The Molecular and Dust Envelope of HD 56126
Abstract
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 Modot yr-1 and a more intense superwind that lasted 840 years with a mass-loss rate of 3x10-5 Modot yr-1 and that ended the star's life on the AGB 1240 years ago. The total mass of this envelope is 0.059 Modot which indicates a lower limit progenitor mass for the system of 0.66 Modot, 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.
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