  We present millimeter interferometry images of the CO J=1-0 line emission
arising in the circumstellar envelope of HD 56126 (a.k.a. IRAS 07134+1005),
which is one of the best studied 21-micron proto-planetary nebulae (PPNs). The
CO emission extends from 1.2" to 7" in radius from the central star and appears
consistent with a simple expanding envelope, as expected for a post-AGB star.
We quantitatively model the molecular envelope using a radiative transfer code
that we have modified for detached shells. Our best fit model reveals that two
sequential winds created the circumstellar envelope of HD 56126: an AGB wind
that lasted 6500 years with a mass-loss rate of 5.1x10^{-6} M_odot yr^{-1} and
a more intense superwind that lasted 840 years with a mass-loss rate of
3x10^{-5} M_odot yr^{-1} and that ended the star's life on the AGB 1240 years
ago. The total mass of this envelope is 0.059 M_odot which indicates a lower
limit progenitor mass for the system of 0.66 M_odot, quite reasonable for this
low-metallicity star which probably resides in the thick disk of the Galaxy.
Comparison with images of the dust emission reveal a similar structure with the
gas in the inner regions. Using 2-DUST, we model the dust emission of this
source so that the model is consistent with the CO emission model and find an
average gas-to-dust mass ratio of 75.

Astrophysics