2017A&A...607A..66K

Context. HD 189733 is an exoplanetary system consisting of a transiting hot Jupiter and an active K2V-type main sequence star. Rich manifestations of a stellar activity, like photometric spots or chromospheric flares were repeatedly observed in this system in optical, UV and X-rays.
Aims. We aim to use VLT/UVES high resolution (R=60000) echelle spectra to study a stellar flare.
Methods. We have performed simultaneous analyses of the temporal evolution in several chromospheric stellar lines, namely, the CaII H & K lines (3933, 3968Å), Hα (6563Å), Hβ (4861Å), Hγ (4341Å), Hδ (4102Å), Hε (3970Å), the CaII infrared triplet lines (8498, 8542 and 8662Å), and HeI D3 (5875.6Å). Observations were carried out with a time resolution of approximately 1 min for a duration of four hours, including a complete planetary transit.
Results. We determine the energy released during the flare in all studied chromospheric lines combined to be about 8.7x10³¹erg, which puts this event at the upper end of flare energies observed on the Sun. Our analysis does not reveal any significant delay of the flare peak observed in the Balmer and CaII H & K lines, although we find a clear difference in the temporal evolution of these lines. The HeI D3 shows additional absorption possibly related to the flare event. Based on the flux released in CaII H & K lines during the flare, we estimate the soft X-ray flux emission to be 7x10³⁰erg.
Conclusions. The observed flare can be ranked as a moderate flare on a K-type star and confirms a rather high activity level of HD 189733 host star. The cores of the studied chromospheric lines demonstrate the same behavior and let us study the flare evolution. We demonstrate that the activity of an exoplanet host star can play an important role in the detection of exoplanet atmospheres, since these are frequently discovered as an additional absorption in the line cores. A possible star-planet interaction responsible for a flare occurrence during a transit can neither be confirmed nor ruled out.