Other
Scientific paper
Jul 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995hst..prop.6019h&link_type=abstract
HST Proposal ID #6019
Other
Hst Proposal Id #6019 Hot Stars
Scientific paper
In a Cycle 1 HST study of eclipses of the dwarf nova OY Car in quiescence, we discovered strong Fe II absorption features that appear to form in the upper atmosphere {chromosphere} of the accretion disk along the line of sight to the white dwarf. Analysis of the spectrum allowed us to determine the temperature, density, column density, and velocity dispersion of the absorbing gas. The velocity dispersion is Mach 6, thought to be impossible for hydrodynamic turbulence. Furthermore, the column density is so high that the gas cannot be supported so far above the disk plane by thermal pressure alone. This is an exciting HST discovery that may be interpreted as evidence that magnetic turbulence supports the vertical structure of quiescent accretion disks. Our previous HST observations of OY Car were confined to a narrow range of phases around the eclipse. We will now obtain FOS and GHRS spectra of OY Car around complete binary orbits to map the supersonic velocity profile and other characteristics of the absorbing gas as functions of azimuth around the rim of the disk. This will establish whether the supersonic dispersion is due to turbulence or bulk motions, and whether it is intrinsic to the disk or induced by impact of the gas stream. We will also obtain FOS coverage of 3 additional short-period eclipsing dwarf novae to determine if the phenomenon is a widespread characteristic of quiescent accretion disks.
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