http://arxiv.org/abs/1808.05950
Recent observations have revealed that the remnants of stellar-coalescence transients are bipolar. This raises the questions of how these bipolar morphologies arise and what they teach us about the mechanisms of mass ejection during stellar mergers and common envelope phases. In this paper, we analyze hydrodynamic simulations of the lead-in to binary coalescence, a phase of unstable Roche lobe overflow that takes the binary from the Roche limit separation to the engulfment of the more-compact accretor within the envelope of the extended donor. As mass transfer runs away to increasing rates, gas trails away from the binary. Contrary to previous expectations, early mass loss remains bound to the binary and forms a circumbinary torus. Later ejecta, generated as the accretor grazes the surface of the donor, have very different morphology and are unbound. These two components of mass loss from the binary interact as later, higher-velocity ejecta collide with the circumbinary torus formed by earlier mass loss. Unbound ejecta are redirected toward the poles and escaping material creates a bipolar outflow. Our findings show that the transition from bound to unbound ejecta from coalescing binaries can explain the bipolar nature of their remnants, with implications for our understanding of the origin of bipolar remnants of stellar coalescence transients and, perhaps, some pre-planetary nebulae.
M. MacLeod, E. Ostriker and J. Stone
Tue, 21 Aug 18
4/71
Comments: Submitted to ApJ, we welcome comments or feedback
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