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

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 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 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, Sco and 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 Ori and 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.

© European Southern Observatory (ESO) 1999

Online publication: April 19, 1999
helpdesk.link@springer.de