The onset of photoionization in Sakurai's Object (V4334 Sgr)
van Hoof P.A.M., Hajduk M., Zijlstra A.A., Herwig F., Evans A., Van de Steene G.C., Kimeswenger S., Kerber F., Eyres S.P.S.
published in: A&A, 471, L9 (2007)
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 8 GHz — from ≤ 90 μJy
and 80 ± 30 μJy in February 2005 to 320 μJy and
280 μJy 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 12 kK in 2006. The mass
ejected in the VLTP eruption is Mej ≥ 10-4 Msol, but could be
as high as 10-2 Msol, depending mainly on the distance and the
clumping factor of the outflow. We derive a distance between 1.8 and
5 kpc. A high mass loss could expose the helium layer and yield
abundances compatible with those of [WC] and PG1159 stars.
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Peter van Hoof
Royal Observatory of Belgium
Ringlaan 3
1180 Brussel
Belgium
email: p DOT vanhoof AT oma DOT be
