Young massive binary θ ¹ OriC: Radial velocities of components

Abstract

We succeeded in separating the absorption lines of both the primary C1 and the secondary C2 component in the spectra of the young massive binary θ ¹ OriC (O6Vp + B0V, mass sum 44 ± 7M _⊙), obtained during the period from November 1995 to February 2013 with different telescopes. These observations allowed us to derive, for the first time, the radial velocities of both components. The orbitalmotion of the secondary star is traced through its weak (the line depth is approximately 0.01–0.02) absorption lines of CII, NII, OII, Si III, which are broadened by fast rotation of the star. Silicon absorptions Si III λλ 4553, 4568, and 4575 are better suited for radial velocity measurements than the other lines. From the velocity curves, we obtained the systemic velocity of the system, γ = 31 ±2 kms⁻¹, and semi-amplitudes of the C1 and C2 velocities: K ₁ = 15 ± 2 kms⁻¹, K ₂ = 43 ± 3 kms⁻¹. This leads to individual component masses of M ₁ = 33 ± 5 M _⊙ and M ₂ = 11 ± 5 M _⊙, based on the adopted mass sum. At present, the combined spectroscopic-interferometric orbital solution cannot be obtained because of the large scatter of velocity measurements caused by chaotic line shifts in the spectrum of the primary star and by the weakness of wide absorptions from the secondary. New spectroscopy with a resolution of R ≥ 30000 and S/N ratio over 200 performed in the period close to the periastron passage in the second half of 2013, as well as additional long-baseline interferometry, will be decisive in refining the parameters of θ ¹ OriC. We expect that as a result of this campaign, masses and luminosities of the components will be determined with an accuracy of 2–3%.