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 $M_B=-19.84\pm 0.40\mathrm{m}\mathrm{a}\mathrm{g}$, with a post-maximum decline rate $\mathrm{\Delta }$m${}_{15}$(B) = 0.92 $\pm$ 0.03,$\mathrm{m}\mathrm{a}\mathrm{g}$. From the maximum-light bolometric luminosity, $L=(2.30 \pm 0.90) \times 10^{43},\rm{erg,s^{-1}}$, we estimate the mass of synthesized \Nifs\ in SN 2011hr to be $M({}^{56}\mathrm{N}\mathrm{i})=1.11\pm 0.43M_{\text{sun}}$. 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 the latter at similar phases. Spectral modeling suggests that SN 2011hr has the IMEs of $\sim$,0.07 M${}_{\text{sun}}$ in the outer ejecta, which is much lower than the typical value of normal SNe Ia (i.e., 0.3 – 0.4 M${}_{\text{sun}}$) and is also lower than the value of SN 1991T (i.e., $\sim$,0.18 M${}_{\text{sun}}$). 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 the superluminous subclass like SN 2007if given that the latter also shows very weak IMEs at all phases.