Exoplanet detection limits using spectral cross-correlation with spectro-imaging

Abstract

Context. The combination of high-contrast imaging and medium- to high-resolution spectroscopy offers new possibilities for the detection and characterization of exoplanets. The so-called molecular mapping technique makes use of the difference between the planetary and stellar spectra. While traditional post-processing techniques for high-contrast imaging are quickly limited by speckle noise at short angular separations, we find that molecular mapping efficiently suppresses speckles and offers new detection possibilities. Aims. The molecular mapping performance depends on multiple parameters such as the star magnitude, adaptive optics residual halo, companion spectrum, and telluric absorption, as well as the telescope and instrument properties. Exploring such a parameter space through end-to-end simulations to predict potential science cases and to optimize future instruments designs is very time-consuming and makes it difficult to draw simple conclusions. We propose quantifying the signal of interest and the noise that propagates in molecular mapping, with explicit dependencies upon the main stellar, planetary, and instrument parameters to define an efficient methodology for such an analysis. Methods. We derived explicit expressions of the estimates of molecular mapping signal and noise and validated them through comparisons with end-to-end simulations. These expressions provide an understanding of the instrumental dependencies, aiding in the discussion of optimal instrumental choices with regard to the targets of interest. We applied them to the case of the ELT/HARMONI integral field spectrograph as an online tool predicting the contrast performance in various observational cases. Results. We confirm the potential of molecular mapping for high-contrast detections, especially for cool planets at short separations. We provide guidelines based on quantified estimates for design trade-offs for future instruments. We discuss the planet detection performance of the HARMONI observing modes, with a spectral resolution varying from 3000 to 17 000 and and a range of corresponding spectral bandwidths in the near infrared. While these modes aptly cover the appropriate requirements for high detection capability of warm exoplanets, the high-contrast mode of HARMONI would benefit from a transmission extended down to the J band. A contrast of a few 10−7 at 50 mas should be within reach for bright targets in a photon noise regime with molecular mapping.