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Astron. Astrophys. 345, 605-610 (1999)

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1. Introduction

The thermal balance of the gas in circumstellar envelopes of red giants has been studied extensively, both theoretically and observationally. In an early study of OH/IR stars Goldreich & Scoville (1976) concluded that the gas is predominantly heated by friction between radiatively driven dust and gas and cooled by molecular line radiation, chiefly from H2O and CO. In the case of supergiants the situation is complicated by the fact that the mass loss rate is usually a few orders of magnitude smaller than for OH/IR stars and by the presence of a chromosphere which can lead to photoionization and dissociation of atoms and molecules close to the photosphere.

A detailed model of [FORMULA] Ori has been proposed by Rodgers & Glassgold (1991, hereafter RG) who calculated the gas kinetic temperature from the energy balance equation, taking into account heating via gas-grain collision, photoelectric heating by dust grains in the outer envelope and radiative cooling via atomic fine-structure lines and molecular transitions. They found that the latter are unimportant in the circumstellar envelope of [FORMULA] Ori because of the low abundances of CO and H2O. The main cooling line is the [O I] 63 µm line.

Observations of atomic fine-structure lines are hampered by absorption in the earth atmosphere. Using the Kuiper Airborne Observatory (KAO) Haas et al. (1995, hereafter HGT, see also Erikson et al. 1995) observed a number of supergiants in [O I] at 63 and 146 µm and in [Si II] at 35 µm. All stars in their sample were detected in the [O I] line. Only two stars were detected in the [Si II] line, [FORMULA] Sco and [FORMULA] Ori. Recently, detection of atomic fine-structure lines has been reported in the ISO-SWS spectra of AGB stars by Aoki et al. (1998).

In this paper we present ISO-SWS spectra of the supergiants [FORMULA] Ori and [FORMULA] Sco along with results of modelling the spectral energy distributions (SED). Three fine-structure lines are detected in our spectra, identified with transitions of the ions [Fe II] and [Si II]. We investigate where these lines originate using the model of RG.

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© European Southern Observatory (ESO) 1999

Online publication: April 19, 1999