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A Second Planet Transiting LTT 1445A and a Determination of the Masses of Both Worlds

J. G. Winters, R. Cloutier, A. A. Medina, J. M. Irwin, D. Charbonneau, N. Astudillo-Defru, X. Bonfils, A. W. Howard, H. Isaacson, J. L. Bean, A. Seifahrt, J. Teske, J. D. Eastman, J. D. Twicken, K. A. Collins, E. L. N. Jensen, S. N. Quinn, M. J. Payne, M. H. Kristiansen, A. Spencer, A. Vanderburg, M. Zechmeister, L. M. Weiss, S. X. Wang, G. Wang, S. Udry, I. A. Terentev, J. Stürmer, G. Stefánsson, A. Shporer, S. A. Shectman, R. Sefako, H. M. Schwengeler, R. P. Schwarz, N. Scarsdale, R. A. Rubenzahl, A. Roy, L. J. Rosenthal, P. Robertson, E. Petigura, F. Pepe, M. Omohundro, Joseph M. A. Murphy, F. Murgas, T. Močnik, B. T. Montet, R. E. Mennickent, A. W. Mayo, B. Massey, J. Lubin, C. Lovis, P. Lewin, D. Kasper, S. R. Kane, J. M. Jenkins, D. Huber, K. Horne, M. L. Hill, P. Gorrini, S. Giacalone, B. J. Fulton, T. Forveille, P. Figueira, T. Fetherolf, C. D. Dressing, R. F. Díaz, X. Delfosse, P. A. Dalba, F. Dai, C. C. Cortés, I. J. M. Crossfield, J. D. Crane, D. M. Conti, K. I. Collins, A. Chontos, R. P. Butler, P. Brown, M. Brady, A. Behmard, C. Beard, N. Batalha, J. -M. Almenara

LTT 1445 is a hierarchical triple M-dwarf star system located at a distance of 6.86 pc. The primary star LTT 1445A (0.257 M) is known to host the transiting planet LTT 1445Ab with an orbital period of 5.36 days, making it the second-closest known transiting exoplanet system, and the closest one for which the host is an M dwarf. Using Transiting Exoplanet Survey Satellite data, we present the discovery of a second planet in the LTT 1445 system, with an orbital period of 3.12 days. We combine radial-velocity measurements obtained from the five spectrographs, Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, High Accuracy Radial Velocity Planet Searcher, High-Resolution Echelle Spectrometer, MAROON-X, and Planet Finder Spectrograph to establish that the new world also orbits LTT 1445A. We determine the mass and radius of LTT 1445Ab to be 2.87 ± 0.25 M and 1.304+0.067−0.060 R, consistent with an Earth-like composition. For the newly discovered LTT 1445Ac, we measure a mass of 1.54+0.20−0.19 M and a minimum radius of 1.15 R, but we cannot determine the radius directly as the signal-to-noise ratio of our light curve permits both grazing and nongrazing configurations. Using MEarth photometry and ground-based spectroscopy, we establish that star C (0.161 M) is likely the source of the 1.4 day rotation period, and star B (0.215 M) has a likely rotation period of 6.7 days. We estimate a probable rotation period of 85 days for LTT 1445A. Thus, this triple M-dwarf system appears to be in a special evolutionary stage where the most massive M dwarf has spun down, the intermediate mass M dwarf is in the process of spinning down, while the least massive stellar component has not yet begun to spin down.

Exoplanets; Solar neighborhood; Multiple stars; M dwarf stars

The Astronomical Journal
Volume 163, Number 4
2022 April

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Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia