CHARACTERIZING THE COOL KOIs. III. KOI 961: A SMALL STAR WITH LARGE PROPER MOTION AND THREE SMALL PLANETS

Abstract

We present the characterization of the star KOI 961, an M dwarf with transit\nsignals indicative of three short-period exoplanets, originally discovered by\nthe Kepler Mission. We proceed by comparing KOI 961 to Barnard's Star, a\nnearby, well-characterized mid-M dwarf. By comparing colors, optical and\nnear-infrared spectra, we find remarkable agreement between the two, implying\nsimilar effective temperatures and metallicities. Both are metal-poor compared\nto the Solar neighborhood, have low projected rotational velocity, high\nabsolute radial velocity, large proper motion and no quiescent H-alpha\nemission--all of which is consistent with being old M dwarfs. We combine\nempirical measurements of Barnard's Star and expectations from evolutionary\nisochrones to estimate KOI 961's mass (0.13 +/- 0.05 Msun), radius (0.17 +/-\n0.04 Rsun) and luminosity (2.40 x 10^(-3.0 +/- 0.3) Lsun). We calculate KOI\n961's distance (38.7 +/- 6.3 pc) and space motions, which, like Barnard's Star,\nare consistent with a high scale-height population in the Milky Way. We perform\nan independent multi-transit fit to the public Kepler light curve and\nsignificantly revise the transit parameters for the three planets. We calculate\nthe false-positive probability for each planet-candidate, and find a less than\n1% chance that any one of the transiting signals is due to a background or\nhierarchical eclipsing binary, validating the planetary nature of the transits.\nThe best-fitting radii for all three planets are less than 1 Rearth, with KOI\n961.03 being Mars-sized (Rp = 0.57 +/- 0.18 Rearth), and they represent some of\nthe smallest exoplanets detected to date.\n