High-resolution transmission spectroscopy study of ultra-hot Jupiters HAT-P-57b, KELT-17b, KELT-21b, KELT-7b, MASCARA-1b, and WASP-189b

¹ Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
e-mail: mstangret@iac.es
² Departamento de Astrofísica, Universidad de La Laguna, 38200 San Cristobal de La Laguna, Spain
³ Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
⁴ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁵ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany

Received: 15 July 2021
Accepted: 21 October 2021

Abstract

Ultra-hot jupiters (UHJs) are giant planets on short orbital periods with high equilibrium temperature (T_eq) values. Their hot, extended atmospheres are perfect laboratories for transmission spectroscopy studies based on high-resolution spectrographs. In recent years, a variety of atoms and molecules were found in their atmospheres, using different methods such as cross-correlation or transmission and emission spectroscopy. Here, we present the studies of six ultra-hot Jupiters: HAT-P-57b, KELT-7b, KELT-17b, KELT-21b, MASCARA-1b, and WASP-189b, based on high-resolution observations obtained with HARPS-N and HARPS spectrographs. By applying line and cross-correlation transmission spectroscopy methods, we searched for the absorption features of a broad range of atomic and molecular species. We did not detect any absorption features in our sample of UHJs, with the exception of WASP-189b, for which we detected Fe I, Fe II, and Ti I using cross-correlation. The transmission spectroscopy of single lines for WASP-189b revealed several absorption features (including Hα, Hβ, and Ca H&K), but they remain tentative pending a better modeling of the gravity darkening deformation of the Rossiter-McLaughlin effect. The non-detections with regard to the rest of the planets can be explained via a combination of stellar pulsations and the Rossiter-McLaughlin effect, which mask possible planetary signals for most of the planets, and by the low signal-to-noise ratios of the observations for KELT-21b. Here, we compare our results with the known population of planets for which atmospheric detections have been reported in the literature. We find that the empirical frontier between hot and ultra-hot planets, based on the detection of atomic and ionized species in their atmospheres, can be established as T_eq = 2150 K.