2013A&A...552A..25A

The origin of hot subdwarf O-type stars (sdOs) remains unclear since their discovery in 1947. Among others, a post-asymptotic giant branch (post-AGB) origin is possible for a fraction of sdOs. We are involved in a comprehensive ongoing study to search for and to analyze planetary nebulae (PNe) around sdOs with the aim of establishing the fraction and properties of sdOs with a post-AGB origin. We used deep Hα and [OIII] images of sdOs to detect nebular emission and intermediate-resolution, long-slit optical spectroscopy of the detected nebulae and their sdO central stars. These data were complemented with other observations (archive images, high-resolution, long-slit spectroscopy) for further analysis of the detected nebulae. We report detecting an extremely faint, complex PN around 2MASSJ19310888+4324577 (2M1931+4324), a star classified as sdO in a binary system. The PN shows a bipolar and an elliptical shell, whose major axes are oriented perpendicular to each other, and high-excitation structures outside the two shells. WISE archive images show faint, extended emission at 12µm and 22µm in the inner nebular regions. The internal nebular kinematics, which is derived from high-resolution, long-slit spectra, is consistent with a bipolar and a cylindrical/ellipsoidal shell, in both cases with the main axis mainly perpendicular to the line of sight. The nebular spectrum only exhibits Hα, Hβ, and [OIII]λλ4959,5007 emission lines, but suggests a very low-excitation ([OIII]/Hβ≃1.5), in strong contrast to the absence of low-excitation emission lines. The spectrum of 2M1931+4324 presents narrow, ionized helium absorptions that confirm the previous sdO classification and suggest an effective temperature ≥60000K. The binary nature of 2M1931+4324, its association with a complex PN and several properties of the system provide strong support for the idea that binary central stars are a crucial ingredient in the formation of complex PNe.