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

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 MB = -19.84 +/- 0.40 mag, with a postmaximum decline rate Delta(m15)(B) = 0.92. +/- 0.03 mag. From the maximum-light bolometric luminosity, L = (2.30 +/- 0.90) x 10(43) erg s(-1), we estimate the mass of synthesized Ni-56 in SN 2011hr to be M(Ni-56) = 1.11 +/- 0.43M(circle dot). 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 similar to 0.07 M-circle dot in the outer ejecta, which is much lower than the typical value of normal SNe Ia (i.e., 0.3-0.4 M-circle dot) and is also lower than the value of SN 1991T (i.e., similar to 0.18 M-circle dot). 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.