A First Look at CRIRES+: Performance Assessment and Exoplanet Spectroscopy

Abstract

High-resolution spectroscopy has proven to be a powerful avenue for atmospheric remote sensing of exoplanets. Recently, ESO commissioned the CRIRES+ high-resolution infrared spectrograph at the Very Large Telescope. CRIRES+ is a cross-dispersed spectrograph with high throughput and wide wavelength coverage across the near-infrared (0.95–5.3 μm), designed to be particularly suited for atmospheric characterization of exoplanets. In this work, we report early insights into the performance of CRIRES+ for exoplanet spectroscopy and conduct a detailed assessment of the data reduction procedure. Because of the novelty of the instrument, we perform two independent data reduction strategies using the official CR2RES pipeline and our new custom-built ExoRES pipeline. Using science verification observations we find that the spectral resolving power of CRIRES+ can reach R ≳ 100,000 for optimal observing conditions. Similarly, we find the signal-to-noise ratio (S/N) to be consistent with expected and empirical estimates for the observations considered. As a case study, we perform the first application of CRIRES+ to the atmospheric characterization of an exoplanet—the ultrahot Jupiter MASCARA-1 b. We detect CO and H2O in the atmosphere of MASCARA-1 b at a S/N of 12.9 and 5.3, respectively, and a temperature inversion revealed through the CO and H2O emission lines, the first for an exoplanet. We find a combined S/N of 13.8 for CO and H2O together, with a preference for lower H2O abundance compared to CO. Our findings demonstrate the scientific potential of CRIRES+ and highlight the excellent opportunity for high-resolution atmospheric spectroscopy of diverse exoplanets.

Robustness measures for molecular detections using high-resolution transmission spectroscopy of exoplanets

“…to do this in several past studies (de Kok et al 2013;Giacobbe et al 2021;Holmberg & Madhusudhan 2022;van Sluijs et al 2022). PCA finds and removes the common modes in the time-variation of each wavelength channel.…”

“…When optimising the number of PCA iterations, we aim to maximise the recovery of the planetary signal itself, rather than select an optimum iteration based on its amplification of noise into a more significant but biased detection. A less commonly used approach involves selecting the number of PCA iterations by optimising the S/N from a noise-subtracted CCF (Spring et al 2022;Holmberg & Madhusudhan 2022). Both CCF obs and CCF inj are found individually for each PCA iteration, as discussed in Section 3.1.…”

“…In Section 2 we introduce the general methodology by which a planetary signal can be extracted from the spectra, using observations of HD 189733 b as a case study. In Section 3 we examine the robustness of different detrending optimisations from across the literature (Birkby et al 2017;Alonso-Floriano et al 2019;Cabot et al 2019;Sánchez-López et al 2019;Giacobbe et al 2021;Spring et al 2022;Holmberg & Madhusudhan 2022). Order weighting and other contributing factors in the determination of the detection S/N are discussed in Section 4.…”

Unifying High- and Low-resolution Observations to Constrain the Dayside Atmosphere of KELT-20b/MASCARA-2b

“…Further observations of KELT-20b could build on this work to determine more precise abundances of these elements as a constraint on the planet's formation. Of particular value would be a measurement of the mass of the planet, broader wavelength space-based spectroscopy from the JWST, and ground-based observations to probe the H 2 O and CO lines at higher spectral resolution (e.g., Line et al 2021;Pelletier et al 2021;Holmberg & Madhusudhan 2022;van Sluijs et al 2022;Yan et al 2022a). In addition, advances in retrieval techniques are needed to ensure that the derived abundances are also accurate.…”

CRIRES+ on sky at the ESO Very Large Telescope

“…Furthermore, the IR is a spectral region where molecular gases, such as CO, CO 2 , NH 3 , CH 4 , H 2 O, among others, produce strong spectral features. Observations of transiting exoplanets with CRIRES + are presented in Leibundgut et al (2022) and Holmberg & Madhusudhan (2022). In addition to the 1.0 to 2.5 µm domain (where several of these molecules have strong transitions and several instruments operate), CRIRES + is one of the very few instruments that covers the 3.0 to 5.2 µm domain at high-spectral resolution.…”