The disrupted molecular envelope of Frosty Leo ^*,**

¹ Institut de RadioAstronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France e-mail: ccarrizo@iram.fr
² Observatorio Astronómico Nacional (IGN), Apdo. 112, 28803 Alcalá de Henares, Spain e-mail: v.bujarrabal@oan.es
³ Caltech Astronomy Department, MS 105-24, California Institute of Technology, Pasadena, CA 91125, USA e-mail: sanchez@astro.caltech.edu
⁴ Jet Propulsion Laboratory, MS 183-900, California Institute of Technology, Pasadena, CA 91109, USA e-mail: Raghvendra.Sahai@jpl.nasa.gov
⁵ Observatorio Astronómico Nacional (IGN), c/ Alfonso XII 3, 28014, Madrid, Spain e-mail: j.alcolea@oan.es

Received: 17 June 2004
Accepted: 19 October 2004

Abstract

We present maps of CO emission in the protoplanetary nebula Frosty Leo. Observations of the rotational transitions ¹²CO and 1–0 have been obtained with the IRAM interferometer and the OVRO array. The molecular envelope of Frosty Leo is found to be complex and compact; most of the gas extends 6´´ and shows a structure that is very different to the extended optical nebula. It is composed of a central ring-like structure, whose symmetry axis is inclined ~-40° with respect to the sky plane and expands at speeds of up to ~30 km s^-1, and high-velocity jets distributed along the symmetry axis of the ring, which reach expansion velocities as high as ~75 km s^-1. The symmetry axis of the molecular jets in the plane of the sky coincides with the direction of some jet-like features seen in the optical, which are not aligned at all with the main symmetry axis of the optical nebula. The brightness distribution of the ring presents a clumpy structure. We have modeled the spatio-kinematical distribution of, and the excitation conditions in, the molecular envelope. For both transitions, the ¹²CO emission is found to be very optically thick in the center of the nebula. From our best-fit model, we find that the nebular particle density varies between ~10⁵ cm^-3 and ~10³ cm^-3, and that the rotational temperature is very low, ~10 K. The kinematical lifetime of the molecular jets is ~1700 yr, long in comparison with the lifetime of the post-AGB winds of most PPNe. It is very remarkable that the bulk of the gas accelerated during the post-AGB phase of Frosty Leo is located within the central ring, reaching expansion velocities of up to ~30 km s^-1. The central ring-like distribution of Frosty Leo is probably not the undisrupted remnant of the previous AGB envelope (as found for most PPNe), but its dynamics likely result from multiple post-AGB interactions.