The Unusual Temporal and Spectral Evolution of SN2011ht. II. Peculiar Type IIn or Impostor?

TL;DR

SN2011ht likely represents a giant eruption driven by super-Eddington radiation pressure rather than a true supernova, based on its spectral evolution, wind velocities, and energy estimates, challenging its classification as a core-collapse event.

Contribution

This study provides detailed spectral and kinematic analysis of SN2011ht, arguing it is a super-Eddington eruption rather than a genuine supernova, which is a novel reinterpretation.

Findings

Spectral evolution from hot to cool dense wind observed.

Wind speeds identified at 500-900 km/s, no evidence above 1000 km/s.

Total energy estimated at ~2 x 10^49 ergs, less than a true supernova.

Abstract

SN2011ht has been described both as a true supernova and as an impostor. In this paper, we conclude that it does not match some basic expectations for a core-collapse event. We discuss SN2011ht's spectral evolution from a hot dense wind to a cool dense wind, followed by the post-plateau appearance of a faster low density wind during a rapid decline in luminosity. We identify a slow dense wind expanding at only 500--600 km/s, present throughout the eruption. A faster wind speed V ~ 900 km/s may be identified with a second phase of the outburst. There is no direct or significant evidence for any flow speed above 1000 km/s; the broad asymmetric wings of Balmer emission lines in the hot wind phase were due to Thomson scattering, not bulk motion. We estimate a mass loss rate of order 0.04 Msun/yr during the hot dense wind phase of the event. There is no evidence that the kinetic energy substantially exceeded the luminous energy, roughly 2 X 10^49 ergs; so the total energy was far less than a true SN. We suggest that SN2011ht was a giant eruption driven by super-Eddington radiation pressure, perhaps beginning about 6 months before the discovery. A strongly non-spherical SN might also account for the data, at the cost of more free parameters.