1997A&A...326..741S

We have obtained high resolution, high S/N spectra of the He I D3 line (5876 Å) for 53 stars. Combining these data with previous measurements, we investigate correlations between the flux absorbed by D3, F_D3, rotation and other stellar activity indicators for a set of 76 G and K dwarfs. We find that F_D3∝P_rot^–1.2 for P_rot≥4days. For P_rot<4days, F_D3 behaviour depends on spectral type, either remaining roughly constant (G stars), decreasing (K stars), or even going into emission (a few late K stars). We study correlations between D3 and chromospheric (Ca II HK), transition region (C IV 1550Å) and coronal emission, and find, for P_rot>4days, F_D3∝{DELTA}F_HK^1.5, F_D3∝F_CIV^0.7, and F_D3∝F_X^0.6, respectively. Thus, D3 has a response intermediate between Ca II HK and C IV in low to moderate activity stars, consistent with its formation in the upper chromosphere. Our data suggest that the maximum flux absorbed by D3 is F_D3∝T_eff^9.3≃2x10⁵erg/cm²/s in G stars (equivalent widths W_λ≤80mÅ), and about 40% of that value (or W_λ≤50mÅ) in K stars, less than predicted by current theoretical models. We discuss the implications of our results for stellar activity and He I line formation, and suggest future avenues of study.