The Carnegie Hubble Program: The Leavitt Law at 3.6 and 4.5 micron in the Milky Way

The Carnegie Hubble Program (CHP) is designed to calibrate the extragalactic distance scale using data from the post-cryogenic era of the Spitzer Space Telescope. The ultimate goal of the CHP is a systematic improvement in the distance scale leading to a determination of the Hubble Constant to within an accuracy of 2%. This paper focuses on the measurement and calibration of the Galactic Cepheid Period-Luminosity (Leavitt) Relation using the warm Spitzer IRAC 1 and 2 bands at 3.6 and 4.5 μm. We present photometric measurements covering the period range 4 - 70 days for 37 Galactic Cepheids. Data at 24 phase points were collected for each star. Three PL relations of the form M=a(Log(P)-1)+b are derived. The method adopted here takes the slope a to be -3.31, as determined from the Spitzer LMC data of Scowcroft et al. (2012). Using the geometric HST guide-star distances to ten Galactic Cepheids we find a calibrated 3.6 micron PL zero-point of -5.80\pm0.03. Together with our value for the LMC zero-point we determine a reddening-corrected distance modulus of 18.48\pm0.04 mag to the LMC. The mid-IR Period-Color diagram and the [3.6] - [4.5] color variation with phase are interpreted in terms of CO absorption at 4.5 μm. This situation compromises the use of the 4.5 μm data for distance determinations.