Abstract
We give the first results of a study in F-type MS stars using the HeiD3 absorption feature at λ5876 A as an indicator of stellar activity and structure. The observations were obtained with the QUB echelle spectrograph at the Cassegrain focus of the 4.2 m William Herschel telescope, and have spectral resolution λ/Δλ≈6×104, and signal-to-noise ratios ≈200. We chose as a first sample 17 F-stars of the Hyades Main Sequence. It was found that HeiD3 and X-ray emission appear aroundB-V ∼0.3, indicating the onset of sub-surface convection zones. Below this value, in stars withB-V≤0.42–0.44 the behaviour of the HeiD3 equivalent width against Rossby number is different from that observed in later type stars, indicating that the presence of nonradiative mechanisms other than dynamo must be important in heating the chromospheres of these hotter stars.
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