Feasibility of High-resolution Transmission Spectroscopy for Low-velocity Exoplanets

Abstract In recent years, high-resolution transmission spectroscopy in the near-infrared has led to detections of prominent molecules in several giant exoplanets on close-in orbits. This approach has traditionally relied on the large Doppler shifts of the planetary spectral lines induced by the high velocities of close-in planets, which were considered necessary for separating them from the quasi-static stellar and telluric lines. In this work, we demonstrate the feasibility of high-resolution transmission spectroscopy for chemical detections in atmospheres of temperate low-mass exoplanets around M dwarfs with low radial velocity variation during transit. We pursue this goal using model injection and recovery tests with H - and K -band high-resolution spectroscopy of the temperate sub-Neptune TOI-732 c, observed using the IGRINS spectrograph on Gemini South. We show that planetary signals in transit may be recovered when the change in the planet’s radial velocity is very small, down to subpixel velocities. This is possible due to the presence of the planetary signal in only a subset of the observed spectra. A sufficient number of out-of-transit spectra can create enough contrast between the planet signal and telluric/stellar contaminants that the planet signal does not constitute a principal component of the time-series spectra and can therefore be isolated using principal-component-analysis-based detrending without relying on a significant Doppler shift. We additionally explore novel metrics for finding such signals, and investigate trends in their detectability. Our work extends the scope of high-resolution transmission spectroscopy and creates a pathway toward the characterization of habitable sub-Neptune worlds with ground-based facilities.