Doppler shifted transient sodium detection by KECK/HIRES

Year: 2025 DOI: 10.1093/mnrasl/slaf031
We carried out the first high-resolution transit observations of the exoplanet WASP-49 Ab with Keck/HIRES. Upon custom wavelength calibration we achieve a Doppler RV precision of < 60 m s⁻¹⁠. This is an improvement in RV stability of roughly 240 m s⁻¹ with respect to the instrument standard. We report an average sodium flux residual of ΔF_(NaD)/F*(λ) ~3.2 ± 0.4 per cent (8.0σ⁠) comparable to previous studies. Interestingly, an average Doppler shift of −6.2 ± 0.5 km s⁻¹ (12.4σ⁠) is identified offset from the exoplanet rest frame. The velocity residuals in time trace a blueshift (vΓ,ingress ~ −10.3 ± 1.9 km s⁻¹⁠⁠) to redshift (vΓ,egress ~ + 4.1 ± 1.5 ⁠km s⁻¹) suggesting the origin of the observed sodium is unlikely from the atmosphere of the planet. The average Na light curves indicate a depth of ΔF_(NaD)/F*(t) ~0.47 ± 0.04 per cent (11.7σ⁠) enduring ≤ 90 min with a half-max duration of ~40.1 min. Frequent high-resolution spectroscopic observations will be able to characterize the periodicity of the observed Doppler shifts. Considering the origin of the transient sodium gas is of unknown geometry, a co-orbiting natural satellite may be a likely source.