Extreme winds on the emerging dayside of an ultrahot Jupiter

TL;DR

This study demonstrates that high-resolution, phase-resolved emission spectroscopy with the Keck Planet Finder can directly measure atmospheric winds and thermal structure on an ultrahot Jupiter. By modeling phase-dependent line strength, width, and Doppler shifts, the authors extract a supersonic day-to-night wind component on KELT-9 b, with a maximum speed of $11.7 \pm 0.6$ km s$^{-1}$, and map dayside chemistry including Fe I/II, Ti I/II, Ca I/II, Mg I, and Si I. The phase-resolved retrieval indicates weak atmospheric drag and efficient heat recirculation, consistent with space-based phase curves, and provides constraints on both dayside and nightside thermal profiles. Overall, the work validates high-resolution phase-curve spectroscopy as a powerful tool for benchmarking extreme physics in exoplanetary atmospheres and informs 3D circulation models of UHJs.

Abstract

High-resolution spectroscopy provides a unique opportunity to directly probe atmospheric dynamics by resolving Doppler shifts of planetary signal as a function of orbital phases. Using the optical spectrometer Keck Planet Finder (KPF), we carry out a pilot study on high-resolution phase curve spectra of the ultra-hot Jupiter KELT-9 b. We spectrally and temporally resolve its dayside emission from post-transit to pre-eclipse (orbital phase phi = 0.1 - 0.45). The signal strength and width increase with orbital phases as the dayside rotates into view. The net Doppler shift varies progressively from -13.4 +/- 0.6 to -0.4 +/- 1.0 km/s, the extent of which exceeds its rotation velocity of 6.4 +/- 0.1 km/s, providing unambiguous evidence of atmospheric winds. We devise a retrieval framework to fit the full time-series spectra, accounting for the variation of line profiles due to the rotation and winds. We retrieve a supersonic day-to-night wind speed up to 11.7 +/- 0.6 km/s on the emerging dayside, representing the most extreme atmospheric winds in hot Jupiters to date. Comparison to 3D circulation models reveals a weak atmospheric drag, consistent with relatively efficient heat recirculation as also supported by space-based phase curve measurements. Additionally, we retrieve the dayside chemistry (including Fe i, Fe ii, Ti i, Ti ii, Ca i, Ca ii, Mg i, and Si i) and temperature structure, and place constraints on the nightside thermal profile. Our high-resolution phase curve spectra and the measured supersonic winds provide excellent benchmarks for extreme physics in circulation models, demonstrating the power of this technique in understanding climates of hot Jupiters.

Figures (3)

• Figure 1: Detection of KELT-9 b's emission as a function of orbital phases. Left panel: phase coverage of our KPF observations in three shaded curves. The black bars mark the timing of the transit and secondary eclipse. Middle panel: CCF map of the spectral data from post-transit to pre-eclipse in star's rest frame. The planetary signal shows as the bright yellow trace following a sinusoidal function of phases (white dashed line) due to the orbital motion. Right panel: CCF map shifted to the planet's rest frame assuming a orbital semi-amplitude of $K_\mathrm{p}$=244.8 km s$^{-1}$ (i.e., the inferred $K_\mathrm{p}$ value from retrievals as detailed in Section \ref{['sec:result_dynamics']}). The planetary emission profile shows clear variation with phases.
• Figure 2: Phase dependency of the line width, net Doppler shift, and amplitude of the KELT-9 b's emission signal as extracted from the CCFs shown in Fig. \ref{['fig:ccf_phase']}. The black lines represent the simple harmonic forms adopted in the retrieval analysis (see Eq.\ref{['eq:line_width']} for the FWHM, Eq. \ref{['eq:line_strength']} for the amplitude, and Eq. \ref{['eq:line_doppler']} for the net Doppler shift). The signal amplitude suffers from stellar contamination for $\phi>0.4$ (gray shaded region).
• Figure 3: Illustration of the modeling framework for phase-resolved time-series spectra. With average dayside (thermal inversion; emission lines) and nightside (non-inverted profile; absorption lines) spectra, we apply phase-dependent scaling, broadening, and Doppler shifting to model the full time-series observations.