Spatial Variations of Dust Opacity and Grain Growth in Dark Clouds: L1689, L1709, and L1712

The far-infrared (FIR) opacity of dust in dark clouds within the Ophiuchus molecular cloud is investigated through multiwavelength infrared observations from UKIDSS, Spitzer, and Herschel. Employing the infrared color excess technique with both near-infrared and mid-infrared photometric data, a high-resolution extinction map in the K band (A K ) is constructed for three dark clouds: L1689, L1709, and L1712. The derived extinction map has a resolution of 1 ' and reaches a depth of A K similar to 3 mag. The FIR optical depths tau 250 at a reference wavelength of 250 mu m are obtained by fitting the Herschel PACS and SPIRE continuum data at 100, 160, 250, 350, and 500 mu m using a modified blackbody model. The average dust opacity per unit gas mass at 250 mu m, r kappa 250, is determined through a pixel-by-pixel correlation of tau 250 with A K , yielding a value of approximately 0.09 cm2 g-1, which is about 2-3 times higher than the typical value in the diffuse interstellar medium. Additionally, an independent analysis across 16 subregions within the Ophiuchus cloud indicates spatial variations in dust opacity, with values ranging from 0.07 to 0.12 cm2 g-1. Although the observed trend of increasing dust opacity with higher extinction implies grain growth, our findings indicate that rapid grain growth has clearly not yet occurred in the dark clouds studied in this work.