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Arxiv: An absolute mass, precise age, and hints of planetary winds for WASP-121 A and b from a JWST NIRSpec phase curve Published: 1/7/2025 4:53:55 PM Updated: 1/7/2025 4:53:55 PM

Paper abstract: We have conducted a planetary radial velocity measurement of the ultra-hotJupiter WASP-121b using JWST NIRSpec phase curve data. Our analysis reveals theDoppler shift of the planetary spectral lines across the full orbit, whichshifts considerably across the detector (~ 10 pixels). Usingcross-correlation techniques, we have determined an overall planetary velocityamplitude of K_{\rm p}=215.7\pm1.1 km/s, which is in good agreement with theexpected value. We have also calculated the dynamical mass for both componentsof the system by treating it as an eclipsing double-line spectroscopic binary,with WASP-121A having a mass of M_{*}=1.330 \pm 0.019 M_{\odot},while WASP-121b has a mass of M_{\rm p}= 1.170 \pm 0.043 M_{\rm Jup}.These dynamical measurements are ~3\times more precise than previousestimates and do not rely on any stellar modeling assumptions which have a~5\% systematic floor mass uncertainty. Additionally, we used stellarevolution modeling constrained with a stellar density and parallax measurementto determine a precise age for the system, found to be 1.11 \pm 0.14 Gyr.Finally, we observed potential velocity differences between the two NIRSpecdetectors, with NRS1 lower by 5.5\pm2.2 km/s. We suggest that differences canarise from day/night asymmetries in the thermal emission, which can lead to asensitivity bias favoring the illuminated side of the planet, with planetaryrotation and winds both acting to lower a measured K_{\rm P}. The planet'srotation can account for 1 km/s of the observed velocity difference, with4.5\pm2.2 km/s potentially attributable to vertical differences in windspeeds.