Voracious vortices in cataclysmic variables: II. Evidence for the expansion of accretion disc material beyond the Roche lobe of the accretor in HT Cassiopeia during its 2017 superoutburst

In our earlier Paper I we showed that the accretion disc radius of the dwarf nova HT Cas in its quiescent state has not changed significantly during many years of observations. It has remained consistently large, close to the tidal truncation radius. This result is inconsistent with the modern understanding of the evolution of the disc radius through an outburst cycle. Here we present a new set of time-resolved spectra of HT Cas obtained in the middle of its 2017 superoutburst. We used Doppler tomography to map emission structures in the system, which we compared with those detected in quiescence. We used solutions of the restricted three-body problem to re-discuss the location of emission structures and the disc size of HT Cas in quiescence. The superoutburst spectrum is similar in appearance to the quiescent spectra, although the strength of most of the emission lines decreased. However, the high-excitation lines significantly strengthened in comparison with the Balmer lines. Many lines show a mix of broad emission and narrow absorption components. H$\alpha$ in superoutburst was much narrower than in quiescence. Other emission lines also narrowed in outburst, but they had not become as narrow as H$\alpha$. Doppler maps of H$\alpha$ in quiescence, and of the H$\beta$ and HeI lines in outburst are dominated by a bright emission arc in the right side of the tomograms, which is located at and even beyond the theoretical truncation limit. However, the bulk of the H$\alpha$ emission in outburst has significantly lower velocities. We show that the accretion disc radius of HT Cas during its superoutburst has become hot but remained the same size as it was in quiescence. Instead, we detected cool gas beyond the Roche lobe of the white dwarf that may have been expelled from the hot disc during the superoutburst.