Towards early-type eclipsing binaries as extragalactic milestones: III. Physical properties of the O-type eclipsing binary OGLE LMC-ECL-21568 in a quadruple system

Abstract

We present the results from a complex study of an eclipsing O-type binary (Aa+Ab) with the orbital period $P_A=3.2254367$ days, that forms part of a higher-order multiple system in a configuration (A+B)+C. We derived masses of the Aa+Ab binary $M_1=19.02\pm 0.12M_{\odot }$, $M_2=17.50\pm 0.13M_{\odot }$, radii $R_1=7.70\pm 0.05R_{\odot }$, $R_2=6.64\pm 0.06R_{\odot }$, and temperatures $T_1=34250\pm 500$ K, $T_2=33750\pm 500$ K. From the analysis of radial velocities, we found a spectroscopic orbit of A in the outer A+B system with $P_{A+B}=195.8$ days ($P_{A+B}/P_A\approx 61$). In the O-C analysis, we confirmed this orbit and found another component orbiting the A+B system with $P_{AB+C}=2550$ days ($P_{AB+C}/P_{A+B}\approx 13$). From the total mass of the inner binary and its outer orbit, we estimated the mass of the third object, $M_B\gtrsim 10.7M_{\odot }$. From the light-travel time effect fit to the O-C data, we obtained the limit for the mass of the fourth component, $M_C\gtrsim 7.3M_{\odot }$. These extra components contribute to about 20% to 30% (increasing with wavelength) of the total system light. From the comparison of model spectra with the multiband photometry, we derived a distance modulus of 18.59 $\pm$ 0.06 mag, a reddening of 0.16 $\pm$ 0.02 mag, and an $R_V$ of $3.2$. This work is part of our ongoing project, which aims to calibrate the surface brightness-color relation for early-type stars.

15 pages, 14 figures, 3 tables, accepted for publication in The Astrophysical Journal