Molecular hydrogen kinematics in the ring-like planetary nebula NGC 6781

Instituto de Astronomía, UNAM Campus Ensenada, Apdo. Postal 877, Ensenada, BC 22800, México e-mail: hiriart@astrosen.unam.mx

Received: 15 September 2004
Accepted: 10 December 2004

Abstract

This paper presents the radial velocity mapping of the molecular hydrogen in the ring-like planetary nebula NGC 6781. Observations of the H₂  S(1) emission line (2.122 μm,  μm) were obtained by Fabry-Pérot spectroscopy. These observations provide a very detailed map of the kinematic structure of the molecular hydrogen envelope in NGC 6781. The kinematic structure is explained with a model consisting of a thin hollow cylinder whose axis is tilted with respect to the line of sight, and gas expanding radially outward with a velocity proportional to the distance to the central star. Molecular hydrogen emission was detected at LSR velocities ranging from -6.2 to +57.1 km s^-1, with peak intensities between +15.7 and +25.5 km s^-1. There is evidence that the molecular hydrogen envelope is excited by shocks from the ionization front. The mass of molecular gas is estimated to be of the order of 0.2 . The emergent picture from the H₂ kinematic structure supports the scenario where the structure of NGC 6781 was formed from the ionization and destruction of an ellipsoidal molecular envelope that began with the destruction and ionization of the least dense polar caps and will continue until the densest molecular material at nebular waist is fully ionized.