Spectroscopic survey of faint planetary-nebula nuclei - II. The subdwarf O central star of Fr 2-30

Fr 2-30 = PN? G126.8-15.5 is a faint emission nebula, hosting a 14th-mag central star that we identify here for the first time. Deep H alpha and [O III] images reveal a roughly elliptical nebula with dimensions of at least 22 arcmin x 14 arcmin, fading into a surrounding network of even fainter emission. Optical spectrograms of the central star show it to have a subdwarf O spectral type, with a Gaia parallax distance of 890 pc. A model-atmosphere analysis gives parameters of T-eff = 60 000 K, log g = 6.0, and a low helium content of n(He)/n(H) = 0.0017. The location of the central star in the log g-T-eff plane is inconsistent with a post-asymptotic-giant-branch evolutionary status. Two alternatives are that it is a helium-burning post-extreme-horizontal-branch object, or a hydrogen-burning post-red-giant-branch star. In either case, the evolutionary ages are so long that a detectable planetary nebula (PN) should not be present. We find evidence for a variable radial velocity (RV), suggesting that the star is a close binary. However, there are no photometric variations, and the spectral-energy distribution rules out a companion earlier than M2 V. The RVs of the star and surrounding nebula are discordant, and the nebula lacks typical PN morphology. We suggest that Fr 2-30 is a 'PN mimic' - the result of a chance encounter between the hot sdO star and an interstellar cloud. However, we note the puzzling fact that there are several nuclei of genuine PNe that are known to be in evolutionary states similar to that of the Fr 2-30 central star.

Automated summary

This paper reports the discovery of a 14th-magnitude central star in the faint emission nebula G126.8-15.5. Spectrograms and model-atmosphere analysis were used to determine the spectral classification and physical parameters of the star. The location of the central star on the log g-T-eff plane suggests it is not in a post-asymptotic-giant-branch evolutionary status. The researchers propose that Fr 2-30 is a 'PN mimic', resulting from a chance encounter between a hot sdO star and an interstellar cloud.