High-velocity features in the spectra of the Type Ia supernova SN 1999ee: a property of the explosion or evidence of circumstellar interaction?

Abstract

The near-maximum spectra of the Type Ia supernova SN 1999ee are reviewed. Two narrow absorption features corresponding to the strongest component of the Ca II infrared (IR) triplet appear in the spectra from 7 d before to 2 d after the B-band maximum, at a high velocity (∼22 000 km s -1 ). Before these features emerge, the Ca II IR triplet has an anomalously high velocity, indicating that the narrow features were still blended with the main, photospheric component. High-velocity Can absorption has been observed in other Type Ia supernovae, but usually detached from the photospheric component. Furthermore, the Si II 6355-A line is observed at a comparably high velocity (∼20 000 km s -1 ) 9 and 7 d before the B maximum, but then it suddenly shifts to much lower velocities. Synthetic spectra are used to reproduce the data under various scenarios. An abundance enhancement requires an outer region dominated by Si and Ca, the origin of which is not easy to explain in terms of nuclear burning. A density enhancement leads to a good reproduction of the spectral evolution if a mass of ∼0.10 M○. is added at velocities between 16 000 and 28 000 km s -1 , and it may result from a perturbation, possibly angular, of the explosion. An improved match to the Can IR triplet at the earliest epoch can be obtained if the outermost part of this modified density profile is assumed to be dominated by H (∼0.004 M○. above 24000 km s -1 ). Line broadening is then the result of increased electron scattering. This H may be the result of interaction between the ejecta and circumstellar material.