Exoplanets around Low-mass Stars Unveiled by K2

Teruyuki Hirano, Fei Dai, Davide Gandolfi, Akihiko Fukui, John H. Livingston, Kohei Miyakawa, Michael Endl, William D. Cochran, Francisco J. Alonso-Floriano, Masayuki Kuzuhara, David Montes, Tsuguru Ryu, Simon Albrecht, Oscar Barragan, Juan Cabrera, Szilard Csizmadia, Hans Deeg, Philipp Eigmüller, Anders Erikson, Malcolm FridlundSascha Grziwa, Eike W. Guenther, Artie P. Hatzes, Judith Korth, Tomoyuki Kudo, Nobuhiko Kusakabe, Norio Narita, David Nespral, Grzegorz Nowak, Martin Pätzold, Enric Palle, Carina M. Persson, Jorge Prieto-Arranz, Heike Rauer, Ignasi Ribas, Bun'Ei Sato, Alexis M.S. Smith, Motohide Tamura, Yusuke Tanaka, Vincent Van Eylen, Joshua N. Winn

Research output: Contribution to journalArticle

23 Scopus citations

Abstract

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 ≲ 2 days, planets with a radius Rp ≳ 2 R are less common than planets with a radius between 1-2 R . We also see a hint of the "radius valley" between 1.5 and 2 R , 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 are found around the most metal-rich M dwarfs.

Original languageEnglish (US)
Article number127
JournalAstronomical Journal
Volume155
Issue number3
DOIs
StatePublished - Mar 2018

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Keywords

  • methods: observational
  • planets and satellites: detection
  • techniques: high angular resolution
  • techniques: photometric
  • techniques: radial velocities
  • techniques: spectroscopic

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    Hirano, T., Dai, F., Gandolfi, D., Fukui, A., Livingston, J. H., Miyakawa, K., Endl, M., Cochran, W. D., Alonso-Floriano, F. J., Kuzuhara, M., Montes, D., Ryu, T., Albrecht, S., Barragan, O., Cabrera, J., Csizmadia, S., Deeg, H., Eigmüller, P., Erikson, A., ... Winn, J. N. (2018). Exoplanets around Low-mass Stars Unveiled by K2. Astronomical Journal, 155(3), [127]. https://doi.org/10.3847/1538-3881/aaa9c1