PHOEBE (PHysics Of Eclipsing BinariEs) by Prsa & Zwitter (2005b) is a modeling package for eclipsing binary stars, built on top of WD program (Wilson & Devinney 1971, Wilson 1979). The introductory paper by Prsa & Zwitter (2005b) overviews most important scientific extensions (incorporating observational spectra of eclipsing binaries into the solution-seeking process, extracting individual temperatures from observed color indices, main sequence constraining and proper treatment of the reddening), numerical innovations (suggested improvements to WD's Differential Corrections method, the new Nelder & Mead's downhill Simplex method), and technical aspects (back-end scripter structure, graphical user interface). While PHOEBE retains 100% WD compatibility, its add-ons are a powerful way to enhance WD by encompassing even more physics and solution reliability. The operability of all these extensions is demonstrated on a synthetic main sequence test binary; applications to real data will be published in follow-up papers. PHOEBE is released under the GNU General Public License, which guaranties it to be free, open to anyone interested to join in on future development.

PHOEBE started as a wrapper, but the authors characterize it now as a standalone EB modeling suite based on the WD model with several physical enhancements among them:

• color indices as indicators of individual temperatures (color constraints);

• spectral energy distribution (SED) as independent data source;

• main sequence constraints; and

• interstellar and atmospheric extinction.

PHOEBE is built in three layers: The lowermost layer is the modeling engine, currently employing WD. On top of it is the extension layer, where all scientific, numerical, and technical extensions are incorporated. The topmost layer is the user interface layer, which serves as a bridge between the user and the model. PHOEBE uses a scripting language especially designed for modeling eclipsing binaries and giving a lot of flexibility to the user. Although the scripter is currently being rewritten into python, we give an example of its syntax:

open_parameter_file ("input.phoebe") mark_for_adjustment (phoebe_incl, 1) set res = minimize_using_dc () print res adopt_minimizer_results (res)

PHOEBE currently uses WD as its back-end, but it can accommodate any physical model instead or in addition to WD. It is written in ANSI C, which makes it fully portable to any platform and any compiler around. Finally, it features a full-fledged graphical user interface displayed in Fig. 8.4, which brings intuitivity and ease of clicking to the EB community.

Fig. 8.4 A screen-shot of the phoebe graphical user interface. Courtesy Andrej Prsa, Villanova University, Villanova, PA

PHOEBE uses a synthetic spectra database to test whether flattened, wavelength-calibrated spectra match synthetic spectra within a given level of significance. Originally, PHOEBE used the Zwitter et al. (2004) grid, but now the internal grid is based on Castelli & Kurucz's (2004) NEWODF SEDs. Accordingly, the scheme to compute limb-darkening coefficients was changed. The computation of stellar spectra of distorted stars is fully supported. The SED analysis is capable to find the values of physical parameters that have not usually been attainable by light-and RV-curve analyses, namely, metallicity and rotational velocity (see Terrell et al. 2003). In Sect. SEDs are used to derive effective temperatures from color indices.

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