Ca II HK emission in rapidly rotating stars. Evidence for an onset of the solar-type dynamo

We present measurements of chromospheric Ca ii H&K activity for 481 solar-like stars. To determine the activity we used the Mount Wilson method and a newly developed method which allows to also measure Ca ii H&K emission features in very rapidly rotating stars. The new technique determines the activity by comparing the line shapes from known inactive slowly rotating template stars that have been artificially broadened to spectra of rapid rotators. We have analyzed solar-like stars ranging from T_eff = 5000 to 7800 K with rotational velocities up to 190 km s^-1 in our sample of FOCES and FEROS spectra. The effects of the rotational broadening on the two methods have been quantified. Our method has proven to produce consistent results where S-Index values are available and offers the possibility to measure the chromospheric activity at the onset of the solar-like dynamo.

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