# ASASSN-15pz: Revealing Significant Photometric Diversity among 2009dc-like, Peculiar SNe Ia

## TL;DR

This study presents detailed observations of a peculiar Type Ia supernova, ASASSN-15pz, revealing significant diversity among 2009dc-like SNe Ia, challenging the notion of a uniform subclass and highlighting their varied luminosities and spectral features.

## Contribution

It provides the first comprehensive multi-wavelength data on ASASSN-15pz, demonstrating the diversity within 2009dc-like SNe Ia and proposing a phenomenological classification over a theoretical one.

## Key findings

- ASASSN-15pz is spectroscopically similar to SN 2009dc but is less luminous.
- 2009dc-like SNe Ia show a wide range of peak luminosities and decline rates.
- These supernovae do not follow the tight width-luminosity relation typical of normal SNe Ia.

## Abstract

We report comprehensive multi-wavelength observations of a peculiar Type Ia-like supernova ("SN Ia-pec") ASASSN-15pz. ASASSN-15pz is a spectroscopic "twin" of SN 2009dc, a so-called "Super-Chandrasekhar-mass" SN, throughout its evolution, but it has a peak luminosity M_B,peak = -19.69 +/- 0.12 mag that is \approx 0.6 mag dimmer and comparable to the SN 1991T sub-class of SNe Ia at the luminous end of the normal width-luminosity relation. The synthesized Ni56 mass of M_Ni56 = 1.13 +/- 0.14 M_sun is also substantially less than that found for several 2009dc-like SNe. Previous well-studied 2009dc-like SNe have generally suffered from large and uncertain amounts of host-galaxy extinction, which is negligible for ASASSN-15pz. Based on the color of ASASSN-15pz, we estimate a host extinction for SN 2009dc of E(B-V)_host=0.12 mag and confirm its high luminosity (M_B, peak[2009dc] \approx -20.3 mag). The 2009dc-like SN population, which represents ~1% of SNe Ia, exhibits a range of peak luminosities, and do not fit onto the tight width-luminosity relation. Their optical light curves also show significant diversity of late-time (>~ 50 days) decline rates. The nebular-phase spectra provide powerful diagnostics to identify the 2009dc-like events as a distinct class of SNe Ia. We suggest referring to these sources using the phenomenology-based "2009dc-like SN Ia-pec" instead of "Super-Chandrasekhar SN Ia," which is based on an uncertain theoretical interpretation.