Mapping the Milky Way with Classical Cephieds

Cepheids have been used to map large-scale features of the Galactic stellar disk, including the spiral arms and the warp. However, while period-luminosity relations are well-defined, distances derived using different photometric indices can show relatively large differences. We show that these differences cannot be explained by uncertain extinction estimates, but are easily explained by the well-established variation of the extinction curve, which can make optical Wesenheit-based distances unreliable when extinction is high. Near or mid-infrared based distances should be preferred, being much less affected by extinction, less sensitive to variations in the extinction curve, and because the intrinsic dispersion of the absolute magnitudes in these bands is smaller than in the optical. We report on the most recent results using distances based on mid-infrared W1 photometry, including the spiral arms, the warp, and the detection of a large-scale vertical corrugation in the outer disk. Using color information of the Cepheids, we are able to estimate the extinction-to-reddening ratios (RV) for sufficiently reddened Cepheids, and improved distance estimates for Cepheids suffering from large extinction. We make a first map of the large-scale variation of RV over the Galactic disk using the Cepheids that show systematic variations of dust properties of the ISM associated with the known spiral arms. We also show an updated map of the spiral structure in the inner disk using these new improved distances.