  During primary transits, the spectral signatures of an exoplanet atmosphere
can be measured using transmission spectroscopy. The goal of this work is to
accurately measure the atomspheric sodium absorption light curve in HD189733b,
correcting for the effects of stellar differential limb-darkening, stellar
activity and a "bump" caused by the changing radial velocity of the exoplanet.
In fact, due to the high cadence and quality of our data, it is the first time
that the last feature can be detected even by visual inspection. We use 244
high-resolution optical spectra taken by the UVES instrument mounted at the
VLT. Our observations cover a full transit of HD 189733b, with a cadence of 45
seconds. To probe the transmission spectrum of sodium we produce excess light-
curves integrating the stellar flux in passbands of 1 \AA, 1.5 \AA, and 3 \AA
inside the core of each sodium D-line. We model the effects of external sources
on the excess light-curves, which correspond to an observed stellar flare
beginning close to mid-transit time and the wavelength dependent limb-darkening
effects. In addition, by characterizing the effect of the changing radial
velocity and Doppler shifts of the planetary sodium lines inside the stellar
sodium lines, we estimate the depth and width of the exoplanetary sodium
feature. We estimate the shape of the planetary sodium line by a Gaussian
profile, with an equivalent width of 0.0023 \pm 0.0010 \AA, thereby confirming
the presence of sodium in the atmosphere of HD189733b with excess absorption
levels of 0.72 \pm 0.25%, 0.34 \pm 0.11%, and 0.20 \pm 0.06% for the
integration bands of 1 \AA, 1.5 \AA, and 3 \AA, respectively. From these, we
produce a first order estimate of the number density of sodium in the exoplanet
atmosphere.
