Characterizing K2 Candidate Planetary Systems Orbiting Low-Mass Stars II: Planetary Systems Observed During Campaigns 1-7

Creators: Dressing, Courtney D.; Vanderburg, Andrew; Schlieder, Joshua E.; Crossfield, Ian J. M.; Knutson, Heather A.; Newton, Elisabeth R.; Ciardi, David R.; Fulton, Benjamin J.; Gonzales, Erica J.; Howard, Andrew W.; Isaacson, Howard; Livingston, John; Petigura, Erik A.; Sinukoff, Evan; Everett, Mark; Horch, Elliott; Howell, Steve B.

Abstract

We recently used near-infrared spectroscopy to improve the characterization of 76 low-mass stars around which K2 had detected 79 candidate transiting planets. 29 of these worlds were new discoveries that had not previously been published. We calculate the false positive probabilities that the transit-like signals are actually caused by non-planetary astrophysical phenomena and reject five new transit-like events and three previously reported events as false positives. We also statistically validate 17 planets (7 of which were previously unpublished), confirm the earlier validation of 22 planets, and announce 17 newly discovered planet candidates. Revising the properties of the associated planet candidates based on the updated host star characteristics and refitting the transit photometry, we find that our sample contains 21 planets or planet candidates with radii smaller than 1.25 R⊕, 18 super-Earths (1.25–2 R⊕), 21 small Neptunes (2–4 R⊕), three large Neptunes (4–6 R⊕), and eight giant planets (>6 R⊕). Most of these planets are highly irradiated, but EPIC 206209135.04 (K2-72e, 1.29^(+0.14)_(-0.13) R⊕), EPIC 211988320.01 (R_p = 2.86^(+0.16)_(-0.15) R⊕), and EPIC 212690867.01 (2.20^(+0.19)_(-0.18) R⊕) orbit within optimistic habitable zone boundaries set by the "recent Venus" inner limit and the "early Mars" outer limit. In total, our planet sample includes eight moderately irradiated 1.5–3 R⊕ planet candidates (F_p ≾ 20 F⊕) orbiting brighter stars (Ks < 11) that are well-suited for atmospheric investigations with the Hubble, Spitzer, and/or James Webb Space Telescopes. Five validated planets orbit relatively bright stars (Kp < 12.5) and are expected to yield radial velocity semi-amplitudes of at least 2 m s^(−1). Accordingly, they are possible targets for radial velocity mass measurement with current facilities or the upcoming generation of red optical and near-infrared high-precision RV spectrographs.

Additional Information

© 2017 The American Astronomical Society. Received 2017 March 21; revised 2017 August 8; accepted 2017 August 24; published 2017 November 3. Many of our targets were provided by the K2 California Consortium (K2C2). We thank K2C2 for sharing their candidate lists and vetting products. We are grateful to Tim Morton for making VESPA publicly available and to Jennifer Winters, Nic Scott, and Lea Hirsch for assisting with speckle imaging. We also acknowledge helpful conversations with Chas Beichman, Eric Gaidos, Jessie Christiansen, Michael Werner, and Arturo Martinez. We thank the anonymous referee for providing helpful suggestions to improve the paper. This work was performed under contract with the Jet Propulsion Laboratory (JPL) funded by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute. A.V. is supported by the NSF Graduate Research Fellowship, grant No. DGE 1144152. This publication was made possible through the support of a grant from the John Templeton Foundation. The opinions expressed here are those of the authors and do not necessarily reflect the views of the John Templeton Foundation. This paper includes data collected by the K2 mission, which is funded by the NASA Science Mission directorate. This research has made use of the NASA Exoplanet Archive, and the Exoplanet Follow-up Observation Program website, which are operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. We appreciate the willingness of ExoFOP users to share their follow-up observations with the broader K2 community. Our stellar characterization follow-up observations were obtained at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract NNH14CK55B with the National Aeronautics and Space Administration and at Palomar Observatory. We thank the staff at both observatories and the Caltech Remote Observing Facilities staff for supporting us during our many observing runs. We are grateful to the IRTF and Caltech TACs for awarding us telescope time. Our contrast curves were based on observations obtained at Gemini Observatory, Kitt Peak National Observatory via the NN-EXPLORE program time allocation at the WIYN telescope, and the W.M. Keck Observatory. Kitt Peak National Observatory, National Optical Astronomy Observatory, is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. Gemini Observatory is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil). The W.M. Keck Observatory 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 Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facilities: Kepler - The Kepler Mission, K2 - , IRTF (SpeX) - , Palomar:Hale (TripleSpec - , PALM-3000/PHARO) - , Gemini-N (DSSI - , NIRI) - , Gemini-S (DSSI) - , Keck:II (NIRC2) - , WIYN (DSSI) - .

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