Letter to the Editor

The nebula around the post-AGB star 89 Herculis ^*

¹ Observatorio Astronómico Nacional (OAN-IGN), Apartado 112, 28803 Alcalá de Henares, Spain e-mail: v.bujarrabal@oan.es
² Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium e-mail: [Pieter.Deroo;Hans.VanWinckel]@ster.kuleuven.be
³ Institut de Radio Astronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 St. Martin d'Hères, France e-mail: [ccarrizo;neri]@iram.fr
⁴ Observatorio Astronómico Nacional (OAN-IGN), C/ Alfonso XII 3, 28014 Madrid, Spain e-mail: j.alcolea@oan.es

Received: 19 December 2006
Accepted: 24 February 2007

Abstract

Aims.We aim to study the structure of the nebula around the post-AGB, binary star 89 Her. The presence of a rotating disk around this star had been proposed but not been yet confirmed by observations.

Methods.We present high-resolution PdBI maps of CO and 1-0. Properties of the nebula are directly derived from the data and model fitting. We also present N-band interferometric data on the extent of the hot dust emission, obtained with the VLTI.

Results.Two nebular components are found: (a) an extended hour-glass-like structure, with expansion velocities of ~7 km s^-1 and a total mass ~ , and (b) an unresolved very compact component, smaller than ~0 4 and with a low total velocity dispersion of ~5 km s^-1. We cannot determine the velocity field in the compact component, but we argue that it can hardly be in expansion, since this would require too recent and too sudden an ejection of mass. On the other hand, assuming that this component is a Keplerian disk, we derive disk properties that are compatible with expectations for such a structure; in particular, the size of the rotating gas disk should be very similar to the extent of the hot dust component from our VLTI data. Assuming that the equator of the extended nebula coincides with the binary orbital plane, we provide new results on the companion star mass and orbit.