Talare: Anders Nyholm, Stockholms Universitet

Titel: Circumstellar matters: Probing star death and AGN host history via supernova surroundings


In my talk, I will discuss supernova (SN) research I have done for my PhD thesis. Type Ia (thermonuclear) SNe are sturdy rungs on the cosmic distance ladder, but no Type Ia progenitor system has yet been securely identified via observations. For nearby Type Ia SN 2014J we used the Nordic Optical Telescope to obtain a late spectrum of the SN, to search for hydrogen and helium ablated off a possible non-compact companion star. We put strong limits on the presence of such gas, making it plausible that the progenitor system was double-degenerate. Apart from this work, the rest of the thesis will be based on the intermediate Palomar Transient Factory (iPTF) survey. Using the iPTF SN catalogue, we examined the occurrence of SNe in active galactic nucleus (AGN) type 1 (broad Balmer lines) and type 2 (narrow Balmer lines) host galaxies. We found a larger number of SNe around AGN type 2 than around AGN type 1, and no core collapse (cc) SNe around AGN type 1. This result indicates that AGN type 2 hosts have younger stellar populations than AGN type 1 hosts. The main area of my PhD thesis work is SNe Type IIn, i.e. cc SNe happening in dense, hydrogen-rich circumstellar media. The interaction between SN ejecta and the circumstellar medium (CSM) can make SNe Type IIn luminous and long-lasting. In SNe IIn, as the ejecta sweeps up the CSM, the ejecta scans the late mass-loss history of the progenitor star. This offers rare glimpses of the last decades of supergiant stars. We studied Type IIn SN iPTF13z, with a long-lasting light curve showing several re-brightenings indicating that the SN ejecta ran into denser strata in the CSM. This, along with a clear detection of a pre-SN outburst, indicated that the iPTF13z progenitor star was restless before its demise. Such outbursts have been observed for a number of SNe Type IIn, offering clues about the progenitor stars. We currently work on a sample study of SNe IIn, using iPTF data, to improve our knowledge of this diverse class of transients.