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

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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 [FORMULA] = 0.0 and [FORMULA] = 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 [FORMULA] ratios.


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

[FIGURE] 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[FORMULA] = 3000 K, [FORMULA] = 0.0, and [FORMULA] = 3 km s-1

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© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999