Contribution of the intrinsic (left), Doppler (centre) and magnetic (right) terms to the second-order moment about their centre of the profiles of the Fe I lines analysed in the HARPS-N spectrum of BD+39 4435. The slopes of the dashed-dotted orange lines are, respectively, the coefficients a₃, a₁ and a₂ of the least-squares fit of the measured values of R_I⁽²⁾(λ_I) by a function of the form given in Eq. (5). The sky blue dashed line in the centre panel represents the minimum contribution of the instrumental profile and of thermal Doppler broadening to the observed line width (see text for details). In BD+39 4435, because the rotational broadening is negligibly small, this line coincides almost exactly with the a₁ slope line, from which it cannot be readily distinguished. The two lines are clearly separated from each other in stars that show significant rotation, as illustrated in Figs. 8 and 9.