A B S T R A C TWe 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 iiiaN iii emission lines with narrow peaks, superimposed on a very blue continuum T , 24 000 KX In the following two weeks the C iiiaN 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±130Y 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 21 X 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 Â 10 25 M ( yr 21 X 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 21 , and may have arisen from a component of the outer CSM shell produced when the progenitor was going through a later q 2001 RAS

We present late‐time near‐infrared (NIR) and optical observations of the Type IIn SN 1998S. The NIR photometry spans 333–1242 d after explosion, while the NIR and optical spectra cover 333–1191 and 305–1093 d, respectively. The NIR photometry extends to the M′ band (4.7 μm), making SN 1998S only the second ever supernova for which such a long IR wavelength has been detected. The shape and evolution of the Hα and He i 1.083‐μm line profiles indicate a powerful interaction with a progenitor wind, as well as providing evidence of dust condensation within the ejecta. The latest optical spectrum suggests that the wind had been flowing for at least 430 yr. The intensity and rise of the HK continuum towards longer wavelengths together with the relatively bright L′ and M′ magnitudes show that the NIR emission was due to hot dust newly formed in the ejecta and/or pre‐existing dust in the progenitor circumstellar medium (CSM). The NIR spectral energy distribution (SED) at about 1 yr is well described by a single‐temperature blackbody spectrum at about 1200 K. The temperature declines over subsequent epochs. After ∼2 yr, the blackbody matches are less successful, probably indicating an increasing range of temperatures in the emission regions. Fits to the SEDs achieved with blackbodies weighted with λ−1 or λ−2 emissivity are almost always less successful. Possible origins for the NIR emission are considered. Significant radioactive heating of ejecta dust is ruled out, as is shock/X‐ray‐precursor heating of CSM dust. More plausible sources are (a) an IR echo from CSM dust driven by the ultraviolet/optical peak luminosity, and (b) emission from newly‐condensed dust which formed within a cool, dense shell produced by the ejecta shock/CSM interaction. We argue that the evidence favours the condensing dust hypothesis, although an IR echo is not ruled out. Within the condensing‐dust scenario, the IR luminosity indicates the presence of at least 10−3 M⊙ of dust in the ejecta, and probably considerably more. Finally, we show that the late‐time (K–L′)0 evolution of Type II supernovae may provide a useful tool for determining the presence or absence of a massive CSM around their progenitor stars.

A B S T R A C TWe present contemporaneous optical and infrared (IR) photometric observations of the Type IIn SN 1998S covering the period between 11 and 146 d after discovery. The IR data constitute the first ever IR light curves of a Type IIn supernova. We use blackbody and spline fits to the photometry to examine the luminosity evolution. During the first 2±3 months, the luminosity is dominated by the release of shock-deposited energy in the ejecta. After ,100 d the luminosity is powered mostly by the deposition of radioactive decay energy from 0X150X05 M ( of 56 Ni which was produced in the explosion. We also report the discovery of an astonishingly high IR excess, K 2 L H 2X5Y that was present at day 130. We interpret this as being due to thermal emission from dust grains in the vicinity of the supernova. We argue that to produce such a high IR luminosity so soon after the explosion, the dust must be preexisting and so is located in the circumstellar medium of the progenitor. The dust could be heated either by the UV/optical flash (IR echo) or by the X-rays from the interaction of the ejecta with the circumstellar material.

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A B S T R A C TWe present contemporary infrared (IR) and optical spectra of the plateau type II SN 1995V in NGC 1087 covering four epochs, approximately 22 to 84 d after shock break-out. The data show, for the first time, the IR spectroscopic evolution during the plateau phase of a typical type II event. In the optical region P Cygni lines of the Balmer series and of metals such as Sc II, Fe II, Sr II, Ca II and Ba II lines were identified. The IR spectra were largely dominated by the continuum, but P Cygni Paschen lines and Brackett g lines were also clearly seen. The other prominent IR features are confined to wavelengths blueward of 11 000 Å , and include Sr II 10327, Fe II 10547, C I 10695 and He I 10830 Å . Helium has never before been unambiguously identified in a type IIp supernova spectrum during the plateau phase. We demonstrate the presence of He I 10830 Å on days 69 and 85. The presence of this line at such late times implies reionization. A likely reionizing mechanism is g-ray deposition following the radioactive decay of 56 Ni. We examine this mechanism by constructing a spectral model for the He I 10830-Å line based on explosion model s15s7b2f of Weaver & Woosley. We find that this does not generate the observed line owing to the confinement of the 56 Ni to the central zones of the ejecta. In order to reproduce the He I line, it was necessary to introduce additional upward mixing or 'dredge-up' of the 56 Ni, with ϳ10 ¹5 of the total nickel mass reaching above the helium photosphere. In addition, we argue that the He I line formation region is likely to have been in the form of pure helium clumps in the hydrogen envelope. The study of He I 10830-Å emission during the photospheric phase of core-collapse supernovae provides a promising tool for the constraint of initial mixing conditions in explosion models.