[Paper Review] A First Look at CRIRES+: Performance Assessment and Exoplanet Spectroscopy

Måns Holmberg, Nikku Madhusudhan|arXiv (Cornell University)|Jun 21, 2022

Spectroscopy and Laser Applications95 references32 citations

TL;DR

This paper presents the first on-sky performance assessment of the CRIRES+ high-resolution infrared spectrograph at the VLT, demonstrating its capability for exoplanet atmospheric spectroscopy. Using two independent data reduction pipelines, it achieves R ≳ 100,000 resolving power and detects CO and H2O in the dayside atmosphere of the ultra-hot Jupiter MASCARA-1 b with S/N of 12.9 and 5.3, respectively, revealing a temperature inversion for the first time in an exoplanet.

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 VLT. CRIRES+ is a cross-dispersed spectrograph with high throughput and wide wavelength coverage across the near-infrared (0.95-5.3 $\mu$m), designed to be particularly suited for atmospheric characterisation 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 $R \gtrsim 100,000$ 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 characterisation of an exoplanet - the ultra-hot Jupiter MASCARA-1 b. We detect CO and H$_2$O 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 H$_2$O emission lines, the first for an exoplanet. We find a combined S/N of 13.8 for CO and H$_2$O together, with a preference for lower H$_2$O 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.

Motivation & Objective

• To evaluate the on-sky performance of the newly commissioned CRIRES+ spectrograph for high-resolution exoplanet atmospheric spectroscopy.
• To validate data reduction pipelines by comparing the official ESO CR2RES pipeline with a custom-built ExoRES pipeline.
• To demonstrate the scientific potential of CRIRES+ through a case study of atmospheric characterization of the ultra-hot Jupiter MASCARA-1 b.
• To assess spectral resolution and signal-to-noise ratio (S/N) across diverse observing conditions and targets.
• To investigate the presence of temperature inversions and molecular abundances in the dayside atmosphere of MASCARA-1 b using high-resolution emission spectroscopy.

Proposed method

• Employing two independent data reduction pipelines—ESO’s official CR2RES and a custom-built ExoRES pipeline—for consistent and robust analysis.
• Using science verification data from four exoplanet programs to assess spectral resolution and signal-to-noise ratio (S/N) under varying observing conditions.
• Measuring spectral resolution by fitting high-resolution telluric models to observed spectra, using the stellar FWHM along the slit as a proxy for spectral resolution when adaptive optics perform well.
• Performing cross-correlation analysis with model spectra to detect molecular features of CO and H2O in the thermal emission of MASCARA-1 b.
• Conducting injection tests and robustness checks using singular value decomposition (SVD) for detrending, with noise-insensitive methods to avoid bias in Kp and Vsys estimation.
• Comparing S/N from reduced data with expectations from the CRIRES+ Exposure Time Calculator (ETC) and empirical metrics to validate performance.

Experimental results

Research questions

• RQ1 What is the actual spectral resolving power of CRIRES+ under real observing conditions, and how does it compare to the advertised R = 80,000?
• RQ2 How do the signal-to-noise ratios (S/N) of CRIRES+ observations compare to theoretical expectations and empirical estimates?
• RQ3 Can CRIRES+ detect molecular species such as CO and H2O in the atmosphere of an ultra-hot Jupiter like MASCARA-1 b with high significance?
• RQ4 Does the data reveal a temperature inversion in the dayside atmosphere of MASCARA-1 b, as indicated by emission line profiles of CO and H2O?
• RQ5 Is the H2O abundance in MASCARA-1 b significantly lower than expected under solar composition and thermochemical equilibrium?

Key findings

• CRIRES+ achieves a spectral resolving power of R ≳ 100,000 under optimal observing conditions, exceeding the advertised minimum of R = 80,000.
• The signal-to-noise ratio (S/N) of the data is consistent with both empirical measurements and predictions from the CRIRES+ Exposure Time Calculator (ETC).
• CO is detected in the dayside atmosphere of MASCARA-1 b with a significance of S/N = 12.9, corresponding to a radial velocity semi-amplitude of Kp = 193+11−8 km s−1.
• H2O is detected with S/N = 5.3, corresponding to a systemic velocity of Vsys = -9+24−8 km s−1, indicating a weaker signal compared to CO.
• A temperature inversion is revealed in the atmosphere of MASCARA-1 b through the emission line profiles of CO and H2O, marking the first such detection in an exoplanet.
• The combined detection of CO and H2O yields a higher significance of S/N = 13.8, and model comparison favors a H2O-depleted atmosphere over a solar-abundance model, suggesting potential thermal dissociation.