Analysis of dust grain properties in two star-forming regions through comparison of theoretical models to observations

Infrared reflection nebulae in two regions of star formation, Orion Molecular Cloud-2 and Cepheus A, were studied both observationally and theoretically. An analytical formula is derived that describes the scattered light intensity under the assumption of single scattering. The grain components, graphite, silicate, and ice, determined the scattering and absorption properties of the dust. Mie theory was used to study the behavior of a Mathis, Rumpl, and Nordsieck (1977) grain-size distribution of particles, which ranged in radius from 0.005 ≤ a ≤ 0.3 and 0.05 ≤ a ≤ 0.5 μm, called the standard and non-standard cases, respectively. Variations of the optical depth of the nebula, the scattering angle, grain sizes, and the effect of ice mantles were investigated. The models show that it is possible to explain the continuum flux level and shape of the near-infrared scattered light with illumination by a near-infrared source and a grain-size distribution of graphite and silicate grains. The 3.1-μm feature seen in the spectra of the reflected light was reproduced by coating the grains in the nebula with water ice