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Stellar and circumstellar properties of visual binaries in the Orion Nebula Cluster⋆⋆⋆


Context. Our general understanding of multiple star and planet formation is primarily based on observations of young multiple systems in low density regions like Tau-Aur and Oph. Since many, if not most, of the stars are born in clusters, observational constraints from young binaries in those environments are fundamental for understanding both the formation of multiple systems and planets in multiple systems throughout the Galaxy. Aims. We build upon the largest survey for young binaries in the Orion Nebula Cluster (ONC), which is based on Hubble Space Telescope observations to derive both stellar and circumstellar properties of newborn binary systems in this cluster environment. Methods. We present adaptive optics spatially-resolved JHKL′-band photometry and K-band R ~ 5000 spectra for a sample of eight ONC binary systems from this database. We characterize the stellar properties of binary components and obtain a census of protoplanetary disks through K - L′ color excess. For a combined sample of ONC binaries including 7 additional systems with NIR spectroscopy from the literature, we derive mass ratio and relative age distributions. We compare the stellar and circumstellar properties of binaries in ONC with those in Tau-Aur and Oph from samples of binaries with stellar properties derived for each component from spectra and/or visual photometry and with a disk census obtained through K - L color excess. Results. The mass ratio distribution of ONC binaries is found to be indistinguishable from that of Tau-Aur and, to some extent, to that of Oph in the separation range 85-560 AU and for primary mass in the range 0.15 to 0.8 M ⊙. A trend toward a lower mass ratio with larger separation is suggested in ONC binaries, which is not seen in Tau-Aur binaries. The components of ONC binaries are found to be significantly more coeval than the overall ONC population and as coeval as components of binaries in Tau-Aur and Oph. There is a hint of a larger fraction of mixed pairs, i.e. systems with a disk around only one component, in wide ONC binaries in comparison to wide binaries in Tau-Aur and Oph within the same primary mass range that could be caused by hierarchical triples. The mass ratio distributions of mixed and unmixed pairs in the overall population of T Tauri binaries are shown to be different. Some of these trends require confirmation with observations of a larger sample of binary systems. © ESO, 2013.

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