Spectral Evolution of the Extraordinary Type IIn Supernova 2006gy

TL;DR

This paper analyzes the spectral evolution of the extremely luminous Type IIn supernova 2006gy, revealing insights into its progenitor, shock interaction with dense circumstellar material, and implications for massive star evolution.

Contribution

It provides a detailed spectral analysis of SN2006gy, highlighting its unique properties and proposing a progenitor scenario involving massive stars with significant mass loss.

Findings

SN2006gy's CSM mass is about 20 solar masses.

The explosion energy is at least 5 x 10^51 erg.

Progenitor likely experienced LBV eruptions or pulsational pair-instability ejections.

Abstract

We present a detailed analysis of the extremely luminous Type IIn supernova SN2006gy using spectra obtained between days 36 and 237 after explosion. We derive the temporal evolution of the effective temperature, radius, expansion speeds, and bolometric luminosity, as well as the progenitor wind density and total swept-up mass overtaken by the shock. SN2006gy can be interpreted in the context of shock interaction with a dense CSM, but with quite extreme values for the CSM mass of 20 Msun and an explosion kinetic energy of at least 5e51 erg. A key difference between SN2006gy and other SNeIIn is that, owing to its large CSM mass, the interaction region remained opaque much longer. At early times, H-alpha widths suggest that the photosphere is ahead of the shock, and photons diffuse out through the opaque CSM. The pivotal transition to optically thin emission begins around day 110, when we start to see a decrease in the blackbody radius and strengthening tracers of the post-shock shell. From the evolution of pre-shock velocities, we deduce that the CSM was ejected by the progenitor in a 1e49 erg precursor event 8yr before explosion. The large CSM mass rules out models involving stars with initial masses around 10Msun. With the full mass budget, even massive M_ZAMS=30-40 Msun progenitor stars are inadequate. At roughly solar metallicity, substantial mass loss probably occurred during the star's life, so SN 2006gy's progenitor is more consistent with LBV eruptions or pulsational pair-instability ejections in stars with initial masses above 100 Msun. This requires significant revision to current paradigms of massive-star evolution. (abridged)