Astron. Astrophys. 335, L69-L72 (1998)

5. Comparison of observed and computed spectra

In Fig. 3 we compare the observed ISO spectra near minimum and maximum light with the model spectra based on two hydrostatic model atmospheres and two phases of the dynamical model. The model spectra are normalized to the theoretical continuum, the ISO spectra by linear interpolation between the fluxes at 2.4, 2.8, 3.6, 4.2, 10.3 and 12.3 µm and scaling this pseudo-continuum to an overall level comparable to the corresponding models. A different mean continuum level or different reference wavelengths change the depths of the various features typically by 10 %. A larger uncertainty exists beyond 6 µm due to the broad molecular features and the dust emission (see also Hron et al. 1997).

Fig. 3. Comparison of normalized, observed ISO spectra (thick full lines) and model spectra based on hydrostatic atmospheres (dotted lines) and on a dynamical model (thin full lines). The upper panel gives the ISO spectra near phase 0.9, the synthetic spectrum for a similar phase of the dynamical model and a hydrostatic model with =2930 K. The lower panel shows phase 0.4 for the SWS data, the dynamical model, and a hydrostatic model with =2650 K, respectively.

In general, the observed molecular features in the 2-6 µm region are qualitatively reproduced by all models. While the spectral differences of the static models result from large-scale differences in the model structure, the spectral variations of a hydrodynamical model are mainly caused by the local shock fronts propagating through the atmosphere. Thus, comparing the variations of features which originate in different atmospheric layers can help to identify dynamical effects and to separate them from global changes of the structure. In this context we note that the ratio of the intensities of the features at 3.8m (mainly C₂H₂) and 5 µm (mainly C₃) changes from less than one to greater than one for the static models while the ratio is always less than one for the dynamical model. An explanation could be that the C₃ feature forms at slightly higher temperatures than the C₂H₂ feature where the pressure differences between the hydrostatic models are smaller than for the dynamical models (due to the shocks). On the other hand, the variations of the 3 µm and 3.8 µm features, which both involve mainly C₂H₂, are comparable for the static and dynamical models. This indicates that adjusting only the temperature and gravity in hydrostatic models will probably not be sufficient to explain the observed spectra and their variations. We note however that K near maximum light, as estimated from angular diameter measurements (Dyck et al. 1996) fits the SWS spectrum of R Scl reasonably well.

The observed variations of the features shortward of 5 µm are significantly smaller than predicted by the dynamic (and hydrostatic) models and the observed 2.5 µm and 3.8 µm features are also significantly weaker than in the models. This could point to a smaller change in the C₂H₂ partial pressure, i.e. weaker shocks than in the models. The weaker features could also be caused by a lower mean C₂H₂ abundance, although this would increase the difference in the 3 µm band intensity near maximum light. Since the 3 µm feature contains also a significant contribution from HCN and this molecule also contributes to the 6-8 µm region, the differences between observations and models could also be due to uncertainties in the data (abundance, opacity). The stronger change in the 3.8 µm band intensity compared to the 2.5 µm band intensity variation is qualitatively reproduced by the models and can be understood by the contributions of CO (and to a smaller extent also C₂ and CN) to the 2.5 µm feature. The absorption due to the diatomic molecules originates in deeper layers than the C₂H₂ absorption and hence also shows weaker changes with pulsational phase (Sect. 4).

© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998
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