The disrupted molecular envelope of Frosty Leo ,

We present maps of CO emission in the protoplanetary nebula Frosty Leo. Observations of the rotational transitions 12CO J = 2−1 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.