EDP Sciences
Free access
Volume 492, Number 2, December III 2008
Page(s) 475 - 480
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:200810019
Published online 27 October 2008

A&A 492, 475-480 (2008)
DOI: 10.1051/0004-6361:200810019

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

E. E. Lekht1, N. A. Silant'ev1, G. M. Rudnitskij2, and G. A. Alexeeva3

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

The results of analyzing asymmetry in the H2O maser lines at $\lambda$ = 1.35 cm are presented. We investigated in detail the known mechanism of the asymmetry origin due to the hyperfine structure of the 616 $\to$ 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 $u_{\rm D}$). A noticeable asymmetry corresponds to $u_{\rm D}\le$ 0.5-1 km s-1. For higher values of $u_{\rm D}$ 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 $u_{\rm D}\le$ 0.5 km s-1, if there are very narrow linewidths ($\Delta u$$\approx$ 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.

Key words: line: profiles -- masers -- radiative transfer

© ESO 2008