Adaptive Optics Nulling Interferometric Constraints on the Mid-Infrared Exozodiacal Dust Emission around Vega

We present the results of mid-infrared nulling interferometric observationsof the main-sequence star alpha Lyr (Vega) using the 6.5 m MMT with itsadaptive secondary mirror. From the observations at 10.6 microns, we find thatthere is no resolved emission from the circumstellar environment (atseparations greater than 0.8 AU) above 2.1% (3 sigma limit) of the level of thestellar photospheric emission. Thus, we are able to place an upper limit on thedensity of dust in the inner system of 650 times that of our own solar system'szodiacal cloud. This limit is roughly 2.8 times better than those determinedwith photometric excess observations such as those by IRAS. Comparison withfar-infrared observations by IRAS shows that the density of warm dust in theinner system (< 30 AU) is significantly lower than cold dust at largerseparations. We consider two scenarios for grain removal, the sublimation ofice grains and the presence of a planetary mass "sweeper." We find that ifsublimation of ice grains is the only removal process, a large fraction (> 80%)of the material in the outer system is ice.Comment: 11 pages, 1 figure, Accepted to The Astrophysical Journal Letter