Thermal infrared constraint to a planetary companion of Vega with the MMT adaptive optics system

Vega may have a massive companion in a wide orbit, as evidenced by structure in its cold dust debris. We have tested this hypothesis by direct imaging with adaptive optics in the M band. The observations were made with a newly commissioned thermal infrared camera, Clio, on the 6.5 MMT AO system with low-background deformable secondary mirror. The observations constrain a planet to be less than 7 M_J at the approximate position angle expected from the dust structure and at a radius >20 AU (2″.5). This result is more stringent than similar previous near-infrared observations of Vega, which achieved limits of 20 and 10 M_J at separations of 7″. The higher sensitivity is due to both the more favorable contrast of gas giant planets at the M band and the higher Strehl ratio and more stable point spread function at longer wavelengths. Future L′ or M band observations could provide a powerful approach for wide-separation planet detection, especially for cooler and thus older or less massive planets. The natural best targets are nearby stars, where planets in the range of 5-15 M_J and as old as several Gyr are expected to be detectable with this technique.