The hydrogen-deficient knot of the ‘born-again’ planetary nebula Abell 58 (V605 Aql)

Abstract

We have analysed deep optical spectra of the ‘born‐again’ planetary nebula Abell 58 and its hydrogen‐deficient knot, surrounding V605 Aql, which underwent a nova‐like eruption in 1919. Our analysis shows that the extinction towards the central knot is much higher than previously thought, with c(Hβ) = 2.0. The outer nebula is less reddened, with c(Hβ) = 1.04. We find that the outer nebula has a Ne/O ratio higher than the average PN value. The electron temperature we derive for the central knot varies widely depending on the diagnostic used. The [O iii] nebular‐to‐auroral transition ratio gives a temperature of 20 800 K, while the ratio of the [N ii] nebular and auroral lines gives Te= 15 200 K. The helium line ratios λ5876/λ4471 and λ6678/λ4471 imply temperatures of 350 and 550 K, respectively. Weakly temperature‐sensitive O ii recombination line ratios imply similarly low electron temperatures. Abundances derived from recombination lines are vastly higher than those found from collisionally excited lines, with the abundance discrepancy factor (ADF) for O2+ reaching 89 – the second highest known value after that found for the hydrogen‐deficient knots in Abell 30. The observed temperature diagnostics and abundances support the idea that, like Abell 30, the knot of Abell 58 contains some very cold ionized material. Although the central star is carbon‐rich (C/O > 1), the knot is found to be oxygen‐rich, a situation not predicted by the single‐star ‘born‐again’ theory of its formation. We compare the known properties of Abell 58 to those of Abell 30, Sakurai's Object and several novae and nova remnants. We argue that the abundances in the ejecta observed in A 30 and A 58 have more in common with neon novae than with Sakurai's Object, which is believed to have undergone a final helium flash. In particular, the C/O ratio of less than unity and the presence of substantial quantities of neon in the ejecta of both Abell 30 and Abell 58 are not predicted by very late thermal pulse models.

The post-common envelope central stars of the planetary nebulae Henize 2-155 and Henize 2-161

“…Perhaps a stronger link to binarity is the high adf, where recent work has shown that the PNe with the highest known adfs all host a central binary star (Corradi et al 2015, Jones et al in prep;and references therein). High adfs also strengthen the link between PNe with binary central stars and other binary phenomena, such as novae, which are also known to display extremely high adfs (Wesson et al 2003(Wesson et al , 2008. We strongly encourage the detailed study of other post-CE CSPN and their host nebulae (as well as the continued search for new systems) in order to improve the statistics and to establish the parameter space for future modelling and study of the CE and its effect on PN formation and evolution.…”

NGC 6778: a disrupted planetary nebula around a binary central star

“…There are several cases of this type of explosive-like, late events in PNe, especially associated to the late helium-shell flash associated to the bornagain class of PNe Wesson et al 2008) or to oxygen-neon-magnesium nova-like events suggested for the born-again PN A 58 (Lau et al 2011) or for BD+30…”

Planetary Nebulae: Observational Properties, Mimics and Diagnostics

“…Wesson (2008a) found a C/O ratio of less than unity and substantial quantities of neon in the ejecta of both Abell 30 and Abell 58, which are not predicted by very late thermal pulse models. The ejecta abundances are more akin to that seen in neon-rich novae!…”