K2-231 b: A Sub-Neptune Exoplanet Transiting a Solar Twin in Ruprecht 147

Creators: Curtis, Jason Lee; Vanderburg, Andrew; Torres, Guillermo; Kraus, Adam L.; Huber, Daniel; Mann, Andrew W.; Rizzuto, Aaron C.; Isaacson, Howard; Howard, Andrew W.; Henze, Christopher E.; Fulton, Benjamin J.; Wright, Jason T.

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Abstract

We identify a sub-Neptune exoplanet (R_p = 2.5 ± 0.2 R_⊕) transiting a solar twin in the Ruprecht 147 star cluster (3 Gyr, 300 pc, [Fe/H] = +0.1 dex). The ~81 day light curve for EPIC 219800881 (V = 12.71) from K2 Campaign 7 shows six transits with a period of 13.84 days, a depth of ~0.06%, and a duration of ~4 hr. Based on our analysis of high-resolution MIKE spectra, broadband optical and NIR photometry, the cluster parallax and interstellar reddening, and isochrone models from PARSEC, Dartmouth, and MIST, we estimate the following properties for the host star: M_★ = 1.01 ± 0.03 M_⊙, R_★ = 0.95 ± 0.03 R_⊙, and T_(eff) = 5695 ± 50 K. This star appears to be single based on our modeling of the photometry, the low radial velocity (RV) variability measured over nearly 10 yr, and Keck/NIRC2 adaptive optics imaging and aperture-masking interferometry. Applying a probabilistic mass–radius relation, we estimate that the mass of this planet is M_p = 7 + 5 – 3 M_⊕, which would cause an RV semi-amplitude of K = 2 ± 1 m s^(-1) that may be measurable with existing precise RV facilities. After statistically validating this planet with BLENDER, we now designate it K2-231b, making it the second substellar object to be discovered in Ruprecht 147 and the first planet; it joins the small but growing ranks of 22 other planets and three candidates found in open clusters.

Additional Information

© 2018. The American Astronomical Society. Received 2017 November 3; revised 2018 February 1; accepted 2018 March 4; published 2018 March 29. A.V. produced the light curve used in this work and corroborated the initial discovery; he is supported by the NASA Sagan Fellowship. G.T. performed the BLENDER false-alarm analysis and acknowledges partial support for this work from NASA grant NNX14AB83G (Kepler Participating Scientist Program). A.W.H. led the acquisition of HIRES spectra of the planet host and faint neighbor. H.I. measured stellar RVs for those targets and checked the spectra for secondary light. D.H. provided the access to Keck HIRES needed to acquire those spectra and performed the isoclassify analysis; he acknowledges support by the National Aeronautics and Space Administration under grant NNX14AB92G issued through the Kepler Participating Scientist Program. A.L.K., A.C.R., and A.W.M. acquired and analyzed the Keck adaptive optics data. A.W.M. fit the light curve to measure the transit properties and contributed Figure 2. A.L.K. obtained the UKIRT/WFCAM imaging. B.J.F. contributed Figure 7. C. Henze ran the BLENDER jobs on the Pleiades supercomputer and preprocessed the output. J.T.W. advised the Ph.D. dissertation work of J.L.C., which amassed much of the basic data presented herein (e.g., proprietary photometry and spectroscopy), and is the submitting and administrative PI of the K2 program GO 7035. The remainder of the work was completed by J.L.C., including the planet discovery, host star characterization, preliminary transit fitting, and synthesis of the data and contributions provided by the coauthors. He successfully led a petition for Campaign 7 to point at Ruprecht 147 and, as science PI of GO 7035, was awarded the program to survey the cluster while a graduate student at Penn State University and member of the Center for Exoplanets and Habitable Worlds. He was granted access to Magellan while serving as an SAO predoctoral fellow at the Harvard-Smithsonian Center for Astrophysics. The planet discovery and characterization work was performed after joining Columbia University. J.L.C. is supported by the National Science Foundation Astronomy and Astrophysics Postdoctoral Fellowship under award AST-1602662 and the National Aeronautics and Space Administration under grant NNX16AE64G issued through the K2 Guest Observer Program (GO 7035). He thanks the referee for feedback, J.T.W. and Marcel Agüeros for serving as his mentors, Luca Malavolta for commenting on a draft of this manuscript and providing early access to the HARPS RVs acquired by the Minniti team, Iván Ramírez and Luca Casagrande for providing their temperature measurements, Fabienne Bastien and Jacob Luhn for commenting on a draft of this manuscript and discussing RV jitter, the Harvard-Smithsonian Center for Astrophysics telescope allocation committee for granting access to Magellan, the K2 Guest Observer office and Ball Aerospace for repositioning the Campaign 7 field to accommodate Ruprecht 147, and the coinvestigators of the "K2 Survey of Ruprecht 147" (GO 7035): J.T.W., Fabienne Bastien, Søren Meibom, Victor Silva Aguirre, and Steve Saar. The Center for Exoplanets and Habitable Worlds is supported by Pennsylvania State University, the Eberly College of Science, and the Pennsylvania Space Grant Consortium. This paper includes data collected by the K2 mission. Funding for the Kepler and K2 missions is provided by the NASA Science Mission directorate. We obtained these data 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 NNX09AF08G and by other grants and contracts. This research has made use of the Periodogram Service 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 (Han et al. 2014). This work is based on observations obtained with MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada–France–Hawai'i Telescope (CFHT), which is operated by the National Research Council (NRC) of Canada, the Institute National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawai'i Observing time was granted by the University of Hawai'i Institute for Astronomy TAC. These data were reduced at the TERAPIX data center located at the Institut d'Astrophysique de Paris. This publication makes use of data products from the Two Micron All-Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA and the NSF. This research was made possible through the use of the AAVSO Photometric All-Sky Survey (APASS), funded by the Robert Martin Ayers Sciences Fund. This publication makes use of data acquired from UKIRT while it was operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the UK. UKIRT is supported by NASA and operated under an agreement among the University of Hawaii, the University of Arizona, and the Lockheed Martin Advanced Technology Center; operations are enabled through the cooperation of the East Asian Observatory. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC; https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with or with funding from the National Aeronautics and Space Administration. This research has also made use of NASA's Astrophysics Data System and the VizieR and SIMBAD databases, operated at CDS, Strasbourg, France. 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. We wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawai'ian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This work also utilized SOLIS data obtained by the NSO Integrated Synoptic Program (NISP), managed by the National Solar Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under a cooperative agreement with the National Science Foundation. Facilities: Kepler (K2) - , CFHT (MegaCam) - , ESO:3.6 m (HARPS) - , Keck:I (HIRES) - , Keck:II (NIRC2) - , Magellan:Clay (MIKE) - , MMT (Hectochelle) - , Shane (Hamilton) - , SOLIS (ISS) - , UKIRT (WFCAM). - Software: BARYCORR (Wright & Eastman 2014), batman (Kreidberg 2015), EXOFAST (Eastman et al. 2013), forecaster (Chen & Kipping 2017) isochrones (Morton 2015), isoclassify (Huber et al. 2017), RVLIN (Wright & Howard 2009), SME (Valenti & Piskunov 1996).

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