Warm Dust around Cool Stars: Field M Dwarfs with WISE 12 or 22 Micron   Excess Emission
  Using the SDSS DR7 spectroscopic catalog, we searched the WISE AllWISE catalog to investigate the occurrence of warm dust, as inferred from IR excesses, around field M dwarfs (dMs). We developed SDSS/WISE color selection criteria to identify 175 dMs (from 70,841) that show IR flux greater than typical dM photosphere levels at 12 and/or 22 $\mu$m, including seven new stars within the Orion OB1 footprint. We characterize the dust populations inferred from each IR excess, and investigate the possibility that these excesses could arise from ultracool binary companions by modeling combined SEDs. Our observed IR fluxes are greater than levels expected from ultracool companions ($>3\sigma$). We also estimate that the probability the observed IR excesses are due to chance alignments with extragalactic sources is $<$ 0.1%. Using SDSS spectra we measure surface gravity dependent features (K, Na, and CaH 3), and find $<$ 15% of our sample indicate low surface gravities. Examining tracers of youth (H$\alpha$, UV fluxes, and Li absorption), we find $<$ 3% of our sample appear young, indicating we are observing a population of field stars $\gtrsim$ 1 Gyr, likely harboring circumstellar material. We investigate age-dependent properties probed by this sample, studying the disk fraction as a function of Galactic height. The fraction remains small and constant to $|Z| \sim 700$ pc, and then drops, indicating little to no trend with age. Possible explanations for disks around field dMs include: 1) collisions of planetary bodies, 2) tidal disruption of planetary bodies, or 3) failed planet formation. 
Christopher A. Theissen and Andrew A. West
astro-ph.SR