Wide binaries in planetary nebulae with Gaia DR2

Abstract

Context. The Gaia Data Release 2 (DR2) was used to select a sample of 211 central stars of planetary nebulae (CSPNe) with good-quality astrometric measurements, which we refer to as Golden Astrometry Planetary Nebulae (GAPN). Gaia astrometric and photometric measurements allowed us to derive accurate distances and radii and to calculate luminosities with the addition of self-consistent literature values. Such information was used to plot the position of these stars in a Hertzsprung-Russel diagram and to study their evolutionary status in comparison with the evolutionary tracks of CSPNe. Aims. The extremely precise measurement of parallaxes and proper motions in Gaia DR2 has allowed us to search for wide binary companions in a region close to each of the central stars in the GAPN sample. We are interested in establishing the presence of binary companions at large separations which could allow to contribute additional information on the influence of binarity on the formation and evolution of planetary nebulae. We aim to study the evolutive properties of the binary pairs to check the consistency of spectral types and masses in order to better constrain the ages and evolutionary stage of the CSPNe. Methods. We limited our search to a region around 20 000 AU of each CSPN to minimise accidental detections. We only considered stars with reasonably good parallax and proper motions data, that is, with errors below 30% in DR2. We determined that the hypothetical binary pairs should show a statistically significant agreement for the three astrometric quantities, that is, parallax and both components of the proper motions. Results. We found eight wide binary systems among our GAPN sample, including one in a triple system. We compiled the astrometric and photometric measurements of these binary systems and discussed them in relation to previously published searches for binaries in PNe. By analysing the position in the HR diagram of the companion stars using Gaia photometry, we are able to estimate their temperatures, luminosities, masses and, for one star, the evolutionary age. The derived quantities yield a consistent scenario when compared with the corresponding values as obtained for the central stars using stellar evolutionary models in the postAGB phase.