A LUMINOUS PECULIAR TYPE IA SUPERNOVA SN 2011HR: MORE LIKE SN 1991T OR SN 2007if?

Abstract

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 ± 0.40 mag ?> , with a postmaximum decline rate Δm 15(B) = 0.92 ± 0.03 mag. From the maximum-light bolometric luminosity, L = ( 2.30 ± 0.90 ) × 10 43 erg s − 1 ?> , we estimate the mass of synthesized 56Ni in SN 2011hr to be M ( 56 Ni ) = 1.11 ± 0.43 M ⊙ ?> . 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 ⊙ ?> in the outer ejecta, which is much lower than the typical value of normal SNe Ia (i.e., 0.3–0.4 M ⊙ ?> ) and is also lower than the value of SN 1991T (i.e., ∼0.18 M ⊙ ?> ). 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.