2006/07/18 by B. Riaz, Basmah Riaz, D. J. Mullan +1
Physics and Astronomy · #Astro and Planetary Science #Astrophysics and Star Formation Studies #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/507446
published as Astrophys.J.650:1133-1139,2006 · 20 pages, 5 figures, accepted in ApJ
arxiv created 2006/07/18 · openalex publication_date 2006/10/18 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/31
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 μm. We find no evidence for IR excess. None of our targets are detected in the MIPS 70 and 160 μm 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 μm, although the reported fluxes were below the 5 σ IRAS detection limit (~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 μm 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%.