A multi-planetary system orbiting the early-M dwarf TOI-1238

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Context. The number of super-Earth and Earth-mass planet discoveries has increased significantly in the last two decades thanks to the Doppler radial velocity and planetary transit observing techniques. Either technique can detect planet candidates on its own, but the power of a combined photometric and spectroscopic analysis is unique for an insightful characterization of the planets, which in turn has repercussions for our understanding of the architecture of planetary systems and, therefore, their formation and evolution. Aims. Two transiting planet candidates with super-Earth radii around the nearby (d = 70.64 +/- 0.06 pc) K7-M0 dwarf star TOI-1238 were announced by NASA's Transiting Exoplanet Survey Satellite (TESS), which observed the field of TOI-1238 in four different sectors. We aim to validate their planetary nature using precise radial velocities taken with the CARMENES spectrograph. Methods. We obtained 55 CARMENES radial velocity measurements that span the 11 months between 9 May 2020 and 5 April 2021. For a better characterization of the parent star's activity, we also collected contemporaneous optical photometric observations at the Joan Oro and Sierra Nevada observatories and retrieved archival photometry from the literature. We performed a combined TESS+CARMENES photometric and spectroscopic analysis by including Gaussian processes and Keplerian orbits to account for the stellar activity and planetary signals simultaneously. Results. We estimate that TOI-1238 has a rotation period of 40 +/- 5 d based on photometric and spectroscopic data. The combined analysis confirms the discovery of two transiting planets, TOI-1238 b and c, with orbital periods of 0.764597(-0.00001)(+0.000013) d and 3.294736(-0.000036)(+0.000034), masses of 3.76(-1.07)(+1.15) M-circle plus and 8.32(-1.88)(+1.90) M-circle plus, and radii of 1.21(-0.10)(+1.11) R-circle plus and 2.11(-1)(.1)(4)(+0.14) R-circle plus. They orbit their parent star at semimajor axes of 0.0137 +/- 0.0004 au and 0.036 +/- 0.001 au, respectively. The two planets are placed on opposite sides of the radius valley for M dwarfs and lie between the star and the inner border of TOI-1238's habitable zone. The inner super-Earth TOI-1238 b is one of the densest ultra-short-period planets ever discovered (rho = 11.7(-3.4)(+4.2) g cm(-3)). The CARMENES data also reveal the presence of an outer, non-transiting, more massive companion with an orbital period and radial velocity amplitude of >= 600 d and >= 70 m s(-1), which implies a likely mass of M >= 2 M root 1 - e(2) M-Jup and a separation >= 1.1 au from its parent star.
©ESO 2021. Artículo firmado por 56 autores. CARMENES is an instrument for the Centro Astronómico Hispano-Alemán de Calar Alto (CAHA, Almeria, 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 fur Astronomie, Instituto de Astrofísica de Andalucía, Landessternwarte Koonigstuhl, Institut de Ciencies de l'Espai, Insitut fur Astrophysik Gottingen, Universidad, Complutense de Madrid, Thuringer 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-Wuttemberg, the German Science Foundation (DFG), the Klaus Tschira Foundation (KTS), and by the Junta de Andalucía. This work was based on data from the CARMENES data archive at CAB (CSIC-INTA). This research has made use 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. Funding for the TESS mission is provided by NASA's Science Mission Directorate. This paper includes data collected by the TESS mission that are publicly available from the Mikulski Archive for Space Telescopes. We acknowledge the use of public TESS data from pipelines at the TESS Science Office and at the TESS Science Processing Operations Center. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products. Some of the observations in the paper made use of the High-Resolution Imaging instrument `Alopeke obtained under Gemini LLP Proposal Number: GN/S-2021A-LP-105. `Alopeke was funded by the NASA Exoplanet Exploration Program and built at the NASA Ames Research Center by Steve B. Howell, Nic Scott, Elliott P. Horch, and Emmett Quigley. `Alopeke was mounted on the Gemini North (and/or South) telescope of the international Gemini Observatory, a program of NSF's OIR Lab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. on behalf of the Gemini partnership: the National Science Foundation (United States), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina), Ministerio da Ciencia, Tecnologia, Inovacoes e Comunicacoes (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea). Data were partly collected with the 90 cm telescope at the Sierra Nevada Observatory (SNO) operated by the Instituto de Astrofísica de Andalucía (IAA-CSIC). We acknowledge the telescope operators from Observatori Astronomic del Montsec, Sierra Nevada Observatory, and Centro Astronómico Hispano-Alemán de Calar Alto (CAHA). E.G.A., M.R.Z.O., J.A.C., J.S.F, and D.M. acknowledge financial support from the Spanish Ministry of Science and Innovation through project PID2019-109522GBC5[1:4]. E.G.A also acknowledges support from AEI Project No: MDM-2017-0737 Unidad de Excelencia "María de Maeztu" -Centro de Astrobiología (CSIC-INTA). V.M.P. acknowledges financial support from NASA through grant NNX17AG24G. S.V.J.; acknowledges the support of the DFG priority program SPP 1992 "Exploring the Diversity of Extrasolar Planets (JE 701/5-1)". M.J.L.-G., E.R., C.R.-L., and P.J.A. acknowledge financial support from the Agencia Estatal de Investigación of the Ministerio de Ciencia e Innovación through projects PID2019-109522GB-C52, PID2019107061GB-C64, PID2019-110689RB-100 and the Centre of Excellence Severo Ochoa Instituto de Astrofísica de Andalucía (SEV-2017-0709). G.M. received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 895525. S.S. and S.R. acknowledge support by the DFG Research Unit FOR 2544 Blue Planets around Red Stars, project no. RE 2694/4-1.
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