# # Program information file # PROGRAM_ID 2022B081 PROGRAM_TITLE Tracking the kinematic ridge features of the Milky Way disk with Cepheids PROGRAM_INV1 Natsuki Funakoshi PROGRAM_INV2 Noriyuki Matsunaga PROGRAM_INV3 Daisuke Taniguchi PROGRAM_INV4 Jian Mingjie PROGRAM_INV5 PROGRAM_SCICAT galactic/interstellar medium PROGRAM_ABSTRACT_BEG The revolutionary Gaia has revealed many diagonal ridge features in the Galactic disk on the Rgc-Vrot diagram, which were produced by hitherto-unclear physical mechanism[s]. Previous studies used the stellar distribution and motions based on GaiaDR2 and EDR3, but the samples are limited to ~3 kpc from the Sun, and the ridges at different regions of the Galactic disk are unknown. In this study, we use Classical Cepheids with the famous period-luminosity relation in order to observe the ridge features at further than 5 kpc. We propose to measure the radial velocities of 108 newly discovered Classical Cepheids for which Gaia DR3 is going to provide proper motions but not line-of-sight velocities. Cepheids are very unique tracers that enable us to trace the ridges over a wide range of Galactic azimuths for the first time. The dependency of the ridge positions on the azimuth and the location of spiral arms would give an insight into the physical mechanism[s] to produce the ridges. We use the 0.75 arcsec slit to get the spectral resolution of R~37,500 and collect H-band spectra [the H2 mode] with S/N~50. The observation time including the overheads is 25.7 hours. Adding a few velocity-template Cepheids to establish the correction of pulsation, we require 4 nights. For the velocity-template Cepheids, we have velocity curves measured from both literature and our measurements, and thus we can use them to confirm the consistency with the iSHELL measured velocities. PROGRAM_ABSTRACT_END