Optical and infrared spectroscopy of the type IInSN1998S: 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 disc overy. During the first week the spectra are characterized by prominent bro ad H, He and C III/N III emission lines with narrow peaks, superimposed on a very blue continuum (T similar to 24 000 K). In the following two weeks t he C III/N III emission vanished, together with the broad emission componen ts of the H and He lines. Broad, blueshifted absorption components appeared in the spectra. The temperature of the continuum also dropped to similar t o 14000 K. By the end of the first month the spectrum comprised broad, blue shifted absorptions in H, He, Si II, Fe II and Sc II. By day 44, broad emis sion components in H and He reappeared in the spectra. These persisted to a s late as days similar to 100-130, becoming increasingly asymmetric. We agr ee with Leonard et al. that the broad emission lines indicate interaction b etween the ejecta and circumstellar material (CSM) emitted by the progenito r. We also agree that the progenitor of SN 1998S appears to have gone throu gh at least two phases of mass-loss, giving rise to two CSM zones. Examinat ion of the spectra indicates that the inner zone extended to less than or e qual to 90 au, while the outer CSM extended from 185 an to over 1800 au. We also present high-resolution spectra obtained at days 17 and 36. These s pectra 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] a nd [Fe III] are also seen. We attribute the narrow lines to recombination a nd heating following ionization of the outer CSM shell by the UV/X-ray flas h at shock breakout. Using these lines, we show that the outer CSM had a ve locity of 40-50 kin s(-1). Assuming a constant velocity, we can infer that the outer CSM wind commenced more than 170 years ago, and ceased about 20 y ears 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 similar to2 x 10(-5) M. yr(-1). This corresponds to a mass- loss of at least similar to0.003 M., suggesting a massive progenitor. The s hallower, broader absorption is of width similar to 350 km s(-1) and may ha ve arisen from a component of the outer CSM shell produced when the progeni tor was going through a later blue supergiant phase. Alternatively, it may have been produced by the acceleration of the outer CSM by the radiation pr essure of the UV precursor. We also describe and model first-overtone emission in carbon monoxide obser ved in SN 1998S. We deduce a CO mass of similar to 10(-3) M. moving at simi lar to 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 b een observed in the K band at post-100 days, and all three have exhibited e mission from CO.