Water Masers Diagnosing Postshocked Conditions in W49N

We present spectroscopic results of 146 water maser outbursts in W49N, obtained with the Metsähovi radio telescope at 22 GHz. We found the following characteristics: (1) Inside (outside) the velocity range of the dense ambient medium, the increase in flux density during an outburst is typically 104 Jy (103 Jy) and covers 1 order (2.5 orders) of magnitude. (2) The outburst durations closely trace space velocities of maser features. (3) Nonthermal velocity fluctuations produce variations in the line velocity of the maser features, which are comparable to the variations measured in the line width during outbursts. (4) There is no correlation between flux density and line width. Combining these data with Gwinn's VLBI results, notably obtained during the same time period and with the same velocity resolution, we were able to fix the free parameters in the shock model of Hollenbach & McKee and the maser model of Elitzur, Hollenbach, & McKee. This enabled a straightforward determination of some 20 shock and maser parameters including, among others, the following typical values: kinetic temperature 350 K, postshock density (3.6-8.7) × 108 cm-3, water abundance (1-5) × 10-4, water density (0.9-1.9) × 105 cm-3, water column density (2.2-7.9) × 1019 cm-2, preshock field strength 0.8-1.6 mG, and total postshock field strength 80-160 mG. A step-by-step presentation of our diagnostic method is given, and the relation between observations and model parameters is discussed. One uniquely powerful outburst feature during 1981-1983, hereafter referred to as the "big flare feature," showed also the narrowest line width (0.5 km s-1). Observations indicate that the velocity of this feature lies in the plane of the sky, whereas preshock and postshock magnetic fields are directed nearly along the line of sight. Consequently, Alfvénic wave fluctuations along the line of sight, and line width, are minimal, and a very high aspect ratio is achieved. Furthermore, the big flare feature stands out through its low space velocity, higher temperature (480 K), and larger preshock magnetic field strength (8.2 mG). These are naturally explained, if the big flare feature was located closer to the shock front than the other masers.