A luminous peculiar Type Ia supernova SN 2011hr: more like SN 1991T or SN 2007if?

Abstract

Photometric and spectroscopic observations of a slowly declining, luminous Type Ia supernova (SN Ia) SN 2011hr in the starburst galaxy NGC 2691 are presented. SN 2011hr is found to peak at ${M}_{B}\;=\;-19.84\pm 0.40\;\mathrm{mag}$, with a postmaximum decline rate Δm15(B) = 0.92 ± 0.03 mag. From the maximum-light bolometric luminosity, $L\;=\;(2.30\pm 0.90)\times {10}^{43}\;\mathrm{erg}\;{{\rm{s}}}^{-1}$, we estimate the mass of synthesized 56Ni in SN 2011hr to be $M{(}^{56}\mathrm{Ni})\;=\;1.11\pm 0.43\;{M}_{\odot }$. SN 2011hr appears more luminous than SN 1991T at around maximum light, and the absorption features from its intermediate-mass elements (IMEs) are noticeably weaker than those of the latter at similar phases. Spectral modeling suggests that SN 2011hr has IMEs of ~0.07 ${M}_{\odot }$ in the outer ejecta, which is much lower than the typical value of normal SNe Ia (i.e., 0.3–0.4 ${M}_{\odot }$) and is also lower than the value of SN 1991T (i.e., ~0.18 ${M}_{\odot }$). These results indicate that SN 2011hr may arise from a Chandrasekhar-mass white dwarf progenitor that experienced a more efficient burning process in the explosion. Nevertheless, it is still possible that SN 2011hr may serve as a transitional object connecting the SN 1991T-like SNe Ia with a superluminous subclass like SN 2007if given that the latter also shows very weak IMEs at all phases.