Binary central stars of planetary nebulae with long orbits: the radial velocity orbit of BD+33°2642 (PN G052.7+50.7) and the orbital motion of HD 112313 (PN LoTr5)

Abstract

We study the impact of binary interaction processes on the evolution of low- and intermediate-mass stars using long-term monitoring of their radial velocity. Here we report on our results on the central stars of two planetary nebulae (PNe): the well-studied spectrophotometric standard BD+33.2642 (central star of PNG 052.7+50.7) and HD112313 (central star of PN LoTr5), the optical light of which is dominated by a rapidly rotating G star. The high-resolution spectra were cross-correlated with carefully selected masks of spectral lines. The individual masks were optimised for the spectral signatures of the dominant contributor of the optical light. We report on the first detection of orbital motion in these two objects. For BD+33.2642 we sampled 1.5 cycles of the 1105 +/- 24 day orbital period. For HD 112313 a full period is not yet covered, despite our 1807 days of monitoring. The radial-velocity amplitude shows that it is unlikely that the orbital plane is co-planar with the one defined by the nebular waist of the bipolar nebula. To our knowledge these are the first detections of orbits in PNe that are in a range from several weeks to a few years. The orbital properties and chemical composition of BD+33.2642 are similar to what is found in post-AGB binaries with circumbinary discs. The latter are probably progenitors of these PNe. For LoTr5 the Ba-rich central star and the long orbital period are similar to the Ba star giants, which hence serve as natural progeny. In contrast to the central star in LoTr5, normal Ba stars are slow rotators. The orbits of these systems have a low probability of occurrence according to recent population synthesis calculations.