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Atmospheric properties of exoplanets can be constrained with transitspectroscopy. The signature of atomic sodium NaI, known to be present above theclouds, is a powerful probe of the upper atmosphere, where it can be bestdetected and characterized at high spectral resolution. Our goal is to obtain ahigh-resolution transit spectrum of HD189733b in the region around theresonance doublet of NaI at 589 nm, to characterize the absorption signaturepreviously detected from space at low resolution. We analyze archival transitdata of HD189733b obtained with the HARPS spectrograph. We retrieve the transitspectrum and light curve of the planet, implementing corrections for telluriccontamination and planetary orbital motion. We spectrally resolve the NaI Ddoublet and measure line contrasts of $0.64\pm0.07\%$ (D2) and $0.40\pm0.07\%$(D1) and FWHMs of $0.52\pm0.08~\AA$. This corresponds to a detection at the10-$\sigma$ level of excess of absorption of $0.32\pm0.03\%$ in a passband of$2\times0.75\ \AA$ centered on each line. We derive temperatures of$2\,600\pm600$ K and $3270\pm330$ K at altitudes of $9\,800\pm2\,800$ km and$12\,700\pm2\,600$ km in the NaI D1 and D2 line cores, respectively. We measurea temperature gradient of $\sim0.2$ K km$^{-1}$ from comparison withtheoretical models. We also detect a blueshift of $0.16\pm0.04\ \AA$ (4$\sigma$) in the line positions. This blueshift may be due to winds blowing at$8\pm2$ km s$^{-1}$ in the upper layers of the atmosphere. We demonstrate therelevance of studying exoplanet atmospheres with high-resolution spectrographsmounted on 4-meter-class telescopes. Our results pave the way towards in-depthcharacterization of physical conditions in the atmospheres of many exoplanetarysystems with future spectrographs such as ESPRESSO on the VLT or HiReS andMETIS on the E-ELT.

Spectrally resolved detection of sodium in the atmosphere of HD189733b with the HARPS spectrograph