The Molecular and Dust Envelope of HD 56126

We present millimeter interferometry images of the CO J=1-0 line emission arising in the circumstellar envelope of HD 56126 (=IRAS 07134+1005), which is one of the best-studied 21 μm 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. The CO envelope is very clumpy with no apparent fast wind to explain these microstructures that must have arisen during the AGB mass loss. 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 yr with a mass-loss rate of 5.1×10^-6 M_solar yr^-1 and a more intense superwind that lasted 840 yr with a mass-loss rate of 3×10^-5 M_solar yr^-1 and that ended the star's life on the AGB 1240 yr ago. The total mass of this envelope is 0.059 M_solar, which indicates a lower limit progenitor mass for the system of 0.66 M_solar, quite reasonable for this low-metallicity star that probably resides in the thick disk of the Galaxy. Comparison with images of the dust emission reveals a structure similar to that of the gas in the inner regions. Using 2-D UST, we model the dust emission of this source so that the model is consistent with the CO emission model and find a total dust mass of 7.8×10^-4 M_solar, a superwind-dust mass-loss rate of 1.9×10^-7 M_solar yr^-1 and an AGB-dust mass-loss rate of 9.6×10^-8 M_solar yr^-1. We derive an average gas-to-dust mass ratio of 75, which is consistent with ISM values but low for what most consider for carbon stars. Our results indicate that TiC nanocrystals are probably not the carrier of the 21 μm feature.