Nebular spectra of the unusual Type Ia supernova 1991T

The nebular optical spectra of the unusual Type Ia supernova 1991T are modeled by treating important atomic processes with reliable atomic data and by assuming both Chandrasekhar-mass and sub-Chandrasekhar-mass white dwarf explosion models. Similar to the case of normal Type Ia supernovae, a better agreement between the calculated and observed spectra of SN 1991T is obtained by assuming a sub-Chandrasekhar-mass model than Chandrasekhar-mass models, though the required white dwarf mass of similar to 1.1 M. is higher for SN 1991T than the similar to 0.9 M. for normal Type Ia supernovae. This demonstrates that variation in the behavior among Type Ia supernovae can be naturally realized by a range of sub-Chandrasekhar-mass models. The optical emission of SN 1991T is well accounted for by the sub-Chandrasekhar-mass model in both brightness and spectral shape from similar to 7 to 14 months for a distance of 14 Mpc to the supernova.