Hubble space telescope/FOS spectrocopy of VW hydri in superoutburst

We present an analysis of two HST/FOS UV spectra of the SU UMa type dwarf nova, VW Hyi, obtained on 1993 October 24, ∼5 days after the optical rise of a superoutburst. The absorption features in the first spectrum appear to consist of two components: a broad-winged component (with velocity width of ∼3000 km s^-1) and a sharp core narrow component. This is the first time the narrow core is clearly resolved in superoutburst spectra of a dwarf nova system. The sharp core appears absent in the second spectrum obtained ∼ 10 minutes later. The broader component is mainly from the accretion disk. By comparing the spectra with a grid of LTE model accretion disk atmospheres constructed with TLUSTY, SYNSPEC, and DISKSYN, we present two possible disk fits to the observed spectra: a steady state disk with solar abundance and Ṁ = 3 × 10^-9 M_⊙ yr^-1 which can account for all the broad absorption features except for N v λ1240, and a model with a discontinuous T_eff(r) distribution in which there is a contribution to the N v λ1240 absorption feature. We provide arguments supporting the possibility that the sharp cores are due to gas streams in the system. We also point out the far less likely possibility that the sharp cores form in a hot, high-gravity atmosphere. The synthetic fitting results may imply that the hot matter is accreted from the inner part of the disk onto the surface of the white dwarf through a highly inhomogeneous gas flow. We relate this discussion to our FOS detection of highly asymmetric inverse P Cygni profile structure in the narrow stellar components at C iv λ1550.