2020ApJ...895L..12A

With the increasing number of large stellar survey projects, the quality and quantity of excellent tracers for studying the Milky Way are rapidly growing, one of which is the classical Cepheids. Classical Cepheids are high-precision standard candles with very low typical uncertainties (<3%) available via the mid-infrared period-luminosity relation. About 3500 classical Cepheids identified from the Optical Gravitational Lensing Experiment, All-Sky Automated Survey for Supernova, Gaia, Wide-field Infrared Survey Explorer, and Zwicky Transient Facility survey data have been analyzed in this work, and their spatial distributions show a clear signature of Galactic warp. Two kinematical methods are adopted to measure the Galactic rotation curve (RC) in the Galactocentric distance range of 4≲R_GC≲19kpc. Gently declining RCs are derived by both the proper motion (PM) method and three-dimensional velocity vector (3DV) method. The largest sample of classical Cepheids with the most accurate 6D phase-space coordinates available to date are modeled in the 3DV method, and the resulting RC is found to decline at the relatively smaller gradient of (-1.33 ± 0.1) kms^–1kpc^–1. Comparing to results from the PM method, a higher rotation velocity ((232.5 ± 0.83) kms^–1) is derived at the position of the Sun in the 3DV method. The virial mass and local dark matter density are estimated from the 3DV method, which is the more reliable method, M_vir=(0.822±0.052)×10¹²M_☉ and ρ_DM,☉=0.33±0.03 GeV, respectively.