DOPPLER SIGNATURES OF THE ATMOSPHERIC CIRCULATION ON HOT JUPITERS

Abstract

The meteorology of hot Jupiters has been characterized primarily with thermal\nmeasurements, but recent observations suggest the possibility of directly\ndetecting the winds by observing the Doppler shift of spectral lines seen\nduring transit. Motivated by these observations, we show how Doppler\nmeasurements can place powerful constraints on the meteorology. We show that\nthe atmospheric circulation--and Doppler signature--of hot Jupiters splits into\ntwo regimes. Under weak stellar insolation, the day-night thermal forcing\ngenerates fast zonal jet streams from the interaction of atmospheric waves with\nthe mean flow. In this regime, air along the terminator (as seen during\ntransit) flows toward Earth in some regions and away from Earth in others,\nleading to a Doppler signature exhibiting superposed blueshifted and redshifted\ncomponents. Under intense stellar insolation, however, the strong thermal\nforcing damps these planetary-scale waves, inhibiting their ability to generate\njets. Strong frictional drag likewise damps these waves and inhibits jet\nformation. As a result, this second regime exhibits a circulation dominated by\nhigh-altitude, day-to-night airflow, leading to a predominantly blueshifted\nDoppler signature during transit. We present state-of-the-art circulation\nmodels including nongray radiative transfer to quantify this regime shift and\nthe resulting Doppler signatures; these models suggest that cool planets like\nGJ 436b lie in the first regime, HD 189733b is transitional, while planets\nhotter than HD 209458b lie in the second regime. Moreover, we show how the\namplitude of the Doppler shifts constrains the strength of frictional drag in\nthe upper atmospheres of hot Jupiters. If due to winds, the ~2-km/sec blueshift\ninferred on HD 209458b may require drag time constants as short as 10^4-10^6\nseconds, possibly the result of Lorentz-force braking on this planet's hot\ndayside.\n