A Dust Model for the Frosty Nebula HH 100 IRS

Abstract

Comprehensive model calculations are presented in order to interpret the observed spectral energy distribution of HH 100 IRS as a function of dust parameters such as the grain size, the ice volume fraction, and the fluffiness of the particles. The radiative transfer calculations treat the spectroscopic signatures of various ice bands in detail. The observed infrared spectrum together with the strength of the water ice band of HH 100 IRS is successfully reproduced if an upper size limit of the grains below 1 μm is used. Large, comet-like grains, with sizes above 1 μm, result in a poor fit to the observations of HH 100 IRS. Contributions from scattering by the grains to the long-wavelength wing of the 3.08 μm H2O absorption are thus excluded. Our best-fit dust model includes grains with sizes a ≤ 1 μm, a vacuum volume fraction of fvac = 0.5, and a mass ratio of ice to refractive materials of Mice/Mref = 0.5. The modeled dust cloud toward HH 100 IRS is characterized by a dust density distribution of ρ(r) ∝ r-1, a total gas mass of 0.3 M☉, an inner ice evaporation zone with extension of r ≤ 23 AU, and an outer boundary of 2300 AU.