The Carnegie Hubble Program: The Leavitt Law at 3.6 \mu m and 4.5 \mu m in the Large Magellanic Cloud

TL;DR

This study refines the period-luminosity relations of Cepheids in the Large Magellanic Cloud at mid-infrared wavelengths, demonstrating the 3.6 μm band as an excellent distance indicator and exploring CO's role in metallicity measurement.

Contribution

It provides new uniform light curves and period-luminosity relations at 3.6 μm and 4.5 μm, and first measurements of the Cepheid color variation due to CO molecules.

Findings

3.6 μm band is a superb distance indicator

First measurement of [3.6]-[4.5] color variation in Cepheids

CO dissociation affects 4.5 μm flux, indicating metallicity potential

Abstract

The Carnegie Hubble Program (CHP) is designed to improve the extragalactic distance scale using data from the post-cryogenic era of Spitzer. The ultimate goal is a determination of the Hubble constant to an accuracy of 2%. This paper is the first in a series on the Cepheid population of the Large Magellanic Cloud, and focusses on the period-luminosity relations (Leavitt laws) that will be used, in conjunction with observations of Milky Way Cepheids, to set the slope and zero--point of the Cepheid distance scale in the mid-infrared. To this end, we have obtained uniformly-sampled light curves for 85 LMC Cepheids, having periods between 6 and 140 days. Period-luminosity and period-color relations are presented in the 3.6 \mu m and 4.5\mu m bands. We demonstrate that the 3.6 \mu m band is a superb distance indicator. The cyclical variation of the [3.6]-[4.5] color has been measured for the first time. We attribute the amplitude and phase of the color curves to the dissociation and recombination of CO molecules in the Cepheid's atmosphere. The CO affects only the 4.5 \mu m flux making it a potential metallicity indicator.