Characterizing the Cool KOIs. III. KOI 961: A Small Star with Large Proper Motion and Three Small Planets

March 2012 • 2012ApJ...747..144M

Authors • Muirhead, Philip S. • Johnson, John Asher • Apps, Kevin • Carter, Joshua A. • Morton, Timothy D. • Fabrycky, Daniel C. • Pineda, John Sebastian • Bottom, Michael • Rojas-Ayala, Bárbara • Schlawin, Everett • Hamren, Katherine • Covey, Kevin R. • Crepp, Justin R. • Stassun, Keivan G. • Pepper, Joshua • Hebb, Leslie • Kirby, Evan N. • Howard, Andrew W. • Isaacson, Howard T. • Marcy, Geoffrey W. • Levitan, David • Diaz-Santos, Tanio • Armus, Lee • Lloyd, James P.

Abstract • We characterize the star KOI 961, an M dwarf with transit signals indicative of three short-period exoplanets discovered by the Kepler mission. We proceed by comparing KOI 961 to Barnard's Star, a nearby, well-characterized mid-M dwarf. We compare colors, optical and near-infrared spectra, and find remarkable agreement between the two, implying similar effective temperatures and metallicities. Both are metal-poor compared to the Solar neighborhood, have low projected rotational velocity, high absolute radial velocity, large proper motion, and no quiescent Hα emission—all of which are consistent with being old M dwarfs. We combine empirical measurements of Barnard's Star and expectations from evolutionary isochrones to estimate KOI 961's mass (0.13 ± 0.05 M _⊙), radius (0.17 ± 0.04 R _⊙), and luminosity (2.40 × 10^{-3.0 ± 0.3} L _⊙). We calculate KOI 961's distance (38.7 ± 6.3 pc) and space motions, which, like Barnard's Star, are consistent with a high scale-height population in the Milky Way. We perform an independent multi-transit fit to the public Kepler light curve and significantly revise the transit parameters for the three planets. We calculate the false-positive probability for each planet candidate, and find a less than 1% chance that any one of the transiting signals is due to a background or hierarchical eclipsing binary, validating the planetary nature of the transits. The best-fitting radii for all three planets are less than 1 R _⊕, with KOI 961.03 being Mars-sized (R_P = 0.57 ± 0.18 R _⊕), and they represent some of the smallest exoplanets detected to date.

Links

• PREPRINT http://arxiv.org/abs/1201.2189
• ELECTR https://doi.org/10.1088%2F0004-637X%2F747%2F2%2F144
• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2012ApJ...747..144M
• PDF https://stacks.iop.org/0004-637X/747/144/pdf
• DATA http://exoplanetarchive.ipac.caltech.edu/cgi-bin/DisplayOverview/nph-DisplayOverview?type=REFERENCE&bibcode=2012ApJ...747..144M
• DATA https://archive.stsci.edu/mastbibref.php?bibcode=2012ApJ...747..144M
• DATA https://irsa.ipac.caltech.edu/bibdata/2012/M/2012ApJ...747..144M.html