The rapid evolution of the exciting star of the Stingray nebula

Abstract

Context. SAO 244567, the exciting star of the Stingray nebula, is rapidly evolving. Previous analyses suggested that it has heated up from an effective temperature of about 21 kK in 1971 to over 50 kK in the 1990s. Canonical post-asymptotic giant branch evolution suggests a relatively high mass while previous analyses indicate a low-mass star.
Aims: A comprehensive model-atmosphere analysis of UV and optical spectra taken during 1988-2006 should reveal the detailed temporal evolution of its atmospheric parameters and provide explanations for the unusually fast evolution.
Methods: Fitting line profiles from static and expanding non-LTE model atmospheres to the observed spectra allowed us to study the temporal change of effective temperature, surface gravity, mass-loss rate, and terminal wind velocity. In addition, we determined the chemical composition of the atmosphere.
Results: We find that the central star has steadily increased its effective temperature from 38 kK in 1988 to a peak value of 60 kK in 2002. During the same time, the star was contracting, as concluded from an increase in surface gravity from log g = 4.8 to 6.0 and a drop in luminosity. Simultaneously, the mass-loss rate declined from log (Ṁ/M_⊙ yr^-1) = -9.0 to -11.6 and the terminal wind velocity increased from v_∞ = 1800 km s^-1 to 2800 km s^-1. Since around 2002, the star stopped heating and has cooled down again to 55 kK by 2006. It has a largely solar surface composition with the exception of slightly subsolar carbon, phosphorus, and sulfur. The results are discussed by considering different evolutionary scenarios.
Conclusions: The position of SAO 244567 in the log T_eff-log g plane places the star in the region of sdO stars. By comparison with stellar-evolution calculations, we confirm that SAO 244567 must be a low-mass star (M< 0.55 M_⊙). However, the slow evolution of the respective stellar evolutionary models is in strong contrast to the observed fast evolution and the young planetary nebula with a kinematical age of only about 1000 years. We speculate that the star could be a late He-shell flash object. Alternatively, it could be the outcome of close-binary evolution. Then SAO 244567 would be a low-mass (0.354 M_⊙) helium pre-white dwarf after the common-envelope phase, during which the planetary nebula was ejected.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26666.Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer.Tables 1 and 3, and Figs. 4-9 are available in electronic form at http://www.aanda.org