Giant planets around M stars

M dwarfs constitute a promising reservoir to survey in order to advance our understanding of planetary formation and evolution. Indeed, M dwarfs outnumber all earlier-type stars together. Their abundance, low close binary fraction, and the ubiquitous presence of massive protoplanetary disks at young agesimply that they are common sites of planet formation. Close separations (< 1 AU) have been extensively probed by Doppler and transit surveys with the following results: the frequency of close-in giant planets(1 − 10 MJup) is only 2.5 ± 0.9%, consistent with core accretion plus migration models. On the other hand, the Kepler survey indicates that Earth- to Neptune-sized planets might be as common as one per star. The outskirts of young M-dwarf systems (10 − 100 AU) are being probed by first-generation direct imaging instruments, and preliminary results show that massive planets are rare: fewer than 10.6% of M-dwarf systems surveyed harbor 1 − 13 MJup giant planets in their outer regions. Disk instability does not seem to be a common mechanism of giant planet formation. The 1 − 10 AU parameter space is thus believed to be the overwhelmingly favored region for planet formation. 

High contrast imaging with a good knowledge of the host star distance and age is therefore the perfect complement to indirect techniques, and holds the promise of filling in this untouched parameter space, and provide excellent characterization opportunities. Last but not least, M dwarfs provide the best star-planet contrast ratios among all stellar masses. Keck-NIRC2 has very recently been upgraded with a pair of new high performance small inner working angle L-band vortex coronagraphs. L-band (centered around 3.8 μm) is a sweet spot for ground-based planet surveys. This wavelength range offers significant advantages compared to shorter wavelengths: (i) The L-band contrast of planetary-mass companions with respect to their host stars is more favorable than in the H and K bands (11) so that lower-mass objects can be probed; and (ii) the L-band provides better and more stable image quality, with Strehl ratios well above 70% and sometimes as high as 90% at Keck, reducing pinned speckles, and thus systematics. These advantages compensate for the increased sky background in the thermal infrared and the loss in resolution, especially if small IWA phase-mask coronagraphs such as the vortex are available. Finally, we note that background contamination rates will be near-zero at small separations in in L-band, so minimal follow-up time will be needed to confirm candidates.

Co-Investigators: H. Ngo (now Plaskett fellow at NRC-Victoria), B. Bowler (UT), J. Wang (Caltech), O. Absil (ULg), M. Bottom (Caltech), E. Choquet (JPL/Caltech), C. Gomez (ULg), E. Huby (ULg), L. Hillenbrand (Caltech), R. Jensen-Clem (Caltech), T. Meshkat (JPL/Caltech), M. Reggiani (ULg), E. Serabyn (JPL/Caltech)

© Dimitri Mawet 2017