Freshly ionized matter around the final helium shell flash object V4334 sagittarii (Sakurai's object)

F. Kerber, Space Telescope European Coordinating Facility
N. Pirzkal, Space Telescope European Coordinating Facility
Orsola De Marco, American Museum of Natural History
M. Asplund, Australian National University, Mount Stromlo Observatory
G. C. Clayton, Louisiana State University
M. R. Rosa, Space Telescope European Coordinating Facility


We report on the discovery of recently ionized hydrogen-deficient gas in the immediate circumstellar environment of the final helium shell flash star V4334 Sgr (Sakurai's object). On spectra obtained with FORS2 multiobject spectroscopy, we have found spatially extended (∼2″) emission from [N II], [O I], [O II], and very faint Hα and [S II]. In the [N II] (λλ6548, 6583) lines, we have identified two components located at velocities -350 ± 50 and +200 ± 50 km s-1, relative to V4334 Sgr itself. The full width of the [N II] λ6583 feature at zero intensity corresponds to a velocity spread of ∼1500 km s-1. On the basis of the available data, it is not possible to conclusively determine the mechanism of ionization. Both photoionization, from a rapidly evolving central star, and shock excitation, resulting from the collision of the fast ouflows with slower circumstellar matter, could account for the observed lines. The central star is still hidden behind strong dust absorption, since only a faint highly reddened continuum is apparent in the spectra. Theory states that it will become hotter and will retrace its post-asymptotic giant branch evolution toward the planetary nebula domain. Our detection of the ionized ejecta from the very late helium shell flash marks the beginning of a new phase in this star's amazingly rapid evolution.