PhD Thesis: The Circumstellar Environment of Type Ia Supernovae
Friday 03 November 2017
to 17:00 at
Raphael Ferretti (Stockholm University, Department of Physics)
Type Ia supernovae (SNe Ia) have proven to be extremely useful for measuring cosmological distances and were used for
the discovery of the accelerated expansion of the universe. Although thousands of SNe Ia have been observed to date,
many questions surrounding the physics of the explosions and the nature of their progenitor systems remain unanswered.
An notable property of many SNe Ia is the relation between extinction due to dust and their colour. For example SN 2014J,
the nearest SN Ia in recent years, has an extinction relation which would be very unusual to observe in the Milky Way. One
possible explanation to the peculiar extinction could be the presence of circumstellar (CS) dust surrounding the explosions.
Incidentally, some proposed progenitor models of SNe Ia suggest that the explosions are surrounded by shells of matter,
which could account for the unusual extinction.
CS gas would be ionised, if it is exposed to the intense ultraviolet (UV) radiation of a SN Ia. The research presented
in this thesis focuses on the search for CS gas by observing the effects of photoionisation on absorption lines commonly
detected in optical spectra. Simple models suggest that the frequently studied sodium doublet (Na I D) should significantly
decrease or even disappear if the gas is in the CS environment. Conversely, the absence of variations implies that the
absorbing gas clouds must be far from the explosion, in the interstellar medium (ISM). To date, few SNe Ia have been
shown to have variable absorption lines, to which we have added another case with SN 2013gh. Yet, we have also shown
that most observations searching for variable absorption lines have been taken at too late phases, when most CS gas will
have already been ionised. Setting out to obtain the earliest possible coverage of a SN Ia with high-resolution spectra, we
have been able to set strong limits on the presence of CS gas surrounding SN 2017cbv.
Along with evidence from other observational methods, these results have shown that there is little matter in the CS
environments of SNe Ia, suggesting that the peculiar extinction likely results from the dust properties of their host galaxy
ISM. Although the progenitor question cannot be resolved by these observations, nondetections of CS gas point to models
which do not deposit large amounts of matter in their surroundings.
Keywords: Type Ia supernovae, extinction, supernova cosmology.