Spitzer observations of nearby M dwarfs

We present Spitzer IRAC and MIPS observations for a sample of eight M dwarfs: six dMe, one dM, and one sdMe star. All of our targets are found to have SEDs that are fitted within the error bars by a purely photospheric spectrum out to 24 mu m. We find no evidence for IR excess. None of our targets are detected in the MIPS 70 and 160 mu m bands. The estimated ages for all are > 10 Myr, suggesting that enough disk dissipation has occurred within the inner several AU of the star. For four of these, Mullan et al. had reported IRAS detections at 12 mu m, although the reported fluxes were below the 5 sigma IRAS detection limit (similar to 0.2 Jy). Mullan et al. also pointed out that V - K colors in dMe stars are larger than those in dM stars, possibly because of the presence of a chromosphere. Here we suggest that metallicity effects provide a better explanation of the V - K data. For reasons of observational selection, our targets are not the most active flare stars known, but being dMe stars indicates the presence of a chromosphere. Scaling from Houdebine's model of the AU Mic chromosphere, we have computed the free-free IR excesses for a range of densities. Our Spitzer 24 mu m data show that the chromospheres in two of our targets are less dense than in AU Mic by a factor of 10 or more. This is consistent with the fact that our sample includes the less active flare stars. Our models also indicate that the chromospheric contribution to the observed AU Mic emission at submillimeter wavelengths is only about 2%.