The superluminous Type IIn supernova ASASSN-15ua: part of a continuum in extreme precursor mass-loss

We present a series of ground-based photometry and spectroscopy of the superluminous Type IIn supernova (SN) ASASSN-15ua, which shows evidence for strong interaction with pre-existing dense circumstellar material (CSM). Our observations constrain the speed, mass-loss rate, and extent of the progenitor wind shortly before explosion. A narrow P Cygni absorption component reveals a progenitor wind speed of ∼100 km s^-1. As observed in previous SNe IIn, the intermediate-width H α emission became more asymmetric and blueshifted over time, suggesting either asymmetric CSM, an asymmetric explosion, or increasing selective extinction from dust within the post-shock shell or SN ejecta. Based on the CSM radius and speed, we find that the progenitor suffered extreme eruptive mass-loss with a rate of 0.1-1 M_· yr^-1 during the ∼12 yr immediately before the death of the star that imparted ∼10⁴⁸ erg of kinetic energy to the CSM. Integrating its V-band light curve over the first 170 d after discovery, we find that ASASSN-15ua radiated at least 3 × 10⁵⁰ erg in visual light alone, giving a lower limit to the total radiated energy that may have approached 10⁵¹ erg. ASASSN-15ua exhibits many similarities to two well-studied superluminous SNe IIn: SN 2006tf and SN 2010jl. Based on a detailed comparison of these three, we find that ASASSN-15ua falls in between these two events in a wide variety of observed properties and derived physical parameters, illustrating a continuum of behaviour across superluminous SNe IIn.