of this is affecting our understanding of the life
of stars. In this Symposium we will bring together observers, modellers and
pure theorists in order to define the various classes of explosions, how they
can be understood, what impact supernovae (SNe) have in the evolution of the Universe, and what are the prospects in the upcoming JWST and ELT era.

Historically, SNe have been divided into two groups, thermonuclear
(Type Ia) and core collapse. SNe Ia are thought to be the explosion of CO white
dwarfs. They are used for distance determination thanks to a relation between
the luminosity and the width of their light curve, but the exact nature of the
progenitor and the explosion is still debated, while it is becoming clear that
multiple paths may lead to SNe Ia. These may involve both a white dwarf
accreting from a less evolved companion (the Single Degenerate scenario) and the
merging of two white dwarfs (the Double Degenerate scenario).Recently other
models have been proposed, one example being the collision of two white dwarfs
that are brought to resonance in a triple-star system. For several of these
models observational predictions have been made. Despite these uncertainties,
SNe Ia remain the best tool for cosmological distance measurements, indicating
that either an overarching physical mechanism unifies their behaviour or that
only one channel dominates.

Core-collapse SNe come from massive stars (M >= 8 Msun) and have
different spectra, indicating progressive stripping of the progenitor, due to
mass loss or binary interaction. The most energetic and massive SNe Ic are
linked to long GRBs. The exact mechanism that drives these explosions is also
debated: the neutrino mechanism may work at low masses, but at higher masses
jet-driving may be required, or even the magnetic energy associated with newly
formed Neutron Stars (Magnetars). This is ultimately leading to a revision of
the ultimate fate of massive stars, the separation between Black Holes and
Neutron Stars having become less clear. At the same time the likely role of
binarity, which is common among massive stars, is being more and more
appreciated, not only in core-collapse SNe, but also as a source of short GRBs
via NS-NS or NS-BH mergers which are also the ideal source of Gravitational
Waves, an area which is likely to develop greatly in the near future.

Adding to this picture, in recent years other explosive events have been
discovered. They are classified according to the old scheme but may represent
different types of events. Among these we mention Super-Luminous SNe, events
whose luminosity exceeds that of Type Ia SNe of GRB/SNe by 1-2 magnitudes. This
class contains different types of phenomena, including potential
Pair-Instability SNe, very luminous SNe which involve large masses and
apparently synthesize large amounts of 56Ni (Gal Yam et al. 2009, see
Dessart et al. 2012, Nicholl et al. 2013 and Kozyreva et al. 2014 for ongoing
debate), SNe Ib which occur in very old populations and may be He-shell
explosions on the surface of accreting WDs (Perets et al. 2010, Valenti et al.
2014), and other "Fast and Faint" transients (Kasliwal et al. 2013). These are
tracers of different populations. Pair-Instability SNe come from very massive - hence young -
stars, which are thought to be more common in the early Universe. Isolated WDs
are old, and may themselves contain traces of the older population.

These new types of explosion also reveal different nucleosynthesis. Pair-Instability SNe
produce large amounts of Fe. Mergers of compact objects are now indicated as
sources of r-process elements, but other sites may be possible. Fast and Faint
transients only produce partial burning, and Ca-rich nucleosynthesis could be
important in the production of elements such as Calcium, Titanium, Chromium.

In this proposed IAU symposium we want to bring together observers, theorists
and modellers in order to review the present status of stellar physics, stellar
explosions, observations and the use of radiation transport tools both to
translate models into observables and to infer quantitave information from the
data. We want to bring the SN and stellar communities together to establish a
closer contact between the study of the properties of stars (eg mass-loss,
binarity) and that of the explosions.

The timing of the Symposium is particularly important: new automatic surveys are
about to come on line which will increase the number of SN discoveries by orders
of magnitude, and at the same time the results of current surveys have produced a
number of questions that are still unanswered about the nature of a variety of
stellar-related transients. In particular, 2016 will mark the formal end of
PESSTO, the ESO public SN survey which involves a large fraction of the SN
community, and the Symposium will be the perfect stage in which those results
can be reviewed.