H2O maser emission in circumstellar envelopes around AGB stars: Physical conditions in gas-dust clouds

Abstract

The pumping of 22.2-GHz H 2 O masers in the circumstellar envelopes of asymptotic giant branch stars has been simulated numerically. The physical parameters adopted in the calculations correspond to those of the circumstellar envelope around IK Tau. The one-dimensional plane-parallel structure of the gas-dust cloud is considered. The statistical equilibrium equations for the H 2 O level populations and the thermal balance equations for the gas-dust cloud are solved self-consistently. The calculations take into account 410 rotational levels belonging to the five lowest vibrational levels of H 2 O. The stellar radiation field is shown to play an important role in the thermal balance of the gas-dust cloud due to the absorption of emission in rotational-vibrational H 2 O lines. The dependence of the gain in the 22.2-GHz maser line on the gas density and H 2 O number density in the gas-dust cloud is investigated. Gas densities close to the mean density of the stellar wind, 10 7 -10 8 cm −3 , and a high relative H 2 O abundance, more than 10 −4 , have been found to be the most likely physical conditions in maser sources.

Model of the vibrationally excited H2O maser at 658 GHz in circumstellar envelopes around asymptotic giant branch stars

“…The gas density in the stellar wind is about NH 2 ∼ 10 8 cm −3 at distances D ∼ 5 stellar radii from the star in a model of the circumstellar envelope of oxygen-rich Mira variable with mass loss rate Ṁ ≈ 7 × 10 −6 M⊙ year −1 (Jeong et al 2003). The gas density and water abundance in the maser clouds can differ substantially from the values calculated in the framework of the isotropic circumstellar wind (Richards et al 2012;Nesterenok 2013a).…”

“…According to the present calculations, gas densities about 10 9 − 10 10 cm −3 and relative H2O abundances 10 −5 − 10 −4 are necessary for the effective pumping of the 658-GHz maser. Such values of the water abundance were considered in the models of 22.2-GHz circumstellar masers (Humphreys et al 2001;Nesterenok 2013a). The gas density in the stellar wind is about NH 2 ∼ 10 8 cm −3 at distances D ∼ 5 stellar radii from the star in a model of the circumstellar envelope of oxygen-rich Mira variable with mass loss rate Ṁ ≈ 7 × 10 −6 M⊙ year −1 (Jeong et al 2003).…”

Modelling CH3OH masers: Sobolev approximation and accelerated lambda iteration method

“…The detailed description of the ALI method for solving the system of master equations for molecule level populations coupled to the radiative transfer equation was given by Rybicki & Hummer (1991). This method was employed in our modelling of the H2O maser pumping in Nesterenok (2013aNesterenok ( ,b, 2015; Nesterenok & Varshalovich (2014). Here, the numerical scheme was modified in order to treat large velocity gradient of the gas in the cloud.…”

H2O maser pumping: The effect of quasi-resonance energy transfer in collisions between H2 and H2O molecules

“…The cloud sizes along two coordinate axes are much larger than that along the third z coordinate axis. As a rule, the model of a flat cloud is considered for modeling the H 2 O maser emission in the envelopes of late-type stars and in star-forming regions (Nesterenok et al 2013;Hollenbach et al 2013).…”

“…In our calculations, we used the algorithms for solving systems of linear equations published in Rybicki and Hummer (1991) and in the book by Press et al (1997). For a description of our calculations, see also Nesterenok (2013).…”