Optical and infrared spectroscopy of the type IIn SN 1998S: days 3-127

We present contemporary optical and infrared spectroscopic observations of the type IIn SN 1998S covering the period between 3 and 127 days after discovery. During the first week the spectra are characterized by prominent broad H, He and C iii/N iii emission lines with narrow peaks, superimposed on a very blue continuum (T∼24 000 K). In the following two weeks the C iii/N iii emission vanished, together with the broad emission components of the H and He lines. Broad, blueshifted absorption components appeared in the spectra. The temperature of the continuum also dropped to ∼14 000 K. By the end of the first month the spectrum comprised broad, blueshifted absorptions in H, He, Si ii, Fe ii and Sc ii. By day 44, broad emission components in H and He reappeared in the spectra. These persisted to as late as days ∼100–130, becoming increasingly asymmetric. We agree with Leonard et al. that the broad emission lines indicate interaction between the ejecta and circumstellar material (CSM) emitted by the progenitor. We also agree that the progenitor of SN 1998S appears to have gone through at least two phases of mass-loss, giving rise to two CSM zones. Examination of the spectra indicates that the inner zone extended to ≤90 au, while the outer CSM extended from 185 au to over 1800 au. We also present high-resolution spectra obtained at days 17 and 36. These spectra exhibit narrow P Cygni H i and He i lines superimposed on shallower, broader absorption components. Narrow lines of [N ii], [O iii], [Ne iii] and [Fe iii] are also seen. We attribute the narrow lines to recombination and heating following ionization of the outer CSM shell by the UV/X-ray flash at shock breakout. Using these lines, we show that the outer CSM had a velocity of 40–50 km s−1. Assuming a constant velocity, we can infer that the outer CSM wind commenced more than 170 years ago, and ceased about 20 years ago, while the inner CSM wind may have commenced less than 9 years ago. During the era of the outer CSM wind the outflow from the progenitor was high – at least ∼2×10−5 M⊙ yr−1. This corresponds to a mass-loss of at least ∼0.003 M⊙, suggesting a massive progenitor. The shallower, broader absorption is of width ∼350 km s−1, and may have arisen from a component of the outer CSM shell produced when the progenitor was going through a later blue supergiant phase. Alternatively, it may have been produced by the acceleration of the outer CSM by the radiation pressure of the UV precursor. We also describe and model first-overtone emission in carbon monoxide observed in SN 1998S. We deduce a CO mass of ∼10−3 M⊙ moving at ∼2200 km s−1, and infer a mixed metal/He core of about 4 M⊙, again indicating a massive progenitor. Only three core-collapse supernovae have been observed in the K band at post-100 days, and all three have exhibited emission from CO.