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arXiv · 1603.03776

The Carnegie Chicago Hubble Program: The Mid-Infrared Colours of Cepheids and the Effect of Metallicity on the CO Band-head at $4.6μ$m

Victoria Scowcroft·Mark Seibert·Wendy L. Freedman·Rachael L. Beaton·Barry F. Madore·Andrew J. Monson·Jeffrey A. Rich·Jane R. Rigby
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Abstract

We compare mid-infrared 3.6 and 4.5 $μ$m 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 CO rotation-vibration band-head at 4.6 $μ$m on the mid-infrared photometry. We confirm the temperature sensitivity of the CO band-head at 4.6 $μ$m and find no evidence for an effect at 3.6 $μ$m. We compare the ([3.6]-[4.5]) period-colour relations in the MW, 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. (2014), demonstrates that the ([3.6]-[4.5]) colour provides a reliable metallicity indicator for Cepheids, with a precision comparable to current spectroscopic determinations.

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Victoria Scowcroft, Mark Seibert, Wendy L. Freedman, Rachael L. Beaton, Barry F. Madore, Andrew J. Monson, Jeffrey A. Rich, Jane R. Rigby. 2016-03-22. The Carnegie Chicago Hubble Program: The Mid-Infrared Colours of Cepheids and the Effect of Metallicity on the CO Band-head at $4.6μ$m. https://doi.org/10.1093/mnras%2Fstw628

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