Evidence for a [WR] or WEL-type binary nucleus in the bipolar planetary nebula Vy 1-2

Abstract

We present high-dispersion spectroscopic data of the compact planetary nebula Vy 1-2, where high expansion velocities up to 100 km s^-1 are found in the Hα, [N II] and [O III] emission lines. Hubble Space Telescope images reveal a bipolar structure. Vy 1-2 displays a bright ring-like structure with a size of 2.4 arcsec × 3.2 arcsec and two faint bipolar lobes in the east-west direction. A faint pair of knots is also found, located almost symmetrically on opposite sides of the nebula at position angle = 305°. Furthermore, deep low-dispersion spectra are also presented and several emission lines are detected for the first time in this nebula, such as the doublet [Cl III] 5517, 5537, [K IV] 6101, C II 6461 and the doublet C IV 5801, 5812Å. By comparison with the solar abundances, we find enhanced N, depleted C and solar O. The central star must have experienced the hot-bottom burning (CN-cycle) during the second dredge-up phase, implying a progenitor star of M ≥ 3 M_⊙. The very low C/O and N/O abundance ratios suggest a likely post-common envelope close binary system. A simple spherically symmetric geometry with either a blackbody or an H-deficient stellar atmosphere model is not able to reproduce the ionization structure of Vy 1-2. The effective temperature and luminosity of its central star indicate a young nebula located at a distance of ∼9.7 kpc with an age of ∼3500 yr. The detection of stellar emission lines, C II 6461, the doublet C IV λλ 5801, 5812 and O III 5592 Å, emitted from an H-deficient star, indicates the presence of a late-type Wolf-Rayet or a WEL-type central star.