A Revised Geometry for the Magnetic Wind of θ1 Orionis C

Abstract

ABSTRACTThe star θ1 Orionis C (O6–7 V) is often cited as a hot analog of Bp variables, because its optical and UV line and X‐ray continuum fluxes modulate over the magnetic/rotational period. In this circumstance, one expects emission and absorption components of the UV resonance lines to vary as a flattened magnetosphere corotates with the star. In this paper, we reexamine the detailed velocity behavior of several strong UV lines. Whereas past work has focused on variations of the full profiles, we find that the blue and red wings of the C iv and N v resonance lines exhibit anticorrelated modulations. These appear as absorption excesses at large blueshifts and as flux elevations at moderate redshifts at the edge‐on phase ϕ = 0.5. No rest‐frame absorption features, which are the typical signatures of cool, static disks surrounding Bp stars, can be detected at any phase. We suggest that this behavior is caused by two geometrically distinct components of the wind, which are defined by the relationship between the extent of a magnetic loop and the local Alfvén radius. Streams on field lines opening outside this radius are first channeled toward the magnetic equator, but after reaching the Alfven radius, they are forced outward by radiative forces, eventually to become an expanding radial outflow. This wind component causes blueshifted absorption as the corotating magnetic equatorial plane crosses the observer’s line of sight (ϕ = 0.5). The geometry of the inner component requires a more complicated interpretation. Wind streams first follow closed loops and collide at the magnetic equator with counterpart streams from the opposite pole. There they coalesce and fall back to the star along their original field lines. The high temperatures in these falling condensations cause the redshifted emission. The rapid circulation of these flows is likely the reason for the absence of signatures of a cool disk (e.g., zero‐velocity absorptions at ϕ∼0.5) in the strong UV lines.