The Mid-Infrared Leavitt Law For Classical Cepheids In The Magellanic Clouds

The Cepheid Leavitt Law (LL), also known as the Period-Luminosity relation, is a crucial tool for assembling the cosmic distance ladder. By combining data from the OGLE-IV catalogue with mid-infrared photometry from the Spitzer Space Telescope, we have determined the 3.6 and 4.5 mu m LLs for the Magellanic Clouds using similar to 5000 fundamental-mode classical Cepheids. Mean magnitudes were determined using a Monte Carlo Markov Chain (MCMC) template fitting procedure, with template light curves constructed from a subsample of these Cepheids with fully phased, well-sampled light curves. The dependence of the Large Magellanic Cloud LL coefficients on various period cuts was tested, in addition to the linearity of the relationship. The zero-point of the LL was calibrated using the parallaxes of MilkyWay Cepheids from the Hubble Space Telescope and Gaia Data Release 2. Our final calibrated relations are M-[3.6]=-3.246(+/- 0.008)(log (P)- 1.0)- 5.784(+/- 0.030) and M-[4.5]=-3.162(+/- 0.008)(log (P) - 1.0) - 5.751(+/- 0.030).