We present results from a high-contrast adaptive optics imaging search for giant planets and brown dwarfs ( \gtrsim 1 M _ { \mathrm { Jup } } ) around 122 newly identified nearby ( \lesssim 40 pc ) young M dwarfs .
Half of our targets are younger than 135 Myr and 90 % are younger than the Hyades ( 620 Myr ) .
After removing 44 close stellar binaries ( implying a stellar companion fraction of > 35.4 \pm 4.3 % within 100 AU ) , 27 of which are new or spatially resolved for the first time , our remaining sample of 78 single M dwarfs makes this the largest imaging search for planets around young low-mass stars ( 0.1–0.6 M _ { \odot } ) to date .
Our H - and K -band coronagraphic observations with Keck/NIRC2 and Subaru/HiCIAO achieve typical contrasts of 12–14 mag and 9–13 mag at 1 ^ { \prime \prime } , respectively , which corresponds to limiting planet masses of 0.5–10 M _ { \mathrm { Jup } } at 5–33 AU for 85 % of our sample .
We discovered four young brown dwarf companions : 1RXS J235133.3+312720 B ( 32 \pm 6 M _ { \mathrm { Jup } } ; L0 ^ { +2 } _ { -1 } ; 120 \pm 20 AU ) , GJ 3629 B ( 64 ^ { +30 } _ { -23 } M _ { \mathrm { Jup } } ; M7.5 \pm 0.5 ; 6.5 \pm 0.5 AU ) , 1RXS J034231.8+121622 B ( 35 \pm 8 M _ { \mathrm { Jup } } ; L0 \pm 1 ; 19.8 \pm 0.9 AU ) , and 2MASS J15594729+4403595 B ( 43 \pm 9 M _ { \mathrm { Jup } } ; M8.0 \pm 0.5 ; 190 \pm 20 AU ) .
Over 150 candidate planets were identified ; we obtained follow-up imaging for 56 % of these but all are consistent with background stars .
Our null detection of planets enables strong statistical constraints on the occurrence rate of long-period giant planets around single M dwarfs .
We infer an upper limit ( at the 95 % confidence level ) of 10.3 % and 16.0 % for 1–13 M _ { \mathrm { Jup } } planets between 10–100 AU for hot-start and cold-start ( Fortney ) evolutionary models , respectively .
Fewer than 6.0 % ( 9.9 % ) of M dwarfs harbor massive gas giants in the 5–13 M _ { \mathrm { Jup } } range like those orbiting HR 8799 and \beta Pictoris between 10–100 AU for a hot-start ( cold-start ) formation scenario .
The frequency of brown dwarf ( 13–75 M _ { \mathrm { Jup } } ) companions to single M dwarfs between 10–100 AU is 2.8 ^ { +2.4 } _ { -1.5 } % .
Altogether we find that giant planets , especially massive ones , are rare in the outskirts of M dwarf planetary systems .
Although the first directly imaged planets were found around massive stars , there is currently no statistical evidence for a trend of giant planet frequency with stellar host mass at large separations as predicted by the disk instability model of giant planet formation .