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

3. Modelling the IR energy distribution

We have modelled the spectral energy distribution (SED) of the two stars. The calculational procedure is described in Haisch (1979) and Justtanont & Tielens (1992). We assume a spherical symmetry with a constant mass loss rate. The grain size distribution used is that for the interstellar grain size distribution (Mathis, Rumpl, Nordsieck 1977). The calculation takes into account the thermal emission and multiple scattering due to silicate grains. Adopted and derived parameters are listed in Table 2.

Table 2. Parameters of SED modelling

The parameters derived for Ori from fitting the SED (Table 2) are quite similar to those derived by Skinner & Whitmore (1987) who also considered the effects of the chromosphere of the star. The values for the stellar radius and effective temperature also agree with interferometric observations by Dyck et al. (1996). The inner radius of the dust shell is taken from Skinner et al. (1997) to be 0".5 rather than 0".9 based on the 11 µm interferometry by Bester et al. (1991). The model fit to the SWS observation, along with the ground based photometry fluxes are shown in Fig. 2. We estimated a dust mass loss rate of 9 M yr^-1 from our model fit. The dust mass loss rate used in RG model is very similar to this. It is an important parameter in calculating the dust-drag heating which is the main heating source in the circumstellar envelope of Ori.

Fig. 2. The fits to SED of Ori (top ) and Sco (bottom ). Note that the spectrum of Sco is shifted down by a factor of 10 in this plot. Crosses are the observed ISO SWS spectra; open circles are photometry from Low et al. (1970) and Epchtein et al. (1980); filled squares are IRAS photometry.

Occultation observation of Sco gives an estimate of the stellar radius of 22.83 mas, i.e., 6.1 cm at a distance of 180 pc (Dyck et al. 1996). Bloemhof & Danen (1995) measured the inner radius of the dust shell to be 0."6, i.e., 1.6 cm at 180 pc. The fit to the SED is shown in Fig. 2 with the model parameters in Table 2. Both supergiants have very similar input parameters. They also have similar derived dust mass loss rates which are very low, as reflected by their weak 10 µm silicate feature.

© European Southern Observatory (ESO) 1999

Online publication: April 19, 1999
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