The structure and dynamics of the molecular envelope of M 2-56

Abstract

M 2-56 is a protoplanetary nebula (PPN) in which strong shocks are taking place, therefore, useful to study the post-AGB wind interaction. It is well known that molecular observations allow studying the mass distribution of PPNe, even in those regions that have been recently shocked. We present high-resolution maps of the emission of 12CO J = 2-1 and J = 1-0 in M 2-56. Such maps show a bipolar, molecular nebula that extends ~28 ´´ along the symmetry axis. The nebula is composed of two contiguous, incomplete shells located along the symmetry axis, which has an inclination of ~17o with respect to the plane of the sky. Those empty lobes intersect in the center of the nebula, where there is a small and dense ring perpendicular to the axis. This central ring expands radially at about 8 and seems to be the remnant of the circumstellar envelope of the AGB star, that has not been accelerated by the interaction with the fast post-AGB jets. The radius of the central ring is of ~ cm, for a distance of 2.1 kpc (deduced from an analysis of the main properties of the object). At ~ cm from the nebular center, the tips of the lobes reach axial expansion velocities of ~200 . We have developed a model for the spatio-kinematical distribution and the excitation conditions of the molecular gas in M 2-56. From the best fitting of the observations with the predictions of the model for both lines, we have estimated the physical conditions of the molecular nebula. It is found that the density varies from to cm-3 from the nebular center to the lobe tips, and that the part of the lobes that has not been detected is probably composed of photodissociated gas, due to the effect of interstellar photons on low-density regions. The rotational temperature is estimated to be approximately constant, ~13-16 K. For the assumed geometry, a velocity field composed by a dominant radial component plus an axial contribution has been deduced. The emission of both lines is found to be optically thin, and therefore probes the whole molecular gas, which has a mass of ~0.05 . The “scalar” momentum and the kinetic energy of the different regions of the molecular nebula have been calculated, finding that the high momentum won by the gas in the post-AGB phase cannot have been supplied by the radiation pressure mechanism. Although the central star of M 2-56 is not very hot yet (~20 000 K), this PPN has a large kinematical age, between 1000 yr and 1700 yr, in comparison with other PPNe that have hotter central stars. M 2-56 may not be a typical PPN, but an intermediate object between the known low-mass post-AGB nebulae and the standard PPNe.