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Session 67 - Stars: Evolution, Atmospheres, Intrinsic.
Display session, Thursday, June 11
VV Cephei (M2 Iab + B0?) is a long-period (20.3 yr) eclipsing binary system containing an M supergiant and a luminous early-type companion. The supergiant primary is of similar spectral class to \alpha Orionis. The spectrum of VV Cep is that of a typical M supergiant in the red (except for prominent H\alpha emission), but becomes composite in the blue-violet region, and the hot companion dominates in the ultraviolet. Although the stellar companion is probably not seen directly, the source of UV photons (likely an accretion region immediately around the companion) is still much more compact than the M supergiant (R_\ast \sim 1600 R_ødot). Thus, near eclipse, the companion provides an excellent ``light'' probe of the extended chromosphere and wind of the supergiant. Absorption along the line of sight near the primary appears superimposed upon the UV continuum of the hot companion. We are using the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) to monitor the UV chromospheric absorption spectrum as VV Cephei emerges from its 1997/98 total eclipse. From repeated HST/STIS observations (12 visits are scheduled in Cycle 7) of the egress phases over the next two years, we will construct a detailed model of the density, temperature and turbulent velocity structure of the chromosphere of VV Cephei.
Here we present preliminary results from our HST/STIS program GO-7269. The first visit occurred on 1997 November 29 near mid-eclipse and the second visit on 1998 March 21, with the third visit scheduled near third contact in late May 1998. We have obtained complete UV spectral coverage of VV Cep (at medium and high resolution) on both visits. This work was supported by STScI grant GO-07269.01-96A to the University of Colorado.
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