An investigation of chromospheric activity spanning the Vaughan-Preston gap: impact on stellar ages

Context: Chromospheric activity is widely used as an age indicator for solar-type stars based on the early evidence that there is a smooth evolution from young and active to old and inactive stars. However, this transition may require modification as chromospheric activity is not a viable age indicator for stars older than 1 Gyr. Aims: We analysed chromospheric activity in five solar-type stars in two open clusters, in order to study how chromospheric activity evolves with time. Methods: We took UVES high-resolution, high S/N ratio spectra of 3 stars in IC 4756 and 2 in NGC 5822, which were combined with a previously studied data-set and reanalysed here. The emission core of the deep, photospheric Ca II K line was used as a probe of the chromospheric activity. Results: All of the 5 stars in the new sample, including those in the 1.2 Gyr-old NGC 5822, have activity levels comparable to those of Hyades and Praesepe. Conclusions: A likely interpretation of our data is that solar-type-star chromospheric activity, from the age of the Hyades until that of the Sun, does not evolve smoothly. Stars change from active to inactive, crossing the activity range corresponding to the Vaughan-Preston gap, on a time-scale that might be as short as 200 Myr. Evolution before and after such a transition is much less significant than cyclical and long-term variations. We show that data presented in the literature to support a correlation between age and activity could be also interpreted differently in the light of our results. Suggestions have been published that relevant stellar structures and/or dynamos are different for active and inactive stars. These provide a natural explanation for the observations presented here. More observations are required in order to strengthen our results, especially a long-term follow up of our two targets in the 1.2-Gyr old cluster NGC 5822.