Temporal Evolution of Coronagraphic Dynamic Range and Constraints on Companions to Vega

The major obstacle to the direct detection of companions to nearby stars isthe overwhelming brightness of the host star. Current instruments employing thecombination of adaptive optics (AO) and coronagraphy can typically detectobjects within 2'' of the star that are 10^{4-5} times fainter. Correlatedspeckle noise is one of the biggest obstacles limiting such high-contrastimaging. We have obtained a series of 284 8 s, AO-corrected, coronagraphicallyocculted H-band images of the star Vega at the 3.63 m AEOS telescope located onHaleakala, Hawaii. This dataset is unique for studying the temporal behavior ofspeckle noise, and represents the first time such a study on highly corrected_coronagraphic_ AO images has been carried out in a quantitative way. We findthe speckle pattern to be highly stable in both position and time in our data.This is due to the fact that the AO system corrects disturbances to the stellarwave front at the level where the instrumental wave front errors dominate.Because of this, we find that our detection limit is not significantly improvedsimply with increased exposure time alone. However, we are able to improve ourdynamic range by 1.5-2 magnitudes through subtraction of static/quasi-staticspeckles in two rotating frames: the telescope pupil frame and the deformablemirror frame. Furthermore, from our data, we are able to constrain the mass ofany purported companion to Vega to be less than 45 M_J at 8 AU and less than 30M_J at 16 AU, radii not previously probed at these sensitivities.Comment: Accepted by ApJ, 23 pages, 7 figues, 1 tabl