arXiv:astro-ph/0011340v1 17 Nov 2000 Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 5 December 2013 (MN L A T E X style file v1.4) Optical and Infrared Spectroscopy of the type IIn SN 1998S : Days 3-127 A. Fassia 1 , W.P.S. Meikle 1 , N. Chugai 2 , T.R. Geballe 3 , P. Lundqvist 4 , N.A. Walton 5 , D. Pollacco 6 , S. Veilleux 7 , G. Wright 8 , M. Pettini 9 , T. Kerr 10 , E. Puchnarewicz 11 , P. Puxley 12 , M. Irwin 9 , C. Packham 13 , S. J. Smartt 9 , D. Harmer 14 1 Astrophysics Group, Blackett Laboratory, Imperial College, Prince Consort Rd, London SW7 2BZ, UK 2 Institute of Astronomy, Russian Academy of Sciences, Ryatniskaya 48, 109017 Moscow, Russia 3 Gemini Observatory Northern Operations Center, 670 N. A’ohoku Place, Hilo, Hawaii, 96720, USA 4 Stockholm Observatory, SE-133 36, Saltj¨obaden, Sweden 5 Isaac Newton Group, Apartado de Correos 321, 38780 Santa Cruz de La Palma, Islas Canarias, Spain 6 Astrophysics and Planetary Sciences Division, The Queen’s University of Belfast, Belfast BT7 1NN 7 Department of Astronomy, University of Maryland, College Park, MD 20742-2421 8 Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, Scotland, UK 9 Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA 10 Joint Astronomy Centre, 660 N. A’Ohoku Place, University Park, Hilo, Hawaii 96720, USA 11 Mullard Space Science Laboratory,University College, London, Holmbury St. Mary, Dorking, Surrey RH5 6NT 12 International Gemini Project, 950 N. Cherry Ave., P.O. Box 26732, Tucson, Arizona, 85726-6732, USA 13 Department of Astronomy, University of Florida, Gainesville, FL 32611 USA 14 National Opt. Astr. Obs., P.O.Box 26732, Tucson, AZ 85726, USA 5 December 2013 ABSTRACT We present contemporary infrared and optical 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 characterised by prominent broad H, He and C III/N III emission lines with narrow peaks superimposed on a very blue continuum (T∼24000 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 ab- sorption components appeared in the spectra. The temperature of the continuum also dropped to ∼14000 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 com- ponents in H and He reappeared in the spectra. These persisted to as late as ∼100-130 days, becoming increasingly asymmetric. We agree with Leonard et al. (2000) that the broad emission lines indicate interaction between the ejecta and circumstellar mate- rial (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 17 and 36 days. These spec- tra 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 ionisation 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. Assuming a constant veloc- ity, 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 and may have arisen from a component of the outer CSM shell produced when the progenitor was going through a later blue supergiant phase. Al- ternatively, 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, and infer a mixed metal/He core of about 4 M , again indicating a massive progenitor. Only c  0000 RAS