The origin of the high-velocity circumstellar gas around SN 1998S

Abstract

ABSTRACT Modelling of high resolution Balmer line profiles in the early-time spectra ofSN 1998S shows that the inferred fast (≈400 km s −1 ) circumstellar (CS) gas ondays 23 and 42 post-explosion is confined to a narrow, negative velocity gradient shelljust above the photosphere. This gas may be identified with a slow (v < 40 km s −1 )progenitor wind accelerated at the ejecta-wind interface. In this scenario, the photo-sphere coincides with a cool dense shell formed in the reverse shock. Acceleration byradiation from the supernova or by a shock-accelerated relativistic particle precursorare both possible explanations for the observed fast CS gas. An alternative, equallyplausible scenario is that the fast CS gas is accelerated within shocked clouds engulfedby the outer shock, as it propagates through the intercloud wind.Key words: supernovae – circumstellar matter – : stars. 1 INTRODUCTIONSupernova (SN) 1998S in NGC 3877 was discovered by Li WFassia et al. 2001), broad emission lines originating from theinteraction (Leonard et al. 2000; Gerardy et al. 2000; Fas-sia et al. 2001), and radio and X-ray emission (Pooley et al.2001). Despite extensive effort in the past, we still have apoor understanding of the origin of the large pre-supernovamass loss and the ejecta-wind interaction phenomenon inSNe IIn.SN 1998S is the most comprehensively observed SN IIn,thus providing us with an excellent opportunity for gaininga deeper insight into the morphology and origin of the denseCS environment and the physics of the interaction. Of par-ticular interest is the origin of two distinct CS wind velocitycomponents deduced from high resolution early-time spec-tra of SN 1998S taken at ∼2–5 weeks after the explosion(Bowen et al. 2000; Fassia et al. 2001). These had velocitiesof u