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

4. Partial pressures and synthetic spectra

For the comparison with our ISO observations we calculated synthetic opacity sampling spectra in the wavelength range between 2 and 12 µm for the maximum and minimum light phases of the dynamical model as well as for two MARCS models. The molecules CO, CH, C₂, CN, HCN, C₃, and C₂H₂ were included in the model atmosphere calculation as well as in the spectrum computation.

In Fig. 2 we show the pressure-temperature structure of the main spectrum forming region at the maximum and minimum phase of the dynamical model. Although there is a general decrease in the ratio of polyatomic to diatomic molecules with the increase in temperature inwards a steep increase of the gas pressure as caused by a shock (e.g. at about 2100 K in Fig. 2) can locally invert this trend. For this reason the hydrodynamical models predict for example two distinct peaks in the partial pressure of C₂H₂ (and other polyatomics), as is clearly seen in the figure.

C₂ forms deep in the atmosphere and its partial pressure drops rapidly at K. The bulk of the C₂ band system is in the visual spectral region, but a relatively strong Ballik-Ramsay electronic transition appears around 2.5 µm originating in regions with 2500 K. The formation of C₃ requires a combination of high pressure and relatively low temperature and its partial pressure therefore has a rather narrow peak around 2000 K. Its main spectral feature is the fundamental C-C stretching at 5 µm which forms in atmospheric layers with temperatures around 2000 K. On the other hand, the partial pressure of C₂H₂ has a very broad distribution stretching toward much lower temperatures than C₃. The formation of C₂H₂ in a hydrogen dominated atmosphere is less sensitive to the density than C₃ but more sensitive to temperature. The main features of this molecule (at 2.5, 3, 3.8, 7-8 and around 14 µm) originate from layers with temperatures around 2000 K or cooler. HCN is more abundant than C₂H₂ and C₃ in the deeper layers of the atmosphere, but its partial pressure is an order of magnitude lower than the one of C₂H₂ in the upper layers. The strongest HCN bands are at 3 µm, in the 7 µm region, and around 14 µm. The absorption originates at temperatures between about 2000 and 3000 K. The partial pressure of CO follows the structure of the gas pressure at a level of about 3 orders of magnitude below the gas pressure, throughout the region presented in Fig. 2. The CO bands at 5 and 2.5 µm are blended with several other bands, but its trace can be seen in both cases. The CO features form in regions hotter than about 2500 K.

© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998
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