CONSTRAINTS ON LONG-PERIOD PLANETS FROM AN<i>L</i>′- AND<i>M</i>-BAND SURVEY OF NEARBY SUN-LIKE STARS: MODELING RESULTS

Abstract

We have carried out an L' and M band Adaptive Optics (AO) extrasolar planet\nimaging survey of 54 nearby, sunlike stars using the Clio camera at the MMT.\nOur survey concentrates more strongly than all others to date on very nearby F,\nG, and K stars, in that we have prioritized proximity higher than youth. Our\nsurvey is also the first to include extensive observations in the M band, which\nsupplemented the primary L' observations. These longer wavelength bands are\nmost useful for very nearby systems in which low temperature planets with red\nIR colors (i.e. H - L', H - M) could be detected. The survey detected no\nplanets, but set interesting limits on planets and brown dwarfs in the star\nsystems we investigated. We have interpreted our null result by means of\nextensive Monte Carlo simulations, and constrained the distributions of\nextrasolar planets in mass $M$ and semimajor axis $a$. If planets are\ndistributed according to a power law with $dN \\propto M^{\\alpha} a^{\\beta} dM\nda$, normalized to be consistent with radial velocity statistics, we find that\na distribution with $\\alpha = -1.1$ and $\\beta = -0.46$, truncated at 110 AU,\nis ruled out at the 90% confidence level. These particular values of $\\alpha$\nand $\\beta$ are significant because they represent the most planet-rich case\nconsistent with current statistics from radial velocity observations. With 90%\nconfidence no more than 8.1% of stars like those in our survey have systems\nwith three widely spaced, massive planets like the A-star HR 8799. Our\nobservations show that giant planets in long-period orbits around sun-like\nstars are rare, confirming the results of shorter-wavelength surveys, and\nincreasing the robustness of the conclusion.\n