17 - Broad lightcurve SNe Ia: asymmetry or something else?

Summary

Abstract

It is the conventional wisdom that overluminous Type Ia supernovae have an overproduction of their elemental powerhouse, ⁵⁶Ni, leading to broader light curves, higher temperatures, higher ionization states, and peculiar spectra similar to that of SN1991T. However, this simple picture is incomplete: we show that a broad lightcurve width does not necessarily predict spectroscopic peculiarity, nor does a spectrum resembling SN1991T guarantee a broad lightcurve. There is circumstantial evidence that asymmetry may play a role in the explanation of the diverse properties of broad lightcurve and SN1991T-like SNe Ia.

As an illustrative example, we present optical and NIR light curves, and Lick 3m and HST STIS spectra of the SN Ia with the broadest light curve observed to date, SN 2001ay. SN 2001ay has Δm₁₅(B) = 0.6 and stretch s = 1.6, yet at maximum light is fairly spectroscopically normal. The exception is an extremely high Si velocity, v = 15,000 km s^–1. The secondary peak in the I-band lightcurve is higher than the primary peak, and the J_s and H lightcurves remain flat over the entire 55 days of observation. SN 2001ay also does not appear to obey lightcurve shape-luminosity relationships, at least as they are currently formulated. Despite its broad lightcurve, the SN has normal absolute magnitudes after correction for Milky Way and host galaxy extinction. Thus, if a stretch or Δm₁₅(B) correction is applied, the resulting magnitude would be overcorrected by ∼1 mag.