The Carnegie Chicago Hubble Program: The mid-infrared colours of Cepheids and the effect of metallicity on the CO band-head at 4.6μm

We compare mid-infrared (IR) 3.6 and 4.5 μm Warm Spitzer observations for Cepheids in the Milky Way and the Large and Small Magellanic Clouds. Using models, we explore in detail the effect of the COrotation-vibration band-head at 4.6 μmon the mid-IR photometry. We confirm the temperature sensitivity of the CO band-head at 4.6 μm and find no evidence for an effect at 3.6 μm.We compare the ([3.6]-[4.5]) period-colour relations in theMW, LMC and SMC. The slopes of the period-colour relations for the three galaxies are in good agreement, but there is a trend in zero-point with metallicity, with the lowest metallicity Cepheids having redder mid-IR colours. Finally, we present a colour-[Fe/H] relation based on published spectroscopic metallicities. This empirical relation, calibrated to the metallicity system of Genovali et al., demonstrates that the ([3.6]-[4.5]) colour provides a reliablemetallicity indicator for Cepheids, with a precision comparable to current spectroscopic determinations.