Confirmation of an He I evaporating atmosphere around the 650-Myr-old sub-Neptune HD 235088 b (TOI-1430 b) with CARMENES

¹ Instituto de Astrofísica de Canarias (IAC), C/ Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
e-mail: jom@iac.es
² Departamento de Astrofísica, Universidad de La Laguna (ULL), Avd. Astrofísico Francisco Sánchez s/n, 38206 La Laguna, Tenerife, Spain
³ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁴ Centro de Astrobiología (CSIC-INTA), ESAC, Camino Bajo del Castillo s/n, Villanueva de la Cañada, 28692 Madrid, Spain
⁵ Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁶ Institut für Astrophysik und Geophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁷ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁸ Department of Space, Earth and Environment, Chalmers University of Technology, 412 96 Gothenburg, Sweden
⁹ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
¹⁰ Centro Astronómico Hispano en Andalucía, Observatorio de Calar Alto, Sierra de los Filabres, 04550 Gérgal, Almería, Spain
¹¹ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181–8588, Japan
¹² Max-Planck-Institute für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹³ Department of Multi-Disciplinary Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153–8902, Japan
¹⁴ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
¹⁵ Universitäts-Sternwarte, Ludwig-Maximilians-Universität München, Scheinerstrasse 1, -81679 München, Germany
¹⁶ Exzellenzcluster Origins, Boltzmannstrasse 2, 85748 Garching, Germany
¹⁷ Departamento de Física de la Tierra y Astrofísica & IPARCOSUCM (Instituto de Física de Partículas y del Cosmos de la UCM), Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁸ Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
¹⁹ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
²⁰ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
²¹ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
²² Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
²³ Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China

Received: 17 March 2023
Accepted: 20 June 2023

Abstract

HD 235088 (TOI-1430) is a young star known to host a sub-Neptune-sized planet candidate. We validated the planetary nature of HD 235088 b with multiband photometry, refined its planetary parameters, and obtained a new age estimate of the host star, placing it at 600–800 Myr. Previous spectroscopic observations of a single transit detected an excess absorption of He I coincident in time with the planet candidate transit. Here, we confirm the presence of He I in the atmosphere of HD 235088 b with one transit observed with CARMENES. We also detected hints of variability in the strength of the helium signal, with an absorption of −0.91 ± 0.11%, which is slightly deeper (2σ) than the previous measurement. Furthermore, we simulated the He I signal with a spherically symmetric 1D hydrodynamic model, finding that the upper atmosphere of HD 235088 b escapes hydrodynamically with a significant mass loss rate of (1.5−5) × 10¹⁰ g s⁻¹ in a relatively cold outflow, with T = 3125 ±375 K, in the photon-limited escape regime. HD 235088 b (R_p = 2.045 ± 0.075 R_⊕) is the smallest planet found to date with a solid atmospheric detection – not just of He I but any other atom or molecule. This positions it a benchmark planet for further analyses of evolving young sub-Neptune atmospheres.