ι Orionis—Evidence for a Capture Origin Binary

Abstract

The star ι Ori (HD 37043, HR 1899) is a 29.1338 day period, double-lined O-type spectroscopic binary with a highly eccentric orbit. The classification of the primary (O9 III) is well established. The secondary, roughly 2 mag fainter, is generally acknowledged as an early-type B star, but claims about its luminosity class have been made only from its estimated light contribution and not from its spectrum. We have collected visual data with the 1 m Multiple-Telescope Telescope (MTT; at the Center for High Angular Resolution at Georgia State University) in four wave bands well distributed in orbital phase, with the periastron passage particularly well covered. We present reconstructed component spectra from our tomographic reconstruction technique. According to three Morgan-Keenan (MK) luminosity criteria, we can unambiguously characterize the secondary as a B1 III-IV giant (MK type or B0.8 III-IV interpolated type). We also have constructed three line-strength indices from eighteen observed individual lines for the secondary and a grid of comparison stars. These results confirm the typing from the MK luminosity criteria. Furthermore, we argue, from the mass ratio (q = 0.5), evolutionary considerations, the orbital scale, and the unremarkable CNO element equivalent widths of the secondary, that this system is not coevolved nor has it undergone significant mass transfer. We postulate an origin for the system in a binary-binary collision and suspect that the runaway stars μ Col and AE Aur may have been the original partners for the current ι Ori secondary and primary, respectively.