3 years ago

Exoplanets around Low-mass Stars Unveiled by K2.

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

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 0.96-33 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 $P\lesssim 2$ days, planets with a radius $R_p\gtrsim 2\,R_\oplus$ are less common than planets with a radius between 1-2$\,R_\oplus$. We also see a hint of the "radius valley" between 1.5 and 2$\,R_\oplus$ that 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: those few planets larger than about 3 $R_\oplus$ are found around the most metal-rich M dwarfs.

Publisher URL: http://arxiv.org/abs/1710.03239

DOI: arXiv:1710.03239v2

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