Figure 1.3. EOS of neutron star matter: BPS (triangles), Haensel & Pichon (1994) (HP94, stars), SLy (dots), OPAL at T = 106, 107, and 108 K (dashed lines), the fit (C.2) (the solid line) and the fit modified at low p (the dotted line). From Haensel & Potekhin (2004).

The model EOSs can be subdivided into the soft, moderate and stiff ones with respect to the compressibility of the matter. Basing on different EOSs, one obtains (Chapter 6) different stellar models, particularly, different maximum masses, from Mmax ~ 1.4 Mq for the softest EOSs to Mmax ~ 2.5 Mq for the stiffest ones. The EOSs can also be subdivided with respect to the composition of the matter (as already outlined in § 1.3.1). Very stiff EOSs can possibly be attributed only to nucleon matter.

Figure 1.3 gives an example of the EOS in a neutron star, from the core to the surface. We show a moderately stiff EOS (SLy, dots, §§ 3.6 and 5.12) of dense nucleon matter derived by Douchin & Haensel (2001) based on a Skyrme-type energy density functional. It is equally valid in the neutron star core and the crust, and is plotted for p > 5 x 1010 g cm-3. At lower densities in the crust, 108 g cm-3 < p < 5 x 1010 g cm-3, we plot the EOS of Haensel & Pichon (1994) (HP94, stars, § 3.2), based on experimental nuclear data. At still lower p we replace it by the EOS of Baym et al. (1971b) (BPS, triangles, § 3.2) for the ground state of the matter at zero temperature. However, at p < 105 g cm-3 the actual EOS becomes temperature dependent as shown by the dashed lines. These lines present the EOS of iron matter for T = 106,107 and 108 K provided by the Opacity Library (OPAL; Rogers et al. 1996; see § 2.4).4 The solid and dotted lines in Fig. 1.3 are analytic interpolations explained in Appendix C.

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