2016ApJ...817..114Z

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 Δm₁₅(B) = 0.92±0.03 mag. From the maximum-light bolometric luminosity, L = (2.30 ± 0.90) x 10⁴³ M_☉, we estimate the mass of synthesized ⁵⁶Ni in SN 2011hr to be M(⁵⁶ Ni) = 1.11 ± 0.43M_☉. 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.