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Announcement of FreeEOS-1.2.0

This stable release of FreeEOS-1.2.0 corresponds to several years of code development. (FreeEOS-1.0.0 and FreeEOS-1.1.0 have recent file release dates at SourceForge, but they actually correspond to stable code versions dated 2000-07-06 and 2001-10-24.) Download FreeEOS-1.2.0 from here.

Since 2001-10-24 (the code date for version 1.1.0) I have made quite a few changes to the code base. All the change details can be found in the ChangeLog.prior_SF and ChangeLog files in the top-level directory of the tarball. Some changes involve a drastic improvement in the documentation. (See README, NEWS, src/README.developers, utils/README.free_eos_test, and the files in the www directory.) Some changes involve the new build procedure which should work well for most/all Unix platforms. (See the README file in the top-level directory of the tarball for details of how to build FreeEOS now.) Many of the changes involved reorganization of the code without a free-energy model change or adding some special option to explore some experimental change in the free-energy model that is normally not used for the standard EOS1 option suite. Also, there were a number of changes to the free-energy model that would have affected EOS1 option suite results, but all but one of these changes have been temporarily reverted back to the equivalent of the 1.1.0 version so that FreeEOS-1.2.0 represents just one change in standard EOS1 free-energy model (the change to the H2+ pressure ionization formulation described below). In the next few weeks further FreeEOS releases are planned that include the Fermi-Dirac integral approximation improvements that have been written up in Paper I and Coulomb approximation and exchange approximation improvements which are being written up as Papers III and IV in the series of papers that document the FreeEOS implementation.

The change in the pressure-ionization formulation for H2+ is easily understood. FreeEOS now follows the MDH (1988, ApJ 331, 815) idea of treating H2+ using a hard-sphere potential (see the commentary following their equation 8) similarly to the way the pressure-ionization of neutral species is modelled in FreeEOS. Ultimately, this change will provide one additional MDH-style interaction radius parameter to help fit other more detailed EOS results for high densities and low temperatures (i.e., envelope conditions for extreme LMS stars), but for now I have made the H2+ "radius" negligible to minimize the change from FreeEOS-1.1.0. Although that change is small it is, nevertheless, still noticeable. The reason is the occupation probability sum given by MDH equation 8 depends on the sum of the radii of the interacting particles. This means that even for a negligible H2+ radius, the new formulation causes changed results.

This figure shows the effect of the new H2+ pressure-ionization formulation. The 1.1.0 and 1.2.0 calculations being compared have been done using the EOS1 option suite and solar metallicity. The effect of the changed formulation (with negligible H2+ interaction radius, see above discussion) is roughly 1 per cent in the worst case that occurs near log T = 4.5 and log rho = 2.5 (in SI units) at the calculational limit of FreeEOS. These extreme conditions correspond to the envelope of a 0.1 solar-mass star where FreeEOS results (and in fact all EOS results) are uncertain in any case. Note, however, that the relative residuals decrease rapidly for lower densities and/or higher temperatures and become negligible for solar conditions. Thus, the outstanding agreement between FreeEOS and OPAL solar results that has been demonstrated in Figures 6 and 7 of Paper II is essentially unaffected by this change in free-energy formulation.

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Alan W. Irwin
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