Eclipsing Binary Solutions in Physical Units and Direct Distance Estimation

Abstract

Opportunities for extraction of distances and temperatures from eclipsing binary light curves that are in standard flux units, as opposed to traditional arbitrary units, are examined. Benefits include (1) distance becomes an ordinary solution parameter with a standard error, (2) temperatures of both stars may be derivable in favorable circumstances, and (3) semidetached and overcontact binaries suffer no loss of distance accuracy, vis-a-vis well-detached binaries. Flux calibrations enter only for the observations, while theoretical fluxes are naturally in standard units, so confrontation of theory and observation is direct, and semiempirical quantities based on color-temperature relations are not needed. The monolithic process, called direct distance estimation (DDE), also saves time and effort by avoiding separate distance estimation steps and should lead to routine distance measurements for large numbers of eclipsing binaries (EBs). Discussions compare DDE with traditional EB distance estimation, which has been restricted to well detached binaries in most publications. Aspect dependence of spectroscopic or color temperature, as affected by tides and irradiation, is treated rigorously. A temperature-distance theorem that specifies requirements for finding temperatures and distance from EB light and velocity curves is checked by several kinds of simulations. Demonstration solutions are carried out for the overcontact binary AW UMa and the semidetached binary RZ Cnc, with discussions of temperature and distance results. Although AW UMa has many observational and structural oddities, its DDE and Hipparcos distances agree. Six DDE distances for RZ Cnc, done in several ways and for three photometric bands, fall well within the 1 σ range of its Hipparcos distance. Interactions among calibrative errors, parameter values, and fitting discrepancies are discussed, as is proper weighting.