Astron. Astrophys. 345, 233-243 (1999)

3. Model atmospheres

The model atmospheres used in the analysis are based on the code developed by Tsuji (1965). They are constructed with four assumptions: plane-parallel geometry, hydrostatic equilibrium, local thermodynamical equilibrium (LTE), and radiative equilibrium. The code incorporates the effect of the molecular absorption due to CO, C₂, CN, HCN, and C₂H₂ using the Band Model Method. Detailed discussion on our model atmospheres can be found in Paper I and Ohnaka & Tsuji (1998, hereafter Paper II).

Our model atmospheres are specified by a set of four parameters: effective temperature, surface gravity, micro-turbulent velocity, and chemical composition. The determination of effective temperatures will be described in the next section. The surface gravity and the micro-turbulent velocity are estimated in the same way as in the cases of N- and SC-type carbon stars (Paper I); we set = 0.0 and = 3.0 km s^-1. The chemical composition is assumed to be solar (Anders & Grevesse 1989), except for carbon abundance. Lambert et al. (1986) analyzed the elemental abundances of carbon, nitrogen, and oxygen in four J-type carbon stars ³, and the average of the C/O ratios in the four stars is 1.3. We adopt this value for all our program stars. Table 2 shows the input parameters of the models used in the analysis. Examples of our model atmospheres are shown in Fig. 1, where the models with C/O = 1.1, 1.3, and 2.0 are plotted. The models with C/O = 1.1 and 2.0 are calculated to check the effect of C/O ratio on the resulting ratios.

Table 2. Stellar parameters adopted for the model atmospheres used in the analysis

Fig. 1. Examples of the model atmospheres. The models calculated with C/O = 2.0, 1.3, and 1.1, are shown by the dashed-dotted, solid, and dashed lines, respectively. The other parameters are T = 3000 K, = 0.0, and = 3 km s-1

© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999
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