* Astronomy

Title: The prototype colliding-wind pinwheel WR 104
Authors: Peter Tuthill, John Monnier, Nicholas Lawrance, William Danchi, Stan Owocki, Kenneth Gayley

Results from the most extensive study of the time-evolving dust structure around the prototype "Pinwheel" nebula WR 104 are presented. Encompassing 11 epochs in three near-infrared filter bandpasses, a homogeneous imaging data set spanning more than 6 years (or 10 orbits) is presented. Data were obtained from the highly successful Keck Aperture Masking Experiment, which can recover high fidelity images at extremely high angular resolutions, revealing the geometry of the plume with unprecedented precision. Inferred properties for the (unresolved) underlying binary and wind system are orbital period 241.5 ± 0.5 days and angular outflow velocity of 0.28 ± 0.02 mas/day. An optically thin cavity of angular size 13.3 ± 1.4 mas was found to lie between the central binary and the onset of the spiral dust plume. Rotational motion of the wind system induced by the binary orbit is found to have important ramifications: entanglement of the winds results in strong shock activity far downstream from the nose of the bowshock. The far greater fraction of the winds participating in the collision may play a key role in gas compression and the nucleation of dust at large radii from the central binary and shock stagnation point. Investigation of the effects of radiative braking pointed towards significant modifications of the simple hydrostatic colliding wind geometry, extending the relevance of this phenomena to wider binary systems than previously considered. Limits placed on the maximum allowed orbital eccentricity of e < 0.06 argue strongly for a prehistory of tidal circularisation in this system. Finally we discuss the implications of Earth's polar (i < 16 deg) vantage point onto a system likely to host supernova explosions at future epochs. 

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