TOI-2458 b: A mini-Neptune consistent with in situ hot Jupiter formation^★

¹ Astronomical Institute, Czech Academy of Sciences, Fričova 298, 251 65 Ondřejov, Czech Republic
² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
³ Dipartimento di Fisica, Universitá degli Studi di Torino, via Pietro Giuria 1, 10125 Torino, Italy
⁴ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁵ NASA Ames Research Center, Moffett Field, CA 94035, USA
⁶ Research Institute for Advanced Computer Science, Universities Space Research Association, Washington, DC 20024, USA
⁷ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudzia̧dzka 5, 87-100 Toruń, Poland
⁸ Astrobiology Center of NINS, 2-21-1, Osawa, Mitaka, Tokyo, 181-8588, Japan
⁹ National Astronomical Observatory of Japan, 2-21-2, Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁰ Institute for Astronomy, University of Hawai‘i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
¹¹ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
¹² SETI Institute, Mountain View, CA 94043, USA
¹³ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
¹⁴ NASA Exoplanet Science Institute-Caltech/IPAC, Pasadena, CA 91125, USA
¹⁵ McDonald Observatory, The University of Texas, Austin, TX, USA
¹⁶ Center for Planetary Systems Habitability, The University of Texas, Austin, TX, USA
¹⁷ Instituto de Astrofísica de Canarias (IAC), C. Vía Láctea S/N, 38205 La Laguna, Tenerife, Spain
¹⁸ Universidad de La Laguna (ULL), Dept. de Astrofísica, 38206 La Laguna, Tenerife, Spain
¹⁹ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
²⁰ Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 118, 22100 Lund, Sweden
²¹ Astronomical Science Program, Graduate University for Advanced Studies, SOKENDAI, 2-21-1, Osawa, Mitaka, Tokyo, 181-8588, Japan
²² Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
²³ Chalmers University of Technology, Department of Space, Earth and Environment, Onsala Space Observatory, 439 92 Onsala, Sweden
²⁴ Astronomy Department and Van Vleck Observatory, Wesleyan University, Middletown, CT 06459, USA
²⁵ Planetary Discoveries, Valencia CA 91354, USA
²⁶ Department of Physics, Engineering and Astronomy, Stephen F. Austin State University, 1936 North St, Nacogdoches, TX 75962, USA

^★★ Corresponding author; jan.subjak@cfa.harvard.edu

Received: 10 June 2024
Accepted: 14 November 2024

Abstract

We report on the discovery and spectroscopic confirmation of TOI-2458 b, a transiting mini-Neptune around an F-type star leaving the main-sequence with a mass of M_★ = 1.05 ± 0.03 M_⊙, a radius of R_★ = 1.31 ± 0.03 R_⊙, an effective temperature of T_eff = 6005 ± 50 K, and a metallicity of −0.10 ± 0.05 dex. By combining TESS photometry with high-resolution spectra acquired with the HARPS spectrograph, we found that the transiting planet has an orbital period of −3.74 days, a mass of M_p = 13.31 ± 0.99 M_⊕ and a radius of R_p = 2.83 ± 0.20 R_⊕. The host star TOI-2458 shows a short activity cycle of ~54 days revealed in the HARPS S-index and Ha times series. We took the opportunity to investigate other F stars showing activity cycle periods comparable to that of TOI-2458 and found that they have shorter rotation periods than would be expected based on the gyrochronology predictions. In addition, we determined TOI-2458’s stellar inclination angle to be i_* = 10.6_−10.6^+13.3 degrees. We discuss that both phenomena (fast stellar rotation and planet orbit inclination) could be explained by in situ formation of a hot Jupiter interior to TOI-2458 b. It is plausible that this hot Jupiter was recently engulfed by the star. Analysis of HARPS spectra has identified the presence of another planet with a period of P = 16.55 ± 0.06 days and a minimum mass of M_p sin i = 10.22 ± 1.90 M_⊕. Using dynamical stability analysis, we constrained the mass of this planet to the range M_c ≃ (10, 25) M_⊕.