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

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 (${\mathrm{T}}_{\mathrm{eq}}\le 1000$ 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 ${\mathrm{T}}_{\mathrm{eq}}\sim 900K$, 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 ($S/{\mathrm{N}}_{\mathrm{epoch}}\sim 1500$) yield the clearest detections and constraints on C/O, emphasizing the need for high-precision near-infrared telluric correction with short integration times.

22 pages, 8 figures, accepted to AJ