Sakurai’s Object, V605 Aquilae, and FG Sagittae: An Evolutionary Sequence Revealed

Abstract

We have completed a grid of stellar evolution calculations to study the behavior of the born-again phenomenon. All our evolutionary sequences begin with a uniform composition 1 M star on the pre-main-sequence Hayashi phase and end on the white dwarf cooling track. The effects of combined helium and hydrogen burning and time-dependent convective mixing are included. We artificially vary the mass-loss rate beginning at the peak of the last thermal pulse on the asymptotic giant branch in order to create a range of He-layer masses for the post-asymptotic giant branch (AGB) evolution. We find that a very late thermal pulse occurs in 10%-15% of cases. Our models supply an answer to the question of why the born-again stars V4334 Sgr (Sakurai's object) and V605 Aql have a significantly shorter evolutionary timescale than the otherwise similar bornagain star FG Sge. FG Sge has been observed to undergo born-again behavior for more than 120 yr, while the other two objects have evolved in a similar way but in less then 10 yr. Models with low convective mixing efficiency, $10 À4 , first evolve quickly to the AGB, return to the blue, and then evolve more slowly back to the AGB for a second time before finally returning to the white dwarf cooling track. The difference in evolution timescales can then be explained by proposing that Sakurai's object is evolving to the AGB for the first time but that FG Sge has been observed during its second return to the AGB. Our models allow us to make some testable predictions: (1) Sakurai's object will increase in effective temperature in the next 20-50 yr and will then resemble V605 Aql's present high effective temperature state; (2) V605 Aql will cool back toward the AGB some time in the next 50-70 yr, at which point it will evolve in the same way as has been observed for FG Sge over the last 120 yr; and (3) FG Sge will show signs of increasing its effective temperature by about 1500-2000 K in as soon as 10-20 yr, depending on the metallicity of its progenitor main-sequence star.

The formation and evolution of hydrogen-deficient post-AGB white dwarfs: The emerging chemical profile and the expectations for the PG 1159-DB-DQ evolutionary connection

“…We remind the reader that in our calculation the double-diffusive MLT for fluids with composition gradients (Grossman & Taam 1996) Another prediction of our calculations is the occurrence of a double-loop in the Hertzsprung-Russell diagram. That is, the star reaches red giant dimensions for a second time after the onset of the last helium thermal pulse and before finally returning to the white dwarf cooling track, a behaviour reported by Lawlor & MacDonald (2003) and in the case of low convective mixing efficiency. In particular, this second return to the AGB takes about 350 yr in our calculations.…”

The end of super AGB and massive AGB stars

“…This type of instability has previously been discussed by Wood & Faulkner (1986) who suggested that the envelope could be ejected and the star would then evolve to the planetary nebula. Eddington limit luminosities as the cause of the numerical problems during low-mass envelope thermal pulses has already been identified by Lawlor & MacDonald (2003) 1 and Miller Bertolami et al (2006).…”

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

“…The later starts only after hydrogen burning has become extinct, and a critical feature of distinction is that the stellar surface is virtually free of hydrogen (Schönberner 2008). The known VLTP examples are V605 Aql, Sakuraiʼs object (V4334 Sgr), and perhaps FG Sge (Lawlor & MacDonald 2003;Schönberner 2008). But the Stingray certainly cannot be a VLTP star because it has only evolved off the AGB by roughly a millennium (when it ejected the PN shell), and because the central star has high abundance of hydrogen (as shown by the prominent Balmer absorption lines visible before 1980).…”