Henning Avenhaus, Hans Martin Schmid, Michael R. Meyer
Circumstellar debris disks are important for their connection to planetary systems. An efficient way to identify such systems is through their infrared excess. Most studies so far concentrated on early-type or solar-type stars, but less effort has gone into M dwarfs. We characterize the mid-infrared photometric behavior of M dwarfs and search for infrared excess in nearby M dwarfs taken from the volume-limited RECONS sample using data from the WISE satellite and the 2MASS catalog. Our sample consists of 85 sources encompassing 103 M dwarfs. We derive empirical infrared colors from these data and discuss their errors. From this, we check the stars for infrared excess and discuss the minimum excess we would be able to detect. Other than the M8.5 dwarf SCR 1845-6357 A, where the excess is produced by a known T6 companion, we detect no excesses in any of our sample stars. The limits we derive for the 22um excess are slightly larger than the usual detection limit of 10-15% for Spitzer studies, but the inclusion of the [12]-[22] color in our analysis allows us to put tight constraints on the fractional dust luminosity L_dust/L_star. We show that this result is not inconsistent with M dwarf excesses in the mid-inrared being as frequent as excesses around earlier-type stars. The low detection rate could be an age effect. We also present a tentative excess detection at 22um around the known cold debris disk M dwarf AU Mic, which is not part of our statistical sample. There is still no clear detection of a mid-infrared excess around any old (>30 Myr) main-sequence M dwarf. It is unclear whether this is due to a different dust evolution around M dwarfs or whether this is an age effect combined with the diffculties involved in searching M dwarfs for infrared excesses. A significantly larger sample of well-studied M dwarfs will be required to solve this question.
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http://arxiv.org/abs/1209.0678
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