An imaging survey for extrasolar planets around 45 close, young stars with the simultaneous differential imager at the very large telescope and MMT

摘要

We present the results of a survey of 45 young (less than or similar to 250 Myr), close ( less than or similar to 50 pc) stars with the Simultaneous Differential Imager ( SDI) implemented at the VLT and the MMT for the direct detection of extrasolar planets. As part of the survey, we observed 54 objects, consisting of 45 close, young stars; two more distant (< 150 pc), extremely young ( <= 10 Myr) stars; three stars with known radial velocity planets; and four older, very nearby ( <= 20 pc) solar analogs. Our SDI devices use a double Wollaston prism and a quad filter to take images simultaneously at three wavelengths surrounding the 1.62 mu m methane absorption bandhead found in the spectrum of cool brown dwarfs and gas giant planets. By differencing adaptive optics-corrected images in these filters, speckle noise from the primary star is significantly attenuated, resulting in photon ( and flat-field)-noise-limited data. In our VLT data, we achieved H-band contrasts greater than or similar to 10 mag ( 5 sigma) at a separation of 0.50" from the primary star on 45% of our targets and H-band contrasts greater than or similar to 9 mag at a separation of 0.5" on 80% of our targets. With these contrasts, we can image ( 5 sigma detection) a 7 M-J planet 15 AU from a 70 Myr K1 star at 15 pc or a 7.8 MJ planet at 2 AU from a 12 Myr M star at 10 pc. We detected no candidates with S/N > 2 sigma which behaved consistently like a real object. From our survey null result, we can rule out ( with 93% confidence) a model planet population where N( a) proportional to constant out to a distance of 45 AU.