Θ¹ Orionis C - A triple system?

Abstract

Context. As the brightest star in the Trapezium cluster, Θ 1 Ori C is the youngest and nearest to us among O-stars. It is considered to be a multiple system where the main component is an oblique magnetic rotator. Aims. Here, we aim at explaining the structure of the Θ 1 Ori C system. We check for a new hypothesis about the presence of a third component and try to derive the corresponding orbital solutions and the absolute masses of the components. Methods. We measured new radial velocities (RVs) of Θ 1 Ori C and and combined them with data from literature. For the analysis, we used multiple frequency search and iterative calculations of orbits based on the method of differential corrections, applying successive prewhitening to the data in both methods. Results are compared with those obtained from speckle observations. Results. We detected the impact of the known distant companion in the RVs of the primary and can now calculate a spectroscopic orbital solution that is consistent with the observed astrometric positions. We find evidence that Θ 1 Ori C is at least a triple system consisting of the primary, the known astrometric companion in a wide eccentric orbit with a period of 11 yr, and a second companion in a close eccentric orbit with a period of 61. d 5. We assign the additionally found period of 15. d 4 d to the rotation of the primary, which is in a 1:4 resonance with the close orbit. Derived masses are 31 M for the primary, 12 M for the distant, and 1 M for the close companion.

On the dynamical evolution of the Orion Trapezium

“…The systemic radial velocity (γ) of θ 1 Ori C has been determined by Kraus et al (2009) to be 23.6 km s -1 , by adjusting the radial velocities observed by Stahl et al (2008) for the primary component to those predicted by their computed interferometric orbit. Lehmann et al (2010) obtained γ = 25 ± 4 km s -1 after correcting for the modulations observed in the radial velocity of the primary component caused by the oblique magnetic rotator (semi amplitude ∼ 6 km s -1 , P ∼ 15.3 d) and by a putative closer component (semi amplitude ∼ 17 km s -1 ), P ∼ 61 d). In their recent pa-pers, Balega et al (2014Balega et al ( , 2015 calculated γ = 31.0 ± 2.0 km s -1 and γ = 29.4 ± 0.6 km s -1 , based on their combined interferometric and spectroscopic orbital solution which included the radial velocities of both the primary and the secondary interferometric components near periastron.…”

“…Now, after the first complete 11-year orbit has been covered with both interferometric and radial velocity observations (Balega et al 2014(Balega et al , 2015, precise values for the mass and systemic radial velocity of the binary should be available. However, even with sufficient radial velocity data for the primary component (Stahl et al 2008, Lehmann et al 2010, Grellmann et al 2013) and for the secondary (Balega et al 2014(Balega et al , 2015, the combined astrometric and spectroscopic orbital solution to the data is not satisfactory, as admitted by the latter authors. Indeed, uncertainties in the observational data, especially in the radial velocities of both interferometric components -which could be due to non-photospheric contributions (Stahl et al, 2008) or to additional stellar light sources in the primary (Vitrichenko et al 2011)-as well as from the large projected rotation velocity of the secondary component (Balega et al 2014(Balega et al , 2015, introduce a large dispersion in the observed velocity curve and, hence, in the orbital parameters.…”

“…Component B is really a mini-cluster, composed by at least 5 stars in close proximity (Close et al 2013). The brightest component, star C is known to be at least a double, and probably attended by a third companion (Lehmann et al 2010). Component D is a spectroscopic binary (Plaskett & Pearce 1931).…”

Gravitational Slingshot of Young Massive Stars in Orion

“…This scenario also explains the anomalously large proper motion of θ 1 C (van Altena et al 1988), which will cause it to leave the central region of the cluster within ∼10 5 years. Based on a multi-frequency radial velocity analysis, Lehmann et al (2010) have suggested θ 1 C may actually harbor an additional star of 1.0 ± 0.16 M in a close (a = 0.98 AU; P = 61.5 day), eccentric (e = 0.49) orbit around θ 1 C 1 . If during the proposed scattering event that ejected BN, θ 1 C 2 , or BN came close to this inner region, then one would expect likely ejection of the solar mass star.…”

HERMES: a high-resolution fibre-fed spectrograph for the Mercator telescope

“…Aerts et al 2010), massive (interacting) binaries (e.g. Hensberge et al 2008;Lehmann et al 2010;Blomme 2010), final phases of stellar evolution in (binary) stars (e.g. Van Winckel et al 2009;Jorissen et al 2009), and photospheric abundance determinations as probes to internal nucleosynthesis (e.g.…”