Hydrodynamic simulations of the colliding winds in Iota Orionis

Abstract

Two-dimensional hydrodynamic simulations of the colliding winds in the eccentric binary Iota Orionis (HR 1889; HD 37043) have been conducted. With the inclusion of radiative driving, the realistic simulation of such a system becomes possible for the first time. The dynamics of the post-shock flow throughout the orbit are explored. Radiative inhibition and sudden radiative braking both occur, lowering the temperature of the post-shock gas. Instabilities in the collision region are ubiquitous, leading to a great deal of structure. Two separate models with different stellar mass-loss rates are examined. In both models the colliding wind shock collapses on to the photosphere of the secondary around periastron, owing to the imbalance between the wind momentum fluxes. However, the shock is able to detach from the surface of the secondary in the less extreme model as the secondary star heads towards apastron. A higher resolution simulation indicates that this result is currently resolution-dependent. The synthetic intrinsic X-ray emission is extremely dependent on the amount of cooling in the post-shock flow, and hence its nature changes substantially if the shock detaches. In such a case it is very soft at periastron, but much harder at apastron. During the former, the secondary star penetrates deep into the wind acceleration region of the primary, and the pre-shock velocity is reduced from ∼2000 to ∼1000 km s−1. The post-shock density also substantially increases, resulting in very strong cooling. In comparison, at apastron the post-shock density is low, and the pre-shock velocity is high, resulting in a very adiabatic wind collision. Synthetic X-ray light curves show a minimum in the 0.4–10.0 keV ASCA band centred on periastron with a duration of a couple of days. If the shock detaches, a reduction in the 0.1–0.5 keV ROSAT emission is also predicted. Such variation, if seen in ‘real’ data, may help in accurately determining the mass-loss rates of the stellar components.