Exoplanet atmospheres at high resolution through a modest-size telescope

Abstract

Ground-based, high-resolution spectrographs are providing us with an unprecedented view of the dynamics and chemistry of the atmospheres of planets outside the Solar System. While there are a large number of stable and precise high-resolution spectrographs on modest-size telescopes, it is the spectrographs at observatories with apertures larger than 3.5 m that dominate the atmospheric follow-up of exoplanets. In this work we explore the potential of characterising exoplanetary atmospheres with FIES, a high-resolution spectrograph at the 2.56 m Nordic Optical Telescope. We observed two transits of MASCARA-2 b (also known as KELT-20 b) and one transit of KELT-9 b to search for atomic iron, a species that has recently been discovered in both neutral and ionised forms in the atmospheres of these ultra-hot Jupiters using large telescopes. Using a cross-correlation method, we detect a signal of Fe II at the 4.5σ and 4.0σ level in the transits of MaSCARA-2 b. We also detect Fe II in the transit of KELT-9 b at the 8.5σ level. Although we do not find any significant Doppler shift in the signal of MASCARA-2 b, we do measure a moderate blueshift (3–6 km s−1) of the feature in KELT-9 b, which might be a manifestation of high-velocity winds transporting Fe II from the planetary dayside to the nightside. Our work demonstrates the feasibility of investigating exoplanet atmospheres with FIES, and it potentially unlocks a wealth of additional atmosphere detections with this and other high-resolution spectrographs mounted on similar-size telescopes.

The GAPS Programme at TNG

“…This provides some confidence that our systemic velocity is accurate within 1 km s −1 . Our systemic velocity is also consistent with values measured on stellar lines by Pai Asnodkar et al ( 2022), Hoeijmakers et al (2019), andBello-Arufe et al (2022), but seems more difficult to reconcile with the value mea-Fig. 8.…”

“…Day-to-night wind flow in KELT-9b was also reported by Pai and Bello-Arufe et al (2022) based on the Doppler shift of Fe ii lines seen in transmission compared to the stellar systemic velocity. While Fe i traces gas motions in the gravitationally bound part of the atmosphere of a planet, Fe ii is likely tracing the dynamics of the evaporating exosphere (Zhang et al 2022).…”

“…At face value, this is surprising, because the two sets of observations likely bridge from the gravitationally bound part of the atmosphere to the exosphere. However, both our analysis and the analyses of Pai and Bello-Arufe et al (2022) only account for Doppler shifts of lines, and yet line shapes are also distorted differently by different wind patterns (Seidel et al 2020). A more detailed analysis that accounts for this effect is therefore necessary in order to properly assess whether or not a transition in the dynamical regime experienced by Fe i and Fe ii takes place.…”

“…It is also important to note that the measurement of systemic velocity from the stellar lines of KELT-9 appears to be challenging. This highlights a potential weakness of transmission spectra as probes of atmospheric dynamics (Bello-Arufe et al 2022) in that they rely on the accuracy of the systemic velocity. On the other hand, our measurement only depends on the relative shift of planetary lines during the orbit, and is therefore independent of the value of the systemic velocity.…”

The stable climate of KELT-9b

“…Lothringer et al 2018;Kitzmann et al 2018;Parmentier et al 2018). Following this, metals are predicted to exist in their atomic form rather than being bound in molecules, a prediction that has been observationally verified for KELT-9b (Hoeijmakers et al 2018Pino et al 2020;Bello-Arufe et al 2022). Due to the high levels of irradiation, UHJs are expected to have extended atmospheres which also lends itself to hydrogen atmospheric escape, a phenomenon also detected for KELT-9b (Yan & Henning 2018;Wyttenbach et al 2020).…”