The evolution of the final helium shell flash star V605 Aquilae, from 1917 to 1997

Geoffrey C. Clayton, Louisiana State University
Orsola De Marco, University College London


Only three stars have been observed going through a final helium shell flash since the advent of modern instrumentation, FG Sagittae (in 1894), V605 Aquilae (in 1919), and Sakurai's Object (in 1996). In this paper, we present for the first time a spectrum taken of V605 Aql, while it was in its cool giant phase in 1921. This spectrum is identical to that typically seen in R Coronae Borealis stars with Teff∼ 5000 K. It shows strong Swan bands of C2 and the violet CN bands as well as weak or absent hydrogen features and little or no evidence of 13C. The star reached MB∼ -4.7 at its outburst peak in 1919 assuming a distance of 3.5 kpc. In addition, we discuss ground-and space-based observations of the present state of V605 Aql. The star is very faint (mv≲23) but a broad C IV λ5800 emission line is seen, indicating V605 Aql now has a Wolf-Rayet-type spectrum with Teff≥50 000 K. We argue that the present faintness of the star is likely to be due to a thick cloud of dust, rather than the star becoming intrinsically fainter after the post-outburst luminosity peak of 1919. The dust is contained in a hydrogen-deficient nebular knot ejected during the final flash and now resolved as a small asymmetrical nebula. The star and knot are at the center of A58, an old Planetary Nebula. Surrounding A58 is a much larger shell-like structure seen in the IRAS 100 μm map. An apparent shell of diameter 40′ centered on V605 Aql can be seen. V605 Aql provides a unique opportunity to investigate the evolution of a final flash star over a period of 80 years. In the observed evolution of V605 Aql, we may be seeing the future of Sakurai's object and also a possible link to the formation of R Coronae Borealis stars © 1997 The American Astronomical Society.