The CARMENES search for exoplanets around M dwarfs A super-Earth planet orbiting HD 79211 (GJ 338 B)

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Aims. We report on radial velocity time series for two M0.0 V stars, GJ 338 B and GJ 338 A, using the CARMENES spectrograph, complemented by ground-telescope photometry from Las Cumbres and Sierra Nevada observatories. We aim to explore the presence of small planets in tight orbits using the spectroscopic radial velocity technique. Methods. We obtained 159 and 70 radial velocity measurements of GJ 338 B and A, respectively, with the CARMENES visible channel between 2016 January and 2018 October. We also compiled additional relative radial velocity measurements from the literature and a collection of astrometric data that cover 200 a of observations to solve for the binary orbit. Results. We found dynamical masses of 0.64 ± 0.07 Mͼ for GJ 338 B and 0.69 ± 0.07 Mͼ for GJ 338 A. The CARMENES radial velocity periodograms show significant peaks at 16.61 ± 0.04 d (GJ 338 B) and 16.3^(+3.5)_(−1.3) d (GJ 338 A), which have counterparts at the same frequencies in CARMENES activity indicators and photometric light curves. We attribute these to stellar rotation. GJ 338 B shows two additional, significant signals at 8.27 ± 0.01 and 24.45 ± 0.02 d, with no obvious counterparts in the stellar activity indices. The former is likely the first harmonic of the star’s rotation, while we ascribe the latter to the existence of a super-Earth planet with a minimum mass of 10.27^(+1.47)_(−1.38) Mͼ orbiting GJ 338 B. We have not detected signals of likely planetary origin around GJ 338 A. Conclusions. GJ 338 Bb lies inside the inner boundary of the habitable zone around its parent star. It is one of the least massive planets ever found around any member of stellar binaries. The masses, spectral types, brightnesses, and even the rotational periods are very similar for both stars, which are likely coeval and formed from the same molecular cloud, yet they differ in the architecture of their planetary systems.
© ESO 2020. Artículo firmado por 30 autores. We wish to thank the anonymous referee for helpful comments and suggestions, which helped to improve the manuscript. We are grateful to Prof. B. D. Mason for providing Washington Double Star astrometric data for the stellar binary. Based on observations collected at the German-Spanish Astronomical Center, Calar Alto, jointly operated by the Junta de Andalaucía and the Instituto de Astrofísica de Andalucía (CSIC), and observations from Las Cumbres Observatory Global Telescope (LCOGT) network. LCOGT observations were partially acquired via program number TAU2019A-002 of the Wise Observatory, Tel-Aviv University, Israel. CARMENES is an instrument for the Centro Astronómico Hispano-Alemán de Calar Alto (CAHA, Almería, Spain). CARMENES is funded by the German Max-Planck- Gesellschaft (MPG), the Spanish Consejo Superior de Investigaciones Científicas (CSIC), the European Union through FEDER/ERF funds, and the members of the CARMENES Consortium (Max-Planck-Institut für Astronomie, Instituto de Astrofísica de Andalucía, Landessternwarte Koönigstuhl, Institut de Ciències de l’Espai, Insitut für Astrophysik Göttingen, Universidad, Complutense de Madrid, Thüringer Landessternwarte Tautenburg, Instituto de Astrofísica de Canarias, Hamburger Sternwarte, Centro de Astrobiología and Centro Astronómico Hispano-Alemán), with additional contributions by the Spanish Ministry of Economy, the state of Baden-Wüttemberg, the German Science Foundation (DFG), the Klaus Tschira Foundation (KTS), and by the Junta de Andalucía. This work is supported by the Spanish Ministery for Science, Innovation, and Universities through projects AYA-2016-79425-C3-1/2/3-P, AYA2015-69350-C3-2-P, ESP2017-87676-C5-2- R, ESP2017-87143-R. The Instituto de Astrofísica de Andalucía is a Centre of Excellence “Severo Ochoa” (SEV-2017-0709). The Centro de Astrobiología (CAB, CSIC-INTA) is a Center of Excellence “Maria de Maeztu”.
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