Hills Model

The program LIGHT2 [Hill (1979), Hill & Rucinski (1993)] is the result of mating Hill's previous modeling program called LIGHT, which combined the Roche model with Wood's (1971, 1972) GauB-Legendre quadrature scheme, and Rucin-ski's WUMA3 (see Sect. 6.3.5) model which was derived from Lucy (1968). It achieves an accurate representation of the system brightness while dealing with horizons and eclipses. The LIGHT2 program has the following characteristics:

• blackbody/semi-empirical hybrid of calculated blackbody, color-index based, and theoretical atmosphere fluxes;

• irradiation computed in sectors and rings [as per Hutchings (1968)];

• differential corrections via CURFIT (Bevington 1969) based on the Marquardt method;

• multiple (< 10), elliptical spot structures; location and AT may be unknowns;

• line profiles are calculated.

The program itself uses a command system of keywords with defaults and is available to potential users. The integration scheme for LIGHT is based on the GauB-Legendre method and that of WUMA3 on GauB-Legendre-Chebyshev quadrature. Improvements by Wade & Rucinski (1985) have involved limb darkening based on Kurucz (1979) stellar atmosphere models , reference to a line profile database for early-type stars, and use of the Simplex algorithm for uniqueness tests.

The line-profile analysis tool is very powerful and has provided much more precise determinations of masses than has hitherto been possible. See Holmgren (1988) for several examples, and Hill et al. (1990) for an excellent demonstration of how the program can uncover the source of system variations (e.g., movement of spots caused by differential rotation or asynchronous rotation) through line profile analysis.

Among unique features is the option to model elliptical spot regions. Although this may well be an improvement over conventional circular spots of all other light curve modeling codes, elliptical spots require more free parameters and thus, according to Sect. 3.4.2, may lead to overparameterization. Semi-empirical stellar atmospheres are used to simulate stellar fluxes. For a more complete review of LIGHT2 and its current capabilities see Hill & Rucinski (1993).

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