GALACTIC CEPHEIDS WITH SPITZER. I. LEAVITT LAW AND COLORS

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.