2007A&A...471L...9V

We investigate the reheating of the very late thermal pulse (VLTP) object V4334 Sgr (Sakurai's Object) using radio observations from the Very Large Array, and optical spectra obtained with the Very Large Telescope. We find a sudden rise of the radio flux at 5 and 8GHz - from ≤90µJy and 80±30µJy in February 2005 to 320µJy and 280µJy in June 2006. Optical line emission is also evolving, but the emission lines are fading. The optical line emission and early radio flux are attributed to a fast shock (and not photoionization as was reported earlier) which occurred around 1998. The fading is due to post-shock cooling and recombination. The recent rapid increase in radio flux is evidence for the onset of photoionization of carbon starting around 2005. The current results indicate an increase in the stellar temperature to 12kK in 2006. The mass ejected in the VLTP eruption is M_ej≥10^–4M_☉, but could be as high as 10^–2M_☉, depending mainly on the distance and the clumping factor of the outflow. We derive a distance between 1.8 and 5kpc. A high mass loss could expose the helium layer and yield abundances compatible with those of [WC] and PG1159 stars.