The GAPS Programme at TNG

LVI. Characterisation of the low-density gas giant HAT-P-67 b with GIARPS^★

¹ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
e-mail: daniela.sicilia@inaf.it
² INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁴ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
⁵ INAF – Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, Padova 35122, Italy
⁶ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento, 1, 90134 Palermo, Italy
⁷ Space Research Institute, Austrian Academy of Sciences, Schmiedl-strasse 6, 8042 Graz, Austria
⁸ Dipartimento di Fisica e Astronomia “Galileo Galilei” – Università degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁹ University of Palermo, Department of Physics and Chemistry “Emilio Segrè”, Via Archirafi 36, Palermo, Italy
¹⁰ Department of Physics, University of Rome “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹² INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
¹³ Fundación Galileo Galilei-INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain

Received: 28 December 2023
Accepted: 2 April 2024

Abstract

Context. HAT-P-67 b is one of the lowest-density gas giants known to date, making it an excellent target for atmospheric characterisation through the transmission spectroscopy technique.

Aims. In the framework of the GAPS large programme, we collected four transit events of HAT-P-67 b with the aim of studying the exoplanet atmosphere and deriving the orbital projected obliquity.

Methods. We exploited the high-precision GIARPS (GIANO-B + HARPS-N) observing mode of the Telescopio Nazionale Galileo (TNG) along with additional archival TESS photometry to explore the activity level of the host star. We performed transmission spec-troscopy, both in the visible (VIS) and in the near-infrared (NIR) wavelength range, and we analysed the Rossiter–McLaughlin (RML) effect when fitting both the radial velocities and the Doppler shadow. Based on the TESS photometry, we redetermined the transit parameters of HAT-P-67 b.

Results. By modelling the RML effect, we derived a sky-projected obliquity of (2.2 ± 0.4)°, indicating an aligned planetary orbit. The chromospheric activity index log R′_HK, the CCF profile, and the variability in the transmission spectrum of the Hα line suggest that the host star shows signatures of stellar activity and/or pulsation. We found no evidence of atomic or molecular species in the optical transmission spectra, with the exception of pseudo-signals corresponding to Cr I, Fe I, Ha, Na I, and Ti I. In the NIR range, we found an absorption signal of the He I triplet of 5.56_−0.30^+0.29% (19.0σ), corresponding to an effective planetary radius of ~3 R_p (where R_p ~ 2 R_J), which extends beyond the planet’s Roche lobe radius.

Conclusions. Owing to the stellar variability and the high uncertainty of the model, we could not confirm the planetary origin of the signals found in the optical transmission spectrum. On the other hand, we were able to confirm previous detections of the infrared He I triplet, providing a 19.0σ detection. Our finding indicates that the planet’s atmosphere is evaporating.