Thick lines: mean of best-fit curves to radial velocity profiles (RVD); dotted lines: measured RVD profiles of samples SII and SI, obtained with the help of the minimization procedure of Sect. 3.1.3 assuming d = 8 and M = 2.4 × 10¹¹  M_⊙. (The curve’s width equals twice the standard deviation from the mean.) The light gray region is the uncertainty in the RVD profile corresponding to vertical bars in Fig. 3. Here, r_s is the left bound for the fitting region, while r_a and r_b determine the triangular support ABC in Fig. 5.