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

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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 [FORMULA] 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 [FORMULA]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.

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© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998