Long exposures from adaptive optic systems show a diffraction limited core superimposed on a halo of uncorrected light from a science target, and the addition of various long-lived speckles that arise from uncorrected aberrations in the telescope system. The presence of these speckles limit the detection of extra-solar planets at a few diffraction widths from the primary source. Focal plane wavefront sensing uses the deformable secondary mirror of the MMT adaptive optics system to systematically remove the presence of long-lived speckles in a high-contrast image, and also test for the incoherent source that represents a separate astronomical target nearby. We use the Clio 5 micron camera (with its coronagraphic capabilities) to modulate long lived speckles and present initial on-sky results of this technique.
We are carrying out a survey to search for giant extrasolar planets around nearby, moderate-age stars in the
mid-infrared L' and M bands (3.8 and 4.8 microns, respectively), using the Clio camera with the adaptive optics
system on the MMT telescope. To date we have observed 7 stars, of a total 50 planned, including GJ 450
(distance about 8.55pc, age about 1 billion years, no real companions detected), which we use as our example
here. We report the methods we use to obtain extremely high contrast imaging in L', and the performance we
have obtained. We find that the rotation of a celestial object over time with respect to a telescope tracking
it with an altazimuth mount can be a powerful tool for subtracting telescope-related stellar halo artifacts and
detecting planets near bright stars. We have carried out a thorough Monte Carlo simulation demonstrating our
ability to detect planets as small as 6 Jupiter masses around GJ 450. The division of a science data set into two
independent parts, with companions required to be detected on both in order to be recognized as real, played a
crucial role in detecting companions in this simulation. We mention also our discovery of a previously unknown
faint stellar companion to another of our survey targets, HD 133002. Followup is needed to confirm this as a
physical companion, and to determine its physical properties.
Clio is an adaptive-optics camera mounted on the 6.5 meter MMT optimized for diffraction-limited L' and M-band imaging over a ~ 15" field. The instrument was designed from the ground up with a large well-depth, fast readout thermal infrared (~ 3_5<i>μ</i>m) 320 by 256 pixel InSb detector, cooled optics, and associated focal plane and pupil masks (with the option for a coronograph) to minimize the thermal background and maximize throughput. When coupled with the MMT's adaptive secondary AO (two warm reflections) system's low thermal background, this instrument is in a unique position to image nearby warm planets, which are the brightest in the L' and M-band atmospheric windows. We present the current status of this recently commissioned instrument that performed exceptionally during first light. Our instrument sensitivities are impressive and are sky background limited: for an hour of integration, we obtain an L'-band 5 σ detection limit of of 17.0 magnitudes ~ 80%) and an M-band limit of 14.5 (Strehl ~ 90%). Our M-band sensitivity is lower due to the increase in thermal sky background. These sensitivities translate to finding relatively young planets five times Jupiter mass (M<i><sub>Jup</sub></i>) at 10 pc within a few AU of a star. Presently, a large Clio survey of nearby stellar systems is underway including a search for planets around solar-type stars, M dwarfs, and white dwarfs. Even with a null result, we can place strong constraints on planet distribution models.
We have designed and built an infrared camera using a Rockwell HAWAII MBE array sensitive from 1-5 microns. This camera is optimized for sensitive imaging in the 3-5 micron wavelength range, i.e. the L’ and M photometric bands. When used with the deformable secondary adaptive optics (AO) system on the 6.5m MMT telescope, the camera will be ideal for direct imaging surveys for extrasolar planets around young, nearby stars. Based on the models of Burrows et al (2001), we calculate that in a 2-hour background-limited integration with MMT AO we will be able to detect, in both M and L’ bands, a planet of 1 billion year (Gyr) age and 5 Jupiter masses (MJ) at a distance of 10 parsecs (pc). Our simulations of atmospheric speckle noise suggest that background limited M and L’ observations are possible at about 1.5 and 2.5 arcseconds, respectively, from a solar-type star at 10pc distance. The speckle limits move inward dramatically for fainter stars, and brighter planets or brown dwarfs can be seen even where the speckles overwhelm the background noise. The camera opens up a region of parameter space that is inaccessible to the radial velocity technique, and thus the two methods are highly complementary.