Planets Around Low-Mass Stars (PALMS). IV. The Outer Architecture of M   Dwarf Planetary Systems
  We present results from a high-contrast adaptive optics imaging search for giant planets and brown dwarfs (>1 MJup) around 122 newly identified nearby (<40 pc) young M dwarfs. Half of our targets are younger than 135 Myr and 90% are younger than the Hyades (620 Myr). 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", respectively, which corresponds to limiting planet masses of 0.5-10 MJup at 5-33 AU for 85% of our sample. We discovered four young brown dwarf companions: 1RXS J235133.3+312720 B (32 $\pm$ 6 MJup; L0$^{+2}_{-1}$; 120 $\pm$ 20 AU), GJ 3629 B (64$^{+30}_{-23}$ MJup; M7.5 $\pm$ 0.5; 6.5 $\pm$ 0.5 AU), 1RXS J034231.8+121622 B (35 $\pm$ 8 MJup; L0 $\pm$ 1; 19.8 $\pm$ 0.9 AU), and 2MASS J15594729+4403595 B (43 $\pm$ 9 MJup; 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 MJup 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 MJup range like those orbiting HR 8799 and $\beta$ Pictoris between 10-100 AU for a hot-start (cold-start) formation scenario. 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. 
Brendan P. Bowler, Michael C. Liu, Evgenya L. Shkolnik, Motohide   Tamura
astro-ph.EP astro-ph.SR