SN 2015da: late-time observations of a persistent superluminous Type IIn supernova with post-shock dust formation

We present photometry and spectroscopy of the slo wly e volving superluminous Type IIn supernova (SN) 2015da. SN 2015da is extraordinary for its very high peak luminosity, and also for sustaining a high luminosity for several years. Even at 8 yr after explosion, SN 2015da remains as luminous as the peak of a normal SN II-P. The total radiated energy integrated over this time period (with no bolometric correction) is at least 1 . 6 ×10 51 erg (or 1.6 FOE). Including a mild bolometric correction, adding kinetic energy of the expanding cold dense shell of swept-up circumstellar material (CSM), and accounting for asymmetry, the total explosion kinetic energy was likely 5-10 FOE. Powering the light curve with CSM interaction requires an energetic explosion and 20 M ⊙of H-rich CSM, which in turn implies a massive progenitor system > 30 M ⊙. Narrow P Cyg features show steady CSM expansion at 90 km s -1 , requiring a high average mass-loss rate of ∼0.1 M ⊙yr -1 sustained for two centuries before explosion (although ramping up toward explosion time). No current theoretical model for single-star pre-SN mass-loss can account for this. The slow CSM, combined with broad wings of H αindicating H-rich material in the unshocked ejecta, disfa v ours a pulsational pair instability model for the pre-SN mass-loss. Instead, violent pre-SN binary interaction is a likely culprit. Finally, SN 2015da exhibits the characteristic asymmetric blueshift in its emission lines from shortly after peak until the present epoch, adding another well-studied superluminous SNe IIn with unambiguous evidence of post-shock dust formation.