Optical Properties of Interstellar Dust around the Orion A Molecular Cloud

We have studied optical properties of interstellar dust around the Orion A molecular cloud to investigate the size distribution and the composition of dust grains. Orion A is one of the most studied molecular clouds in the solar vicinity ( d ≃ 400 pc). In this paper, we used optical and near-infrared photometric data. The optical data were obtained by BVRI bands imaging observations. The near-infrared data consisting of JHK S bands were taken from 2MASS point source catalog. We produced some color excess maps around Orion A and measured their ratios such as E ( R − I )/ E ( B − V ). In order to investigate dust properties, we compared the observed ratios with results of simulation performed by Naoi et al. who calculated the extinction in the optical to near-infrared wavelengths based on a standard dust model. They assumed a power-law grain-size distribution with an upper cutoff radius and assumed graphite and silicate as dominant components. As a result, we found that the upper cutoff radius around Orion A is ≃0.3 μ m, and silicate predominates compared with graphite (with the fraction of silicate grater than 93%). In addition, we further derived the total-to-selective extinction ratio R V from the observed extinction of A V and the color excess E ( B − V ), and compared it with the model calculations. Dust properties (i.e., the upper cutoff radius and the ratio of graphite / silicate ) derived from R V are almost consistent with those derived from the color excess ratios.