Optical spectroscopy of XMEGA targets in the Carina Nebula - III. The multiple system Tr 16-104 (equivalent to CPD-59 degrees 2603)

We discuss the orbital elements of the multiple system Tr 16-104 which is u sually believed to be a member of the open cluster Trumpler 16 in the Carin a complex. We show that Tr 16-104 could be a hierarchical triple system con sisting of a short-period (2.15 d) eclipsing O7 V + O9.5 V binary bound to a B0.2 IV star. Our preliminary orbital solution of the third body indicate s that the B star most probably describes an eccentric orbit with a period of similar to 285 or similar to 1341 d around the close binary. Folding pho tometric data from the literature with our new ephemerides, we find that th e light curve of the close binary exhibits rather narrow eclipses indicatin g that the two O stars must be well inside their Roche lobes. Our analysis of the photometric data yields a lower limit on the inclination of the orbi t of the close binary of i greater than or equal to 77 degrees. The stellar radii and luminosities of the O7 V and O9.5 V stars are significantly smal ler than expected for stars of this spectral type. Our results suggest that Tr 16-104 lies at a distance of the order of 2.5 kpc and support a fainter absolute magnitude for zero-age main-sequence O stars than usually adopted . We find that the dynamical configuration of Tr 16-104 corresponds to a hi erarchical system that should remain stable provided that it suffers no str ong perturbation. Finally, we also report long-term temporal variations of high-velocity interstellar Ca II absorptions in the line of sight towards T r 16-104.