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

Creators: Muirhead, Philip S.; Johnson, John Asher; Morton, Timothy D.; Pineda, J. Sebastian; Bottom, Michael; Crepp, Justin R.; Kirby, Evan N.; Howard, Andrew W.; Levitan, David; Díaz-Santos, Tanio; Armus, Lee

Abstract

We present the characterization of the star KOI 961, an M dwarf with transit signals indicative of three short-period exoplanets, originally discovered by the Kepler Mission. We proceed by comparing KOI 961 to Barnard's Star, a nearby, well-characterized mid-M dwarf. By comparing colors, optical and near-infrared spectra, we 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 H-alpha emission--all of which is 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 x 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 Re_⊕, with KOI 961.03 being Mars-sized (Rp = 0.57 ± 0.18 R_⊕), and they represent some of the smallest exoplanets detected to date.

Additional Information

We would like to thank Peter Dawson, who provided the multi-wavelength spectrum of Barnard's Star. We would like to thank Sarah Ballard and Michael Line for the thoughtful discussions concerning the paper. We would like to thank Bruce Gary of Hereford Arizona Observatory for taking the B,V and RC photometric measurements of KOI 961. This work includes observations taken at the Palomar Obervatory 200-inch Hale Telescope granted by Cornell University. The TripleSpec spectrograph was built at Cornell and delivered to Palomar as part of the Cornell- Caltech-Palomar arrangement. This work includes observations taken at the W. M. Keck Observatory and Palomar Observatory granted by the California Institute of Technology. Some of the Palomar 200-inch Telescope time was provided by NASA/JPL. We thank Melodie Kao, Matthew Giguere and Ming Zhao for assisting with the Keck observations. J.A.C, K.R.C., D.C.F. and E.N.K. acknowledge support for this work from the Hubble Fellowship Pro- gram, provided by NASA through Hubble Fellowship grants HF-51267.01-A, HST-HF-51253.01-A, HF- 51272.01-A and HST-HF-51256.01-A, awarded by the Space Telescope Science Institute, which is operated by the AURA, Inc., for NASA, under contract NAS 5-26555. K.G.S., J.P. and L.H. acknowledge support through the Vanderbilt Initiative in Data-intensive Astrophysics and NSF grants AST-0849736 and AST-1009810. This paper includes data collected by the Kepler mission. Funding for the Kepler mission is provided by the NASA Science Mission directorate. Some of the data presented in this paper were obtained from the Multimission Archive at the Space Telescope Science Institute (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts. The United Kingdom Infrared Telescope (UKIRT) is operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the U.K. The Digitized Sky Surveys were produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. The images of these surveys are based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope. The plates were processed into the present compressed digital form with the permission of these institutions. The National Geographic Society - Palomar Observatory Sky Atlas (POSS-I) was made by the California Institute of Technology with grants from the National Geographic Society. The Second Palomar Observatory Sky Survey (POSS-II) was made by the California Institute of Technology with funds from the National Science Foundation, the National Geographic Society, the Sloan Foundation, the Samuel Oschin Foundation, and the Eastman Kodak Corporation. The Oschin Schmidt Telescope is operated by the California Institute of Technology and Palomar Observatory.

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