Guiding on the edge (V∼19):: Results from an AO survey of very low mass stars searching for extremely faint companions

Use of a highly sensitive curvature wavefront sensor (WFS) with avalanche photodiode (APD) detectors at the Gemini North telescope has allowed direct AO guiding on very low mass stars (V=18-20, I=14-16) producing 0.1" images in W. This resolution (which is near to the 0.07" diffraction limit) has enabled us to conduct ground-based searches for substellar companions (< 0.075 Me) to within similar to 3 AU of their primary stars. We have been able to use the faint science targets themselves to close the AO loop. The key to this capability is the zero readout noise, photon-counting APDs used in the curvature WFS. The amplification of incident photons from low luminosity sources allows sufficient signal at the subapertures to reconstruct the aberrated wavefronts with relatively little reconstruction error. This is advantageous compared to typical Shack-Hartmann WFSs which require greater signal from their guide stars to compensate for their CCDs' readout noise. The technique presented here is currently the only working one from the ground studying such cool and faint targets. Additionally, we report the discovery of 3 new binary stellar systems from a survey of 30 low mass stars (similar to 0.095 Me, spectral type M6.0-M7.5, V=18-20) at separations between 0.12-0.29" (3.5-8.3 AU) using this technique.