2023ApJS..269....6Z

The total-to-selective extinction ratio, R_V, is a key parameter for tracing the properties of interstellar dust, as it directly determines the variation of the extinction curve with wavelength. By utilizing accurate color excess measurements from the optical to the mid-infrared range, we have derived R_V values for approximately three million stars from LAMOST data release 7 using a forward-modeling technique. This extensive data set enables us to construct a comprehensive 2D R_V map of the Milky Way within the LAMOST footprint at a spatial resolution of ∼27.'5. Based on reliable sight lines of E(B -V) > 0.1, we find that R_V exhibits a Gaussian distribution centered around 3.25 with a standard deviation of 0.25. The spatial variability of R_V in the Galactic disk exhibits a wide range, spanning from small scales within individual molecular clouds to large scales up to kiloparsecs. A striking correlation is observed between the distribution of R_V and molecular clouds. Notably, we observe lower R_V values within the regions of nearby molecular clouds compared to their surrounding areas. Furthermore, we have investigated the relationships between R_V and various parameters, including dust temperature, dust emissivity spectral index, column densities and ratios of atomic and molecular hydrogen, as well as the gas-to-dust ratio. We find that these relationships vary with the level of extinction. These analyses provide new insights into the properties and evolution of dust grains in diverse interstellar environments and also hold significant importance for achieving accurate extinction corrections.