Roche-lobe filling factor of mass-transferring red giants: the PIONIER view^⋆

¹ ESO, Alonso de Córdova 3107, Casilla 19001, Santiago, Chile
e-mail: hboffin@eso.org
² Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
³ ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁴ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, Campus Plaine CP 226, Bd du Triomphe, 1050 Bruxelles, Belgium
⁵ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse, 85741 Garching, Germany
⁶ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
⁷ Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland

Received: 4 December 2013
Accepted: 31 January 2014

Abstract

Using the PIONIER visitor instrument that combines the light of the four Auxiliary Telescopes of ESO’s Very Large Telescope Interferometer, we precisely measure the diameters of several symbiotic and related stars: HD 352, HD 190658, V1261 Ori, ER Del, FG Ser, and AG Peg. These diameters – in the range of 0.6−2.3 milli-arcsec – are used to assess the filling factor of the Roche lobe of the mass-losing giants and provide indications on the nature of the ongoing mass transfer. We also provide the first spectroscopic orbit of ER Del, based on CORAVEL and HERMES/Mercator observations. The system is found to have an eccentric orbit with a period of 5.7 years. In the case of the symbiotic star FG Ser, we find that the diameter changes by 13% over the course of 41 days, while the observations of HD 352 are indicative of an elongation. Both these stars are found to have a Roche filling factor close to 1, as is most likely the case for HD 190658 as well, while the three other stars have factors below 0.5–0.6. Our observations reveal the power of interferometry for the study of interacting binary stars; the main limitation in our conclusions is the poorly known distances of the objects.