Exoplanets around Low-mass Stars Unveiled by K2

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We present the detection and follow-up observations of planetary candidates around low-mass stars observed by the K2 mission. Based on light-curve analysis, adaptive-optics imaging, and optical spectroscopy at low and high resolution (including radial velocity measurements), we validate 16 planets around 12 low-mass stars observed during K2 campaigns 5-10. Among the 16 planets, 12 are newly validated, with orbital periods ranging from 0.96 to 33 days. For one of the planets (K2-151b), we present ground-based transit photometry, allowing us to refine the ephemerides. Combining our K2 M-dwarf planets together with the validated or confirmed planets found previously, we investigate the dependence of planet radius R-p on stellar insolation and metallicity [Fe/H]. We confirm that for periods P less than or similar to 2 days, planets with a radius R-p greater than or similar to 2 R-circle plus are less common than planets with a radius between 1-2 R-circle plus. We also see a hint of the "radius valley" between 1.5 and 2 R-circle plus, which has been seen for close-in planets around FGK stars. These features in the radius/period distribution could be attributed to photoevaporation of planetary envelopes by high-energy photons from the host star, as they have for FGK stars. For the M dwarfs, though, the features are not as well defined, and we cannot rule out other explanations such as atmospheric loss from internal planetary heat sources or truncation of the protoplanetary disk. There also appears to be a relation between planet size and metallicity: the few planets larger than about 3 R-circle plus are found around the most metal-rich M dwarfs.
© 2018. The American Astronomical Society. All rights reserved. Artículo firmado por 41 autores. This paper is based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan, on observations collected at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly by the Max-Planck Institut fur Astronomie and the Instituto de Astrofísica de Andalucía, and on observations obtained (a) with the Nordic Optical Telescope (NOT), operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos (ORM) of the Instituto de Astrofísica de Canarias (IAC); (b) with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundacion Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canaria. The data analysis was in part carried out on a common use data analysis computer system at the Astronomy Data Center, ADC, of the National Astronomical Observatory of Japan. We thank Akito Tajitsu, Joanna Bulger, and Ji Hoon Kim, the support astronomers at Subaru, and Jun Hashimoto, Shoya Kamiaka, Yohei Koizumi, and Shota Sasaki for their helps to carry out the Subaru observations. We also thank Santos Pedraz for carrying out the CAFOS observations at the Calar Alto observatory. We are very grateful to the NOT and TNG staff members for their unique and superb support during the observations. T.H. is grateful to Samuel Yee for providing instructions to install SpecMatch-Emp. We are thankful to Christophe Lovis, who provided the numerical mask for the spectral cross-correlation analysis. The discussions with Eric Gaidos, Hiroyuki Kurokawa, José Caballero, Alexis Klutsch, and Kento Masuda were very fruitful. This work was supported by Japan Society for Promotion of Science (JSPS) KAKENHI Grant Number JP16K17660. D.G. gratefully acknowledges the financial support of the Programma Giovani Ricercatori-Rita Levi Montalcini-Rientro dei Cervelli (2012) awarded by the Italian Ministry of Education, Universities and Research (MIUR). The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2013-2016) under grant agreement No. 312430 (OPTICON). This work was partially supported by Japan Society for Promotion of Science (JSPS) Fellowship for Research (No. 25-8826). D.M. acknowledges financial support from the Universidad Complutense de Madrid (UCM), the Spanish Ministry of Economy and Competitiveness (MINECO) from project AYA2016-79425-C3-1-PIR and acknowledges support by the Spanish Ministry of Economy and Competitiveness (MINECO) and the Fondo Europeo de Desarrollo Regional (FEDER) through grant ESP2016-80435-C2-1-R, as well as the support of the Generalitat de Catalunya/CERCA programme. We acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous people in Hawai'i.
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