Study of eclipsing binary and multiple systems in OB associations. I. Orion OB1a-IM Monocerotis

Abstract

All available photometric and spectroscopic observations were collected and used as the basis of a detailed analysis of the close binary IM Mon. The orbital period of the binary was refined to 1 $.\!\!\!^{ m d}$19024249 (0.00000014). The Roche equipotentials, fractional luminosities (in $B$, $V$, and Hp-bands) and fractional radii for the component stars in addition to the mass ratio, $q$, inclination, $i$, of the orbit, and the effective temperature, $T_{ m eff}$, of the secondary cooler less massive component were obtained by the analysis of light curves. IM Mon is classified to be a detached binary system in contrast to the contact configuration estimations in the literature. The absolute parameters of IM Mon were derived by the simultaneous solutions of light and radial-velocity curves as $M_{1,2}$ $=$ 5.50 (0.24)$\ M_{\odot}$ and 3.32 (0.16)$\ M_{\odot}$, $R_{1,2}$ $=$ 3.15 (0.04)$\ R_{\odot}$ and 2.36 (0.03)$\ R_{\odot}$, $T_{{ m eff} 1,2}$ $=$ 17500 (350) K and 14500 (550) K implying spectral types of B4 and B6.5 ZAMS stars for the primary and secondary components, respectively. The modelling of the high-resolution spectrum revealed the rotational velocities of the component stars as $V_{{ m rot}1}$ $=$ 147 (15) km s$^{-1}$ and $V_{{ m rot}2}$ $=$ 90 (25) km s$^{-1}$. The photometric distance of 353 (59) pc was found to be more precise and reliable than the Hipparcos distance of 341 (85) pc. An evolutionary age of 11.5 (1.5) Myr was obtained for IM Mon. Kinematical and dynamical analyses support the membership of the young thin-disk population system IM Mon to the Ori OB1a association dynamically. Finally, we derived the distance, age, and metallicity information of Ori OB1a sub-group using information of the IM Mon parameters.