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Mapping the Galactic spiral structure is a difficult task since the Sun islocated in the Galactic plane and because of dust extinction. For thesereasons, molecular masers in radio wavelengths have been used with greatsuccess to trace the Milky Way spiral arms. Recently, Gaia parallaxes havehelped in investigating the spiral structure in the Solar extendedneighborhood. In this paper, we propose to determine the location of the spiralarms using Cepheids since they are bright, young supergiants with accuratedistances (they are the first ladder of the extragalactic distance scale). Theycan be observed at very large distances; therefore, we need to take theGalactic warp into account. Thanks to updated mid-infrared photometry and tothe most complete catalog of Galactic Cepheids, we derived the parameters ofthe warp using a robust regression method. Using a clustering algorithm, weidentified groups of Cepheids after having corrected their Galactocentricdistances from the (small) effects of the warp. We derived new parameters forthe Galactic warp, and we show that the warp cannot be responsible for theincreased dispersion of abundance gradients in the outer disk reported inprevious studies. We show that Cepheids can be used to trace spiral arms, evenat large distances from the Sun. The groups we identify are consistent withprevious studies explicitly deriving the position of spiral arms using youngtracers (masers, OB(A) stars) or mapping overdensities of upper main-sequencestars in the Solar neighborhood thanks to Gaia data.

Tracing the Milky Way warp and spiral arms with classical Cepheids