SIMBAD references

A series of early-time optical spectra of SN 1991T have been modelled with our Monte Carlo code, in an effort to determine the properties of this peculiar SN Ia. The spectra, obtained from various observatories at epochs between 2 weeks before and 4 weeks after maximum, cover essentially the entire photospheric epoch of this SN. Synthetic spectra are presented, along with a reasonably complete identification of the spectral lines giving rise to the observed features. The fits were obtained adopting a reddening E(B-V)=0.13 and a Tully-Fisher distance modulus m-M=30.65 to NGC 4527. With these parameters, the model luminosities for SN 1991T are consistently about 0.5mag higher than in the `standard' SN Ia 1990N, while the photospheric velocities are slightly larger. The early suggestion that radioactive ⁵⁶Ni and its daughter nuclei, ⁵⁶Co and ⁵⁶Fe, dominate the composition in the outer part of the ejecta, thus giving rise to pre-maximum spectra where Fe III lines are the dominant features, is confirmed, but we show that the absence of the Si II and Ca II lines at that epoch is due simply to the high envelope temperature. This in turn is caused by the high luminosity of SN 1991T, which is a direct consequence of the overproduction of ⁵⁶Ni. At times around and after maximum, however, the abundance of the Fe group elements drops relative to that of the intermediate mass elements. Eventually, the spectra obtained 3-4 weeks after maximum can be well fitted using the composition obtained by mixing the products of a deflagration model such as W7, although Si and Ca are underabundant. This indicates that the explosion mechanism active deep within the progenitor white dwarf, which was assumed to have a Chandrasekhar mass, must have been similar to the standard deflagration model. Thus, there is strong evidence that the peculiar explosion mechanism active in SN 1991T probably belonged to the family of delayed detonations, and led to Ni production in the outer layers of the exploding white dwarf. The spectral models suggest that about 0.6M_☉ of ⁵⁶Ni have been synthesised in the outermost 1M_☉ of the exploding white dwarf, so that a factor between 1.5 and 1.7 more ⁵⁶Ni was produced in SN 1991T than in SN 1990N. This result is fully consistent with the high luminosity required to fit the spectra.