2005A&A...431..979C

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 ∼30km/s, and high-velocity jets distributed along the symmetry axis of the ring, which reach expansion velocities as high as ∼75km/s. 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 ∼10⁵cm^–3 and ∼10³cm^–3, and that the rotational temperature is very low, ∼10K. The kinematical lifetime of the molecular jets is ∼1700yr, 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 ∼30km/s. 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.