2012A&A...548A.105A

Circumstellar debris disks are important because of their connection and interaction with planetary systems. An efficient way to identify these systems is through their infrared excess. Most studies so far concentrated on early-type or solar-type stars, but less effort has gone into investigating M-dwarfs, which pose the additional problem that their mid-infrared colors are poorly known. We characterize the infrared photometric behavior of M-dwarfs and search for infrared excess in nearby M-dwarfs taken from the volume-limited RECONS sample, concentrating on mid-infrared wavelengths corresponding to warm (>100K) dust. We then check whether the population of these late-type stars has a significantly different fraction of infrared excess compared to earlier-type stars. We used data recently released from the WISE satellite, which provides the most sensitive mid-infrared all-sky survey to date. Our sample consists of 85 sources encompassing 103 M-dwarfs. We compared this sample to Spitzer data and matched it to the 2MASS catalog. We derived empirical infrared colors from these data and discuss the intrinsic spread within these colors as well as the errors from WISE and 2MASS. Based on this, we checked 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 mid-infrared excess is produced by a known T6 companion, we detect no excesses in any of our sample stars. The limits we derive for the 22µm excess are slightly higher than the usual detection limit of ∼10-15% for Spitzer studies, but including the 12µm band and the [12]-[22] color in our analysis allows us to derive tight constraints for the fractional dust luminosity L_dust/L_*. Comparing our findings to earlier-type stars, we show that this result is consistent with the assumption that M-dwarf excesses in the mid-inrared is as frequent as excesses around earlier-type stars. The low detection rate of 0^+1.3_–0.0% we derive for our sample in that case is an age effect. We also present a tentative excess detection at 22µm 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 (>30Myr) main-sequence M-dwarf. It is unclear whether this is due to a different dust evolution around M-dwarfs compared to earlier-type stars or whether this is an age effect combined with the difficulties involved in searching M-dwarfs for infrared excesses. A significantly larger sample of well-studied M-dwarfs is required to solve this question. At the same time, their behavior at longer wavelengths, which are sensitive to colder dust, needs further investigation.