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要旨

In this work we present the results of a direct imaging survey for brown
dwarf companions around the nearest stars at the mid-infrared 10 micron range
($\lambda_{c}$=8.7$\mu$m, $\Delta\lambda$=1.1$\mu$m) using the CanariCam
instrument at the 10.4 m Gran Telescopio Canarias (GTC). We imaged the 25
nearest stellar systems within 5 pc of the Sun at declinations $\delta >
-25^{\circ}$ (at least half have planets from radial velocity), reaching a mean
detection limit of 11.3$\pm$0.2 mag (1.5 mJy) in the Si-2 8.7$\mu$m band over a
range of angular separations from 1 to 10 arcsec. This would have allowed us to
uncover substellar companions at projected orbital separations between $\sim$2
and 50 au, with effective temperatures down to 600 K and masses greater than 30
$M_{Jup}$ assuming an average age of 5 Gyr and down to the deuterium-burning
mass limit for objects with ages $<$1 Gyr. From the non-detection of such
companions, we determined upper limits on their occurrence rate at depths and
orbital separations yet unexplored by deep imaging programs. For the M dwarfs,
main components of our sample, we found with a 90% confidence level that less
than 20% of these low-mass stars have L and T-type brown dwarf companions with
$m \gtrsim 30 M_{Jup}$ and $T_{eff} \gtrsim$ 600 K at $\sim$3.5--35 au
projected orbital separations.