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

Abstract

We present models that reproduce the observed double-shell structure ofthe Homunculus Nebula around {η} Carinae, including the stratificationof infrared H_{2} and [Fe II] emission seen in data obtainedwith the Phoenix spectrograph on Gemini South, as well as thecorresponding stratified grain temperature seen in thermal-infrareddata. Tuning the model to match the observed shell thickness allows usto determine the threshold density that permits survival ofH_{2}. An average density ofn_{H}~=(0.5-1){\times}10^{7} cm^{-3} in the outerzone is required to allow H_{2} to exist at all latitudes in thenebula, and for Fe^{+} to recombine to Fe^{0}. Thisgives independent confirmation of the very large mass of the Homunculus,indicating a total of roughly 15-35 M_{solar} (although we notereasons why the lower end of this range is favored). At the interfacebetween the atomic and molecular zones, we predict a sharp drop in thedust temperature, in agreement with the bimodal dust color temperaturesobserved in the two zones. In the outer molecular shell, the dusttemperature drops to nearly the blackbody temperature, and becomesindependent of grain size because of self-shielding at shorter UVwavelengths and increased heating at longer wavelengths. This relaxesconstraints on large grain sizes suggested by near-blackbody colortemperatures. Finally, from the strength of infrared [Fe II] emission inthe inner shell we find that the gas-phase Fe abundance is roughlysolar. This is astonishing in such a dusty object, where one normallyexpects gaseous iron to be depleted by 2 orders of magnitude.Based in part on observations obtained at the Gemini Observatory, whichis operated by AURA, under a cooperative agreement with the NSF onbehalf of the Gemini partnership: the National Science Foundation (US),the Particle Physics and Astronomy Research Council (UK), the NationalResearch Council (Canada), CONICYT (Chile), the Australian ResearchCouncil (Australia), CNPq (Brazil), and CONICET (Argentina).