A study of the asymmetry in the H₂O maser line at λ = 1.35 cm on the base of the hyperfine structure

¹ Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro No. 1, Apartado Postal 51 y 216, CP 72840 Tonantzintla, Puebla, México e-mail: lekht@inaoep.mx
² Sternberg Astronomical Institute, 13 Univetsitetskij prospect, Moscow, 119992, Russia
³ Main (Pulkovo) Astronomical observatory of Russian Academy of Sciences, Pulkovskoe shosse 65, St.-Petersburg, 196140, Russia

Received: 21 April 2008
Accepted: 30 September 2008

Abstract

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 0.5-1 km s^-1. 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 0.5 km s^-1, if there are very narrow linewidths (≈ 0.2-0.4 km s^-1). 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.