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Figure 2.9. Comparison of the adiabatic gradient isotherms (lg T [K] = 5.0 and 5.5) for the three EOSs shown in Fig. 2.8 and the OPAL data (Rogers etal., 1996), obtained using the physical picture of the plasma.

(i.e., at low densities), the model of Potekhin (1996b) is in good agreement with the present one. The differences from the predictions of Saumon et al. (1995) are somewhat larger. In all the models, the isotherms "wiggle" in the region of consecutive pressure destruction of excited atomic states. However, such wiggles are absent in the OPAL data (Rogers et al., 1996), based on the physical picture of the plasma and also shown in Fig. 2.9. This clearly shows that second-order thermodynamic quantities are more sensitive to the details of a thermodynamic model than first-order ones.

The example of the hydrogen EOS described here is rather simple. More elaborated theoretical models, as well as numerical Monte Carlo simulations (e.g., Bezkrovniy et al., 2004, and references therein) are available in the literature. Nevertheless, the relative simplicity of the described model makes it suitable for astrophysical modeling, and for including the effects of quantizing magnetic fields, which will be considered in Chapter 4.

Chapter 3

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