2008A&A...492..475L

The results of analyzing asymmetry in the H₂O maser lines at λ=1.35 cm are presented. We investigated in detail the known mechanism of the asymmetry origin due to the hyperfine structure of the 6₁₆-5₂₃ transition and different (not equidistant) positions of the corresponding triplet components. The triplet of the lines is characterized by the sum of extinction factors that are continuously transformed into a single-peaked asymmetric extinction coefficient, with the increase in the Doppler widths (characterizing by effective Doppler velocity u_D). A noticeable asymmetry corresponds to u_D≤0.5-1km/s. For higher values of u_D the extinction factor approaches a Gaussian shape with the decreasing magnitude of the profile asymmetry. An appreciable asymmetry in unsaturated maser lines can only arise at u_D≤0.5km/s, if there are very narrow linewidths (Δu≃0.2-0.4km/s). But observed saturated masers possess a fairly high degree of the line asymmetry, 10-40%. Most probably, this asymmetry is a result of rebroadening of the linewidth up to the value that is comparable to the width of the asymmetric extinction coefficient.