Small but mighty: High-resolution spectroscopy of ultra-hot Jupiter atmospheres with compact telescopes

Transmission spectrum of KELT-9 b with Wendelstein’s FOCES spectrograph

¹ Lund Observatory, Division of Astronomy and Theoretical Physics, Lund University, Box 43, 221 00 Lund, Sweden
e-mail: n.borstato@astro.lu.se
² Universitäts-Sternwarte München, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstrasse 1, 81679 München, Germany
³ University of Bern, Center for Space and Habitability, Gesellschaftsstrasse 6, 3012, Bern, Switzerland
⁴ Research Centre for Astronomy, Astrophysics and Astrophotonics, Macquarie University, Balaclava Road, Sydney, NSW 2109, Australia
⁵ ARTORG Center for Biomedical Engineering Research, Murtenstrasse 50, 3008, Bern, Switzerland
⁶ Astronomy & Astrophysics Group, Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁷ Exzellenzcluster Origins, Boltzmannstrase 2, 85748 Garching, Germany

Received: 9 August 2023
Accepted: 20 November 2023

Abstract

When observing transmission spectra produced by the atmospheres of ultra-hot Jupiters (UHJs), large telescopes are typically the instrument of choice given the very weak signal of the planet’s atmopshere. The aim of the present study is to demonstrate that, for favourable targets, smaller telescopes are fully capable of conducting high-resolution cross-correlation spectroscopy. We apply the cross-correlation technique to data from the 2.1 m telescope at the Wendelstein Observatory, using its high-resolution spectrograph FOCES, in order to demonstrate its efficacy in resolving the atmosphere of the UHJ KELT-9 b. Using three nights of observations with the FOCES spectrograph and one with the HARPS-N spectrograph, we conduct a performance comparison between FOCES and HARPS-N. This comparison considers both single-transit and combined observations over the three nights. We then consider the potential of 2 m class telescopes by generalising our results to create a transit emulator capable of evaluating the potential of telescopes of this size. With FOCES, we detected seven species in the atmosphere of KELT-9b: Ti II, Fe I, Fe II, Na I, Mg I, Na II, Cr II, and Sc II. Although HARPS-N surpasses FOCES in performance thanks to the mirror of the TNG, our results reveal that smaller telescope classes are capable of resolving the atmospheres of UHJs given sufficient observing time. This broadens the potential scope of such studies, demonstrating that smaller telescopes can be used to investigate phenomena such as temporal variations in atmospheric signals and the atmospheric loss characteristics of these close-in planets.