A planetary system with two transiting mini-Neptunes near the radius valley transition around the bright M dwarf TOI-776
Creators: name: Luque, R.
name: Serrano, L. M.
name: Molaverdikhani, K.
name: Nixon, M. C.
name: Livingston, J. H.
name: Guenther, E. W.
name: Pallé, E.
name: Madhusudhan, N.
name: Nowak, G.
name: Korth, J.
name: Cochran, W. D.
name: Hirano, T.
name: Chaturvedi, P.
name: Goffo, E.
name: Albrecht, S.
name: Barragán, O.
name: Briceño, C
name: Cabrera, J.
name: Charbonneau, D.
name: Cloutier, R.
name: Collins, K. A.
name: Collins, K. I.
name: Colón, K. D.
name: Crossfield, I. J. M.
name: Csizmadia, Sz.
name: Dai, F.
name: Deeg, H. J.
name: Esposito, M.
name: Fridlund, M.
name: Gandolfi, D.
name: Georgieva, I.
name: Glidden, A.
name: Goeke, R. F.
name: Grziwa, S.
name: Hatzes, A. P.
name: Henze, C. E.
name: Howell, S. B.
name: Irwin, J.
name: Jenkins, J. M.
name: Jensen, E. L. N.
name: Kábath, P.
name: Kidwell, R. C.
name: Kielkopf, J. F.
name: Knudstrup, E.
name: Lam, K. W. F.
name: Latham, D. W.
name: Lissauer, J. J.
name: Mann, A. W.
name: Matthews, E. C.
name: Mireles, I.
name: Narita, N.
name: Paegert, M.
name: Persson, C. M.
name: Redfield, S.
name: Ricker, G. R.
name: Rodler, F.
name: Schlieder, J. E.
name: Scott, N. J.
name: Seager, S.
name: Šubjak, J.
name: Tan, T. G.
name: Ting, E. B.
name: Vanderspek, R.
name: Van Eylen, V.
name: Winn, J. N.
name: Ziegler, C.
Publisher: name: arXiv
Year: 2020
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: We report the discovery and characterization of two transiting planets around the bright M1 V star LP 961-53 (TOI-776, J = 8.5 mag, M = 0.54+-0.03 Msun) detected during Sector 10 observations of the Transiting Exoplanet Survey Satellite (TESS). Combining the TESS photometry with HARPS radial velocities, as well as ground-based follow-up transit observations from MEarth and LCOGT telescopes, we measured for the inner planet, TOI-776 b, a period of 8.25 d, a radius of 1.85+-0.13 Re, and a mass of 4.0+-0.9 Me; and for the outer planet, TOI-776 c, a period of 15.66 d, a radius of 2.02+-0.14 Re, and a mass of 5.3+-1.8 Me. The Doppler data shows one additional signal, with a period of 34 d, associated with the rotational period of the star. The analysis of fifteen years of ground-based photometric monitoring data and the inspection of different spectral line indicators confirm this assumption. The bulk densities of TOI-776 b and c allow for a wide range of possible interior and atmospheric compositions. However, both planets have retained a significant atmosphere, with slightly different envelope mass fractions. Thanks to their location near the radius gap for M dwarfs, we can start to explore the mechanism(s) responsible for the radius valley emergence around low-mass stars as compared to solar-like stars. While a larger sample of well-characterized planets in this parameter space is still needed to draw firm conclusions, we tentatively estimate that the stellar mass below which thermally-driven mass loss is no longer the main formation pathway for sculpting the radius valley is between 0.63 and 0.54 Msun. Due to the brightness of the star, the TOI-776 system is also an excellent target for the James Webb Space Telescope, providing a remarkable laboratory to break the degeneracy in planetary interior models and to test formation and evolution theories of small planets around low-mass stars.
description: 26 pages, 15 figures, 8 tables. Accepted for publication in Astronomy &amp; Astrophysics