Θ¹ Orionis C - A triple system?

¹ Thüringer Landessternwarte Tautenburg, Karl-Schwarzschild-Observatorium, 07778 Tautenburg, Germany e-mail: lehm@tls-tautenburg.de
² Space Research Institute, Russian Academy of Sciences, 117880 Moscow, Russia e-mail: vitrik@gmail.com
³ Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnij Arkhyz, Zelenchuk region, 357147 Karachai-Cherkesia, Russia e-mail: Vagek@yandex.ru

Received: 4 January 2010
Accepted: 26 January 2010

Abstract

Context. As the brightest star in the Trapezium cluster, Θ¹ 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 Θ¹ 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 Θ¹ 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 Θ¹ 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 5. We assign the additionally found period of 15 4 d to the rotation of the primary, which is in a 1:4 resonance with the close orbit. Derived masses are 31  for the primary, 12  for the distant, and 1  for the close companion.