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HD 189733 is an exoplanetary system consisting of a transiting hot Jupiterand an active K2V-type main sequence star. We aim to use VLT/UVES highresolution echelle spectra to study a stellar flare. We have performedsimultaneous analyses of the temporal evolution in several chromosphericstellar lines, namely, the Ca II H and K lines, Halpha, Hbeta, Hgamma, Hdelta,Hepsilon, the Ca II infrared triplet line, and He I D3. Observations werecarried out with a time resolution of approximately 1 min for a duration offour hours, including a complete planetary transit. We determine the energyreleased during the flare in all studied chromospheric lines combined to beabout 8.7e31 erg, which puts this event at the upper end of flare energiesobserved on the Sun. Our analysis does not reveal any significant delay of theflare peak observed in the Balmer and Ca II H and K lines, although we find aclear difference in the temporal evolution of these lines. The He I D3 showsadditional absorption possibly related to the flare event. Based on the fluxreleased in Ca II H and K lines during the flare, we estimate the soft X-rayflux emission to be 7e30 erg. The observed flare can be ranked as a moderateflare on a K-type star and confirms a rather high activity level of HD 189733host star. The cores of the studied chromospheric lines demonstrate the samebehavior and let us study the flare evolution. We demonstrate that the activityof an exoplanet host star can play an important role in the detection ofexoplanet atmospheres, since these are frequently discovered as an additionalabsorption in the line cores. A possible star-planet interaction responsiblefor a flare occurrence during a transit can neither be confirmed nor ruled out.

Time-resolved UVES observations of a stellar flare on the planet host HD 189733 during primary transit