Contrast and Temperature Dependence of Multi-Epoch High-Resolution Cross-Correlation Exoplanet Spectroscopy

TL;DR

This study demonstrates that multi-epoch high-resolution cross-correlation spectroscopy can detect and characterize atmospheres of a broader range of exoplanets, especially cooler ones, using existing instruments and techniques.

Contribution

The paper introduces a simulation-based analysis of multi-epoch HRCCS, expanding the detectable exoplanet population and enabling atmospheric characterization, including C/O ratios, in the near-infrared.

Findings

Planets larger than Saturn within 0.2 AU are detectable in the L-band.

Cooler exoplanets show higher spectroscopic contrast due to chemical changes.

Multiple modest S/N epochs improve detection and atmospheric constraints.

Abstract

While high-resolution cross-correlation spectroscopy (HRCCS) techniques have proven effective at characterizing the atmospheres of transiting and non-transiting hot Jupiters, the limitations of these techniques are not well understood. We present a series of simulations of one HRCCS technique, which combines the cross-correlation functions from multiple epochs, to place temperature and contrast limits on the accessible exoplanet population for the first time. We find that planets approximately Saturn-size and larger within $\sim$0.2 AU of a Sun-like star are likely to be detectable with current instrumentation in the $L$-band, a significant expansion compared with the previously-studied population. Cooler ($ \rm T_{eq} \leq 1000$ K) exoplanets are more detectable than suggested by their photometric contrast alone as a result of chemical changes which increase spectroscopic contrast. The $L$-band CH$_4$ spectrum of cooler exoplanets enables robust constraints on the atmospheric C/O ratio at $\rm T_{eq} \sim 900K$, which have proven difficult to obtain for hot Jupiters. These results suggest that the multi-epoch approach to HRCCS can detect and characterize exoplanet atmospheres throughout the inner regions of Sun-like systems with existing high-resolution spectrographs. We find that many epochs of modest signal-to-noise ($\rm S/N_{epoch} \sim 1500$) yield the clearest detections and constraints on C/O, emphasizing the need for high-precision near-infrared telluric correction with short integration times.