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

TL;DR

SN2015da is a superluminous Type IIn supernova with extraordinary long-lasting brightness, high explosion energy, and evidence of post-shock dust formation, challenging existing models of pre-supernova mass loss.

Contribution

This study provides detailed late-time observations of SN2015da, revealing its high energy, extensive circumstellar material, and dust formation, offering new insights into superluminous supernovae and their progenitors.

Findings

SN2015da remains as luminous as a normal SNII-P after 8 years.

The explosion's kinetic energy is estimated at 5-10 FOE.

High mass-loss rate and binary interaction likely caused the dense CSM.

Abstract

We present photometry and spectroscopy of the slowly evolving superluminous Type IIn SN2015da. SN2015da is extraordinary for its very high peak luminosity, and also for sustaining a high luminosity for several years. Even at 8\,yr after explosion, SN2015da remains as luminous as the peak of a normal SNII-P. The total radiated energy integrated over this time period (with no bolometric correction) is at least 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 Msun of H-rich CSM, which in turn implies a massive progenitor system above 30 Msun. Narrow P Cyg features show steady CSM expansion at 90 km/s, requiring a high average mass-loss rate of roughly 0.1 Msun/yr sustained for 2 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$\alpha$ indicating H-rich material in the unshocked ejecta, disfavor a pulsational pair instability model for the pre-SN mass loss. Instead, violent pre-SN binary interaction is a likely cuprit. Finally, SN2015da exhibits the characteristic asymmetric blueshift in its emission lines from shortly after peak until the present epoch, adding another well-studied superluminous SNeIIn with unambiguous evidence of post-shock dust formation.