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

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 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 M ej ≥ 10 −4 M , but could be as high as 10 −2 M , 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.

Photometry of the Stingray Nebula (V839 Ara) From 1889 to 2015 Across the Ionization of Its Planetary Nebula

“…There is precedent for this fading and this interpretation for Sakuraiʼs Object (V4334 Sgr, a very-late-thermal-pulse born-again star). Its emission line fluxes have also been seen to decline fairly rapidly (with a half-life just under two years) from 2001 to 2006, with the fading attributed to cooling and recombination in the shell after the heating and ionization ended (Van Hoof et al 2007).…”

“…Gesicki et al (2014) has found similar problems in accounting for the white dwarf mass distribution, and they have had to postulate an acceleration by a factor of 3 in the evolutionary timescales of Blocker (1995). In his major review on post-AGB stars, Van Winckel (2003) says "the detailed description of the badly known external post-AGB mass loss on top of the mass loss from the nucleosynthetic consumption is crucial for the transition time estimates" for the star crossing the HR diagram in various phases. It might yet be possible for some new model calculation to match the Stingrayʼs evolution, but until then, the LTP idea has a fundamental problem.…”

“…So for this solution, the Stingray would have to be an LTP star. The LTP can make the star trace out a wide variety of loops through the HR diagram (Schönberner 1983;Blocker 1995;Blocker & Schönber-ner 1997;Van Winckel 2003). In an extreme case (see Figure 2 of Blocker & Schönberner 1997), the starʼs track has nine reversals of direction in the HR diagram.…”

Investigation of the recombination of the retarded shell of “born-again” CSPNe by time-dependent radiative transfer models

“…The initial conditions were calculated by CLOUDY. For the evolution of the "born-again" star in the Hertzsprung-Russell-Diagramm the tracks as used in van Hoof et al (2007) were applied.The nebulae of the "born-again" central stars recombine extremely slowly. The hydrogen and helium timescales dominate the whole process.…”

“…The initial conditions were calculated by CLOUDY. For the evolution of the "born-again" star in the Hertzsprung-Russell-Diagramm the tracks as used in van Hoof et al (2007) were applied.…”