Far-Infrared Properties of M Dwarfs

We report the mid- and far-infrared properties of nearby M dwarfs. Spitzer MIPS measurements were obtained for a sample of 62 stars at 24 μm, with subsamples of 41 and 20 stars observed at 70 and 160 μm, respectively. We compare the results with current models of M star photospheres and look for indications of circumstellar dust in the form of significant deviations of K-[24 μm] colors and 70 μm/24 μm flux ratios from the average M star values. At 24 μm, all 62 of the targets were detected; 70 μm detections were achieved for 20 targets in the subsample observed, and no detections were seen in the 160 μm subsample. No clear far-infrared excesses were detected in our sample. The average far-infrared excess relative to the photospheric emission of the M stars is at least 4 times smaller than the similar average for a sample of solar-type stars. However, this limit allows the average fractional infrared luminosity in the M-star sample to be similar to that for more massive stars. We have also set low limits (10^-4 to 10^-9 M_⊕ depending on location) for the maximum mass of dust possible around our stars.