Highlights on eclipsing binary variables from Araucaria Project

Abstract

The Araucaria Project, which main goal is to provide precise determination of the cosmic distance scale, has recently made a set of discoveries involving variable stars in binary systems. Among these discoveries we highlight three: 1% precise measurement of a Cepheid's dynamical mass and its projection factor, accurate determination of both stellar and orbital parameters of eclipsing binary consisting of two Cepheid variables, and discovery of new class of variable stars, mimicking RR Lyrae pulsators.Key words. stars: Cepheids -stars: pulsation -stars: eclipsing binaries -stars: distances -stars: oscillations The Araucaria Project and the recent resultsThe Araucaria project is a long-term observational program which aims to provide an improved local calibration of the extragalactic distance scale. An application of a number of different stellar standard candles (Cepheid variables, RR Lyrae stars, red clump giants, and blue supergiants) allows to independently determine distances to nearby galaxies. Furthermore, the distances obtained with different methods i.e. using different stellar candles, are compared to trace down the dependencies on environmental properties of the various standard candles. Standard candles in eclipsing binary systems are of particular interest, as they provide both orbital and stellar parameters of high accuracy, which may have an impact upon the theory of stellar pulsation and evolution. The following is the summary of selected results recently obtained by the Araucaria project, specifically focused on pulsating variables in binary systems.The two ways to determine the Cepheid mass -from stellar pulsation theory and from stellar evolution theory -yield the results different by 30% (the Cepheid mass discrepancy problem [1]). The accuracy of previous efforts to establish a dynamical Cepheid mass from single-lined non-eclipsing binaries was typically about 15 − 30%, which was not good enough to resolve the mass discrepancy problem [2]. The discovery of an eclipsing binary system OGLE-LMC-CEP0227 [3] consisting of a classical Cepheid pulsating with a period of 3.8 days together with a stable red giant in a 310-day orbit allowed to determine the mass of the pulsator to 4.165 M with a precision of 1% [4]. This determination agrees with its pulsation mass, providing strong evidence that pulsation theory correctly and precisely predicts the masses of classical Cepheids. For the first time a direct, geometrical and distance-independent determination of the Cepheid projection factor needed for Baade-Wesselinktype analyses was possible. The p-factor value p = 1.21 ± 0.03 (stat.) ± 0.04 (syst.) is consistent with theoretical expectations for a short period Cepheid and interferometric measurements for δ Cep [5]. a