2010ApJ...709..120M

Classical Cepheid variable stars have been important indicators of extragalactic distance and Galactic evolution for over a century. The Spitzer Space Telescope has opened the possibility of extending the study of Cepheids into the mid- and far-infrared, where interstellar extinction is reduced. We have obtained photometry from images of a sample of Galactic Cepheids with the Infrared Array Camera (IRAC) and Multiband Infrared Photometer for Spitzer instruments on Spitzer. Here we present the first mid-infrared period-luminosity relations for Classical Cepheids in the Galaxy, and the first ever Cepheid period-luminosity relations at 24 and 70 µm. We compare these relations with theoretical predictions, and with period-luminosity relations obtained in recent studies of the Large Magellanic Cloud. We find a significant period-color relation for the [3.6] - [8.0] IRAC color. Other mid-infrared colors for both Cepheids and non-variable supergiants are strongly affected by variable molecular spectral features, in particular deep CO absorption bands. We do not find strong evidence for mid-infrared excess caused by warm (∼500 K) circumstellar dust. We discuss the possibility that recent detections with near-infrared interferometers of circumstellar shells around δ Cep, ℓ Car, Polaris, Y Oph, and RS Pup may be a signature of shocked gas emission in a dust-poor wind associated with pulsation-driven mass loss.