Kepler-445, Kepler-446 and the Occurrence of Compact Multiples Orbiting Mid-M Dwarf Stars

Creators: Muirhead, Philip S.; Mann, Andrew W.; Vanderburg, Andrew; Morton, Timothy D.; Kraus, Adam; Ireland, Michael; Swift, Jonathan J.; Feiden, Gregory A.; Gaidos, Eric; Gazak, J. Zachary

Abstract

We confirm and characterize the exoplanetary systems Kepler-445 and Kepler-446: two mid-M dwarf stars, each with multiple, small, short-period transiting planets. Kepler-445 is a metal-rich ([Fe/H] = +0.25 ± 0.10) M4 dwarf with three transiting planets, and Kepler-446 is a metal-poor ([Fe/H] = –0.30 ± 0.10) M4 dwarf also with three transiting planets. Kepler-445c is similar to GJ 1214b: both in planetary radius and the properties of the host star. The Kepler-446 system is similar to the Kepler-42 system: both are metal-poor with large galactic space velocities and three short-period, likely rocky transiting planets that were initially assigned erroneously large planet-to-star radius ratios. We independently determined stellar parameters from spectroscopy and searched for and fitted the transit light curves for the planets, imposing a strict prior on stellar density in order to remove correlations between the fitted impact parameter and planet-to-star radius ratio for short-duration transits. Combining Kepler-445, Kepler-446, and Kepler-42, and isolating all mid-M dwarf stars observed by Kepler with the precision necessary to detect similar systems, we calculate that 21^(+7)_(-5)% of mid-M dwarf stars host compact multiples (multiple planets with periods of less than 10 days) for a wide range of metallicities. We suggest that the inferred planet masses for these systems support highly efficient accretion of protoplanetary disk metals by mid-M dwarf protoplanets.

Additional Information

© 2015 American Astronomical Society. Received 2014 October 6; accepted 2014 December 31; published 2015 February 26. We thank the anonymous referee for the thoughtful report and useful suggestions. We thank Andrew West for the suggestions regarding measuring accurate SDSS magnitudes. A.V. is supported by the National Science Foundation Graduate Research Fellowship under grant No. DGE1144152. This paper includes data collected by the Kepler Mission. Funding for the Kepler Mission is provided by the NASA Science Mission Directorate. The Kepler data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (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 NNX13AC07G and by other grants and contracts. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. This research has made use of the Exoplanet Orbit Database and the Exoplanet Data Explorer at exoplanets.org (Wright et al. 2011). These results made use of the Discovery Channel Telescope at Lowell Observatory, supported by Discovery Communications, Inc., Boston University, the University of Maryland, the University of Toledo, and Northern Arizona University. The Large Monolithic Imager was funded by the National Science Foundation via grant AST-1005313. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. SNIFS on the UH 2.2m telescope is part of the Nearby Supernova Factory project, a scientific collaboration among the Centre de Recherche Astronomique de Lyon, Institut de Physique Nuclaire de Lyon, Laboratoire de Physique Nuclaire et des Hautes Energies,Lawrence Berkeley National Laboratory, Yale University, University of Bonn, Max Planck Institute for Astrophysics, Tsinghua Center for Astrophysics, and the Centre de Physique des Particules de Marseille. The infrared spectrum of Barnard's Star was acquired using the SpeX instrument on the Infrared Telescope Facility, which is operated by the University of Hawaii under Cooperative agreement No. NNX-08AE38A with the National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program. These results made use of the Sloan Digital Sky Survey (SDSS) III. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science. The SDSS-III Web site is http://www.sdss3.org/. SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University. The SDSS data were accessed using the VizieR catalog access tool, CDS, Strasbourg, France (Ochsenbein et al. 2000). Facilities: DCT (LMI), Hale (TSPEC), IRTF (SpeX), Keck:II (NIRC2, ESI), Kepler, UH:2.2m (SNIFS)

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