The central star of the Stingray Nebula

The Stingray Nebula is one of the youngest known plantetary nebula and what makes it more interesting is that the central star (SAO 244567) evolves much more rapid than predicted by stellar evolution theory. Normally it is not possible to watch a star evolving due to the long stellar evolutionary time scales. Space- and ground-based observations obtained over the last decades have, however, revealed that the spectrum of SAO 244567 changes noticeably over just a few years, showing stellar evolution in real time. Based on a spectral classification of the optical spectrum obtained in 1971, it was found to be a B-type star (Parthasarathy 1995). Optical spectra in 1990 and 1992 as well as the IUE spectra (1992-1996) however display many nebula emission lines, indicating that SAO 244567 has ionizied its nebula within twenty years only.

Stingray Nebula
Picture from the NASA Database

Fortunately, SAO 244567 was observed with various space-based telescopes from 1988 to 2006. To address the evolution of the properties of SAO 244567 quantitatively for the first time, I carried out a spectral analysis based on all available optical and UV spectra.
The main results are, that the central star has steadily increased its effective temperature from 38kK in 1988 to a peak value of 60kK 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 (Mdot/M/yr-1)=-9.0 to -11.6 and the terminal wind velocity increased from v = 1800km/s to 2800km/s. Since around 2002, the star stopped heating and has cooled down again to 55kK by 2006. It has a largely solar surface composition with the exception of slightly subsolar carbon, phosphorus, and sulfur.
The position of SAO 244567 in the log Teff - 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 prewhite dwarf after the common-envelope phase, during which the planetary nebula was ejected. The evolutionary status of SAO244567 remains unclear, with a late thermal pulse or a close binary evolution being the most likely explanations.

N. Reindl, T. Rauch, M. Parthasarathy, K. Werner, J. W. Kruk, W.-R. Hamann, A. Sander and H. Todt: The rapid evolution of the exciting star of the Stingray nebula, 2014, A&A 565, A40, arXiv:1403.6056

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New HST/COS observations

In order to follow the evolution of the surface properties of SAO 244567 and to find out more about its evolutionary history, we applied for further observations with HST/COS. The observations were performed on the 9th of August 2015, more than nine years after the star was observed by an UV telescope for the last time.
These new observations finally allow us to shed light on the evolutionary history of this extraordinary object. Our non-LTE spectral analysis of the observations HST/COS reveals that the star cooled significantly since 2002 and that its envelope is now expanding. Therefore, we conclude that SAO 244567 is currently on its way back towards the AGB, which strongly supports the LTP hypothesis. We stress that SAO 244567 is the only LTP object, that was observed during its blue- and red-wards motion through the Hertzsprung-Russell diagram. The evolutionary speed suggests a central star mass between 0.53 and 0.56 M. However, none of the current LTP models can fully reproduce the evolution of all surface parameters simultaneously. Refined LTP evolutionary calculations may not only help to explain the nature of SAO 244567, but could also provide a deeper insight into the evolution of central stars of PNe as well as the formation of H-deficient stars.

N. Reindl, T. Rauch, M. M. Miller Bertolami, H. Todt, K. Werner: Breaking news from the HST: The central star of the Stingray Nebula is now returning towards the AGB, 2016, MNRAS, in press

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