2022ApJS..259...12W

The dust extinction laws and dust properties in M31 are explored with a sample of reddened O-type and B-type supergiants obtained from the Local Group Galaxies Survey (LGGS). The observed spectral energy distributions (SEDs) for each tracer are constructed with multiband photometry from the LGGS, PS1 Survey, UKIRT, PHAT Survey, Swift/UVOT, and XMM-SUSS. We model the SED for each tracer in combination with the intrinsic spectrum obtained from the stellar model atmosphere extinguished by the model extinction curves. Instead of mathematically parameterizing the extinction functions, the model extinction curves in this work are directly derived from the silicate-graphite dust model with a dust size distribution of {dn}/{da}∼{a}^{-α}exp(-a/0.25), 0.005\lt a\lt 5 µm. The extinction tracers are distributed along the arms in M31, with the derived Milky Way (MW)-type extinction curves covering a wide range of R_V (≃ 2-6), indicating the complexity of the interstellar environment and the inhomogeneous distribution of interstellar dust in M31. The average extinction curve with R_V ≃ 3.51 and dust size distribution {dn}/{da}∼{a}^{-3.35}exp(-a/0.25) is similar to that of the MW but rises slightly less steeply in the far-UV bands, implying that the overall interstellar environment in M31 resembles the diffuse region in the MW. The extinction in the V band of M31 is up to 3 mag, with a median value of A_V ≃ 1 mag. The multiband extinction values from the UV to IR bands are also predicted for M31, which will provide a general extinction correction for future works.