H alpha clarifies picture of IIn supernova SN 2015da

Observational data on type IIn supernova SN 2015da are revisited to clarify some ambiguous issues of this phenomenon. The H alpha--emitting site is found to switch around day 90 from the circumstellar matter to the unshocked ejecta. This picture makes it possible to recover the expansion velocity of late time ejecta and to locate the dust formation in the inner zone of unshocked ejecta.The bolometric light curve and expansion velocities combined with modelling the circumstellar interaction imply that the supernova exploded with an energy of 4 times10 51 erg inside the circumstellar envelope with the mass of about 14 Msun. The recovered density of the circumstellar matter is consistent with the Thomson optical depth inferred from H alpha modelling on day 46.The observed circumstellar envelope is produced by an average mass loss rate of 0.035 Msun/yr during 400 yr before the supernova explosion.