Doppler shifted transient sodium detection by KECK/HIRES

Abstract

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 ms−1 . This is an improvement in RV stability of roughly 240 ms−1 with respect to the instrument standard. We report an average sodium flux residual of ΔFNaD/F⋆(λ)∼ 3.2 ± 0.4 per cent (8.0 σ ) comparable to previous studies. Interestingly, an average Doppler shift of ‑6.2 ± 0.5 kms−1 (12.4 σ ) is identified offset from the exoplanet rest frame. The velocity residuals in time trace a blueshift (v Γ,ingress∼ ‑10.3 ± 1.9 kms−1 ) to redshift (v Γ,egress∼ + 4.1 ± 1.5 kms−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 ΔFNaD/F⋆(t)∼ 0.47 ± 0.04 per cent (11.7 σ ) 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.