Morphological and kinematic signatures of a binary central star in the planetary nebula Hu 2-1

Abstract

We present narrow-band optical images, VLA-A 3.6 cm continuum and H92alpha emission line data and high resolution long-slit [N II] spectra of Hu 2-1. The formation of Hu 2-1 appears dominated by anisotropic mass ejection during the late AGB stage of its progenitor and by variable, ``precessing'' collimated bipolar outflows during the proto-PN and/or early PN phases. The data strongly support the existence of a binary central star at center of Hu 2-1: (1) the brightness distribution of the bipolar lobes and inner shell is point-symmetric, and the bipolar lobes show departures from axial symmetry; (2) the position of the central star is off-center; (3) mass ejection towards the equatorial plane is detected; (4) ``precessing'' collimated outflows are present. Besides all this circumstantial evidences, the analysis of the kinematics shows that the systemic velocity of the bipolar outflows does not coincide with the systemic velocity of the nebula. We propose that this velocity difference is a direct evidence of orbital motion of the ejection source in a binary system. Assuming a reasonable range of masses for the progenitor stars, the orbital velocity of ~10 km/s corresponds to a semi-major axis of 9-27 AU and to a period of 25-80 yr. The formation of Hu 2-1 is analyzed within current scenarios of PN formation with binary central stars.