The Optical–Mid-infrared Extinction Law of the l = 165° Sightline in the Galactic Plane: Diversity of the Extinction Law in the Diffuse Interstellar Medium

Understanding the effects of dust extinction is important to properly interpret observations. The optical total-to-selective extinction ratio, $R_V=A_V/E(B-V)$, is widely used to describe extinction variations in ultraviolet and optical bands. Since the $R_V=3.1$ extinction curve adequately represents the average extinction law of diffuse regions in the Milky Way, it is commonly used to correct observational measurements along sightlines toward diffuse regions in the interstellar medium. However, the $R_V$ value may vary even along different diffuse interstellar medium sightlines. In this paper, we investigate the optical–mid-infrared (mid-IR) extinction law toward a very diffuse region at $l={165}^{\circ }$ in the Galactic plane, which was selected based on a CO emission map. Adopting red clump stars as extinction tracers, we determine the optical–mid-IR extinction law for our diffuse region in two APASS bands ($B,V$), three XSTPS-GAC bands ($g,r,i$), three 2MASS bands ($J,H,K_s$), and two WISE bands ($W1,W2$). Specifically, 18 red clump stars were selected from the APOGEE–RC catalog based on spectroscopic data in order to explore the diversity of the extinction law. We find that the optical extinction curves exhibit appreciable diversity. The corresponding $R_V$ ranges from 1.7 to 3.8, while the mean $R_V$ value of 2.8 is consistent with the widely adopted average value of 3.1 for Galactic diffuse clouds. There is no apparent correlation between $R_V$ value and color excess $E(B-V)$ in the range of interest, from 0.2 to 0.6 mag, or with specific visual extinction per kiloparsec, $A_V/d$.