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Astron. Astrophys. 331, 317-327 (1998)

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1. Introduction

The OH ground state maser lines have been studied extensively since their first detection by Weaver et al. (1965). These maser lines were detected in both interstellar medium and the circumstellar envelope of late type stars. The gross feature of the pumping mechanism of the 1612 MHz maser line is now rather well understood (Elitzur et al. 1978). The absorption of far-infrared (FIR) photons at [FORMULA] and [FORMULA] excites the OH molecules from the [FORMULA] ground state to the [FORMULA] ladder and subsequent cascading downward inverts the populations of the ground state [FORMULA] doubling levels (Fig. 1). But due to the atmospheric absorption in the FIR region, OH-FIR rotational lines are inaccessible from the ground and the pumping scheme could only be checked indirectly using the infrared continuum flux extrapolated to [FORMULA]. It was shown that there are enough FIR photons to pump the 1612 MHz satellite maser (Evans & Beckwith 1977; Nguyen-Q-Rieu et al. 1979; Epchtein et al. 1980). Recently, Skinner et al. (1997) and Sylvester et al. (1997) reported the first direct detection of the [FORMULA] OH absorption lines in the megamaser galaxy Arp 220 and in the circumstellar envelope of the supergiant star IRC+10420, respectively. In the latter source Sylvester et al. (1997) detected several other OH-FIR lines.

[FIGURE] Fig. 1. OH energy level and FIR transition diagram.

Theoretical works on OH masers have so far concentrated either on multi-level models using a simplified treatment (LVG approximation) of the radiative transfer (Elitzur et al. 1978, Bujarrabal et al. 1980) or on an exact solution for the radiative transfer equation in a two-level maser (Alcock & Ross 1985, Spaans & van Langevelde 1992). Because the maser emission depends critically on the pumping rates (or the pumping conditions) throughout the envelope, a correct treatment of both pumping conditions and maser radiative transfer is required. Collison & Nedoluha (1995) made an attempt to improve the treatment of the radiative transfer in the maser transitions.

In this paper we present a detailed modeling of both pumping conditions and maser radiative transfer calculations in the circumstellar envelope. We apply our model to the case of the supergiant star IRC+10420 where OH-FIR lines have been detected by ISO. Maser pumping efficiencies are calculated and compared to those obtained from the observations.

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© European Southern Observatory (ESO) 1998

Online publication: February 4, 1998