MV lyrae in low, intermediate, and high states

Archival IUE spectra of the VY Sculptoris system MV Lyrae, taken during an intermediate state, can be best fit by an isothermal accretion disk extending half-way to the tidal cutoff radius. In contrast, a recent HST spectrum, while MV Lyr was in a high state, can be best fit with a standard T(R) profile for an accretion disk extending from an inner truncation radius to an intermediate radius with an isothermal accretion disk beyond. These fits use component-star parameters determined from a study of MV Lyr in a low state. Model systems containing accretion disks with standard T(R) profiles have continua that are too blue. The observed high-state absorption-line spectrum exhibits excitation higher than provided by the T(R) profile, indicating likely line formation in a high-temperature region extending vertically above the accretion disk. The absorption lines show a blueshift and line broadening corresponding to formation in a low-velocity wind apparently coextensive with the high-temperature region. Lines of N V, Si IV, C IV, and He II are anomalously strong relative to our synthetic spectra, indicating possible composition effects, but unmodeled excitation effects could also produce the anomalies. An analysis of a low state of MV Lyr, considered in an earlier study and extended in this paper, sets a limit of 2500 K for the T_eff of an accretion disk that may be present in the low state. This limit is in conflict with two recent models of the VY Sculptoris phenomenon.