The Structure of the Homunculus. II. Modeling the Physical Conditions in η Carinae’s Molecular Shell

We present models that reproduce the observed double-shell structure of theHomunculus Nebula around eta Carinae, including the stratification of infraredH2 and [FeII] emission seen in data obtained with the Phoenix spectrograph onGemini South, as well as the corresponding stratified grain temperature seen inthermal-infrared data. Tuning the model to match the observed shell thicknessallows us to determine the threshold density which permits survival of H2. Anaverage hydrogen density of n_H=(0.5-1)x10^7 cm-3 in the outer zone is requiredto allow H2 to exist at all latitudes in the nebula, and for Fe+ to recombine.This gives independent confirmation of the very large mass of the Homunculus,indicating a total of roughly 15--35 Msun (although we note reasons why thelower end of this range is favored). At the interface between the atomic andmolecular zones, we predict a sharp drop in the dust temperature, in agreementwith the bimodal dust color temperatures observed in the two zones. In theouter molecular shell, the dust temperature drops to nearly the blackbodytemperature, and becomes independent of grain size because of self-shielding atshorter UV wavelengths and increased heating at longer wavelengths. Thisrelaxes constraints on large grain sizes suggested by near-blackbody colortemperatures. Finally, from the strength of infrared [FeII] emission in theinner shell we find that the gas-phase Fe abundance is roughly solar. This isastonishing in such a dusty object, where one normally expects gaseous iron tobe depleted by two orders of magnitude.Comment: 18 pages, 5 figures. Accepted by Ap