Program: GS-2021B-Q-218

Classical Cepheids are sensitive probe for the precision stellar astrophysics and also for the distance scale thanks to their well-defined Period-Luminosity relations (PLRs). Cepheid variables are used extensively for the primary calibration of the traditional Cepheid-Supernovae distance ladder. The ongoing discord in the Hubble constant measurements based on the distance ladder and the Planck mission values warrants a detailed investigation of systematic uncertainties in these measurements. One of the major systematic uncertainty in Cepheid-based distance scale comes from the unknown metallicity effects on their PLRs. To remedy this uncertainty, we propose high-resolution IGRINS NIR spectroscopic follow-up of carefully selected 15 and 10 long-period, less-extincted Cepheid standards in the non-crowded regions of the LMC and the Milky Way, respectively, for which optical and NIR photometry was obtained with HST. Our observations will provide spectroscopic metallicites for LMC Cepheid standards for the first-time and allow a direct investigation of metallicity effects on their PLRs, and in turn, resolve the systematic uncertainties due to metallicity on the Cepheid-based distance scale. Our IGRINS spectra will lead to pioneering studies on LMC and Galactic Cepheids with such high-resolution, high signal-to-noise observations for the first-time in the HKs-spectral range, thus, opening new windows for studies on stellar evolutionary and pulsational properties of Cepheid variables.