Freshly ionized matter around the final Helium shell flash object V4334 Sgr (Sakurai's object)

摘要

We report on the discovery of recently ionized hydrogen-deficient gas in theimmediate circumstellar environment of the final helium shell flash star V4334Sgr (Sakurai's object). On spectra obtained with FORS2 multi-objectspectroscopy we have found spatially extended (about 2") emission from [N II],[O I], [O II] and very faint Halpha and [S II]. In the [N II] (ll6548,83) lineswe have identified two components located at velocities -350 +/-50 and +200+/-50 km/s, relative to V4334 Sgr itself. The full width of the [N II] l6583feature at zero intensity corresponds to a velocity spread of about 1500 km/s.Based on the available data it is not possible to conclusively determine themechanism of ionization. Both photo-ionization, from a rapidly evolving centralstar, and shock excitation, as the result of the collision of the fast ouflowswith slower circumstellar matter, could account for the observed lines. Thecentral star is still hidden behind strong dust absorption, since only a fainthighly reddened continuum is apparent in the spectra. Theory states that itwill become hotter and will retrace its post-asymptotic giant branch evolutiontowards the planetary nebula domain. Our detection of the ionized ejecta fromthe very late helium shell flash marks the beginning of a new phase in thisstar's amazingly rapid evolution.