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 MJ 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 MJ 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 MJ and as old as several Gyr are expected to be detectable with this technique.