2024ApJ...965...29L

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^{\prime} $ and reaches a depth of A_K ∼ 3 mag. The FIR optical depths τ₂₅₀ at a reference wavelength of 250 μm are obtained by fitting the Herschel PACS and SPIRE continuum data at 100, 160, 250, 350, and 500 μm using a modified blackbody model. The average dust opacity per unit gas mass at 250 μm, rκ₂₅₀, is determined through a pixel-by-pixel correlation of τ₂₅₀ 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.