2011A&A...529A..43C

Understanding the formation of collimated outflows is one of the most debated and controversial topics in the study of the late stages of stellar evolution. M 2-9 is an outstanding representative of extreme aspherical flows. It presents unique features such as a pair of high-velocity dusty polar blobs and a mirror-symmetric rotating pattern in the inner lobes. Their study provides important information on the nature of the poorly understood central source of M 2-9 and its nebula. Imaging monitoring at sub-arcsec resolution of the evolution of the nebula in the past decade is presented. Spectroscopic data provide complementary information. We determine the proper motions of the dusty blobs, which infer a new distance estimate of 1.3±0.2kpc, a total nebular size of 0.8pc, a speed of 147km/s, and a kinematical age of 2500yr. The corkscrew geometry of the inner rotating pattern is confirmed and quantified. Different recombination timescales for different ions explain the observed surface brightness distribution. According to the images taken after 1999, the pattern rotates with a period of 92±4years. On the other hand, the analysis of images taken between 1952 and 1977 measures a faster angular velocity. If the phenomenon were related to orbital motion, this would correspond to a modest orbital eccentricity (e=0.10±0.05), and a slightly shorter period (86±5years). New features have appeared after 2005 on the west side of the lobes and at the base of the pattern. The geometry and travelling times of the rotating pattern support our previous proposal that the phenomenon is produced by a collimated spray of high velocity particles (jet) from the central source, which excites the walls of the inner cavity of M 2-9, rather than by a ionizing photon beam. The speed of such a jet would be remarkable: between 11000 and 16000km/s. The rotating-jet scenario may explain the formation and excitation of most of the features observed in the inner nebula, with no need for additional mechanisms, winds, or ionization sources. All properties point to a symbiotic-like interacting binary as the central source of M 2-9. The new distance determination implies system parameters that are consistent with this hypothesis.