Letter to the Editor

The onset of photoionization in Sakurai's Object (V4334 Sagittarii)^*

¹ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium e-mail: p.vanhoof@oma.be
² Centrum Astronomii UMK, ul.Gagarina 11, 87-100 Torun, Poland
³ University of Manchester, School of Physics & Astronomy, PO Box 88, Manchester M60 1QD, UK
⁴ South African Astronomical Observatory, PO Box 7935, Observatory, South Africa
⁵ Astrophysics Group, School of Physical and Geographical Sciences, Keele University, Staffordshire ST5 5BG, UK
⁶ Institute of Astro and Particle Physics, University Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria
⁷ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁸ Centre for Astrophysics, University of Central Lancashire, Preston PR1 2HE, UK

Received: 23 May 2007
Accepted: 15 June 2007

Abstract

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 320Jy and 280Jy 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 10^-4 , but could be as high as 10^-2 , 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.