The patchy accretion disc in HT Cassiopeiae

We reconstruct the temperatures and surface densities in the quiescent accretion disc in HT Cas by performing a Physical Parameter Eclipse Mapping analysis of archival UBVR observations. Using a simple hydrogen slab model and demanding a smooth, maximally artefact-free reconstruction, we derive a formal distance to HT Cas of 207+/-10pc, significantly larger than the 133+/-14pc we derive from a re-analysis of the data in the literature. The accretion disc is small (0.3-0.4R _L1 ) and moderately optically thin, but becomes nearly optically thick near the white dwarf. The temperatures and surface densities in the disc range from 9500K and 0.013gcm^-2 in the centre to about 4000K and 0.04gcm^-2 at the disc edge. The mass-accretion rate in the disc is roughly constant but - at the derived distance - uncomfortably close to the rate that would prohibit the dwarf nova eruptions. We argue that the larger derived distance is probably incorrect but is not produced by inaccuracies in our spectral model or optimization method. The discrepancy can be resolved if the emission regions on the disc are patchy with a filling factor of about 40 per cent of the disc's surface. This solves the problem with the high effective temperatures in the disc - reducing them to around 6500K within a radius of 0.2R _L1 - and reduces the derived temperature of the white dwarf and/or boundary layer from 22600 to 15500K. The viscosity parameters α derived from all reconstructed temperatures and surface densities are of order 10-100 and cannot be lowered significantly by invoking a lower distance or the filling factor. This situation is easily explained using the same patchy nature of the emitting material, since the quiescent disc cannot consist of optically thin regions alone, but also of a dark and hence cold and dense disc which could easily contain most of the matter. If we require global values of α of order 0.1, the implied total surface densities are 1-100gcm^-2 - just like those expected for quiescent discs awaiting the next eruption. We discuss several possible sources of the chromospheric emission and its patchiness, including irradiation of the disc, thermal instabilities, spiral-wave-like global structures, and magnetically active regions associated with dynamo action and/or Balbus-Hawley instabilities.