Spectroscopic Observations and Analysis of the Peculiar SN 1999aa

Abstract

We present an extensive new time-series of spectroscopic data of the peculiar\nSN 1999aa in NGC 2595. Our data set includes 25 optical spectra between -11 and\n+58 days with respect to B-band maximum light, providing an unusually complete\ntime history. The early spectra resemble those of a SN 1991T-like object but\nwith a relatively strong Ca H&K absorption feature. The first clear sign of Si\nII 6355, characteristic of Type Ia supernovae, is found at day -7 and its\nvelocity remains constant up to at least the first month after B-band maximum\nlight. The transition to normal-looking spectra is found to occur earlier than\nin SN 1991T suggesting SN 1999aa as a possible link between SN 1991T-like and\nBranch-normal supernovae. Comparing the observations with synthetic spectra,\ndoubly ionized Fe, Si and Ni are identified at early epochs. These are\ncharacteristic of SN 1991T-like objects. Furthermore, in the day -11 spectrum,\nevidence is found for an absorption feature which could be identified as high\nvelocity C II 6580 or H-alpha. At the same epoch C III 4648.8 at photospheric\nvelocity is probably responsible for the absorption feature at 4500 A. High\nvelocity Ca is found around maximum light together with Si II and Fe II\nconfined in a narrow velocity window. Implied constraints on supernovae\nprogenitor systems and explosion hydrodynamical models are briefly discussed.\n