Temperature-pressure profile of the hot Jupiter HD 189733b from HST sodium observations: Detection of upper atmospheric heating

We present transmission spectra of the hot Jupiter HD 189733b taken with the Space Telescope Imaging Spectrograph (STIS) aboard Hubble Space Telescope (HST). The spectra cover the wavelength range 5808-6380Å with a resolving power of R= 5000. We detect absorption from the Nai doublet within the exoplanet's atmosphere at the 9σ confidence level within a 5Å band (absorption depth 0.09 ± 0.01 per cent) and use the data to measure the doublet's spectral absorption profile. We detect only the narrow cores of the doublet. The narrowness of the feature could be due to an obscuring high-altitude haze of an unknown composition or a significantly sub-solar Nai abundance hiding the line wings beneath an H ₂ Rayleigh signature. These observations are consistent with previous broad-band spectroscopy from Advanced Camera for Surveys (ACS) and STIS, where a featureless spectrum was seen. We also investigate the effects of starspots on the Nai line profile, finding that their impact is minimal and within errors in the sodium feature. We compare the spectral absorption profile over 5.5 scale heights with model spectral absorption profiles and constrain the temperature at different atmospheric regions, allowing us to construct a vertical temperature profile. We identify two temperature regimes: a 1280 ± 240K region derived from the Nai doublet line wings corresponding to altitudes below ~500 km, and a 2800 ± 400K region derived from the Nai doublet line cores corresponding to altitudes from ~500 to 4000 km. The zero altitude is defined by the white-light radius of R _P/R _{{black star}}= 0.15628 ± 0.00009. The temperature rises with altitude, which is likely evidence of a thermosphere. The absolute pressure scale depends on the species responsible for the Rayleigh signature and its abundance. We discuss a plausible scenario for this species, a high-altitude silicate haze and the atmospheric temperature-pressure profile that results. In this case, the high-altitude temperature rise for HD 189733b occurs at pressures of 10 ^-5 to 10 ^-8 bar.