Galactic Cepheids with Spitzer: I. Leavitt Law and Colors

TL;DR

This study establishes mid-infrared period-luminosity relations for Galactic Cepheids using Spitzer data, compares them with theoretical and LMC relations, and explores their colors and circumstellar environments.

Contribution

First mid-infrared period-luminosity relations for Galactic Cepheids are presented, extending Cepheid studies into new wavelength regimes with implications for distance measurement.

Findings

Significant period-color relation at 3.6-8.0 μm.

Mid-infrared colors affected by molecular spectral features.

No strong evidence for warm circumstellar dust excess.

Abstract

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 IRAC and MIPS 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 um. 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 delta Cep, l Car, Polaris, Y Oph and RS Pup may be a signature of shocked gas emission in a dust-poor wind associated to pulsation-driven mass loss.