Optical Properties of Interstellar Dust around the Orion A Molecular Cloud

Abstract

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 ($d \simeq 400\ \rm{pc}$). 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 T. et al. (2021) 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 $\simeq 0.3$ $\rmμ$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 $R_{V}$ is almost consistent with those derived from the color excess ratios.