3D simulations of Betelgeuse’s bow shock^⋆

S. Mohamed^1,2, J. Mackey¹ and N. Langer¹

¹ Argelander Institut für Astronomie, Auf dem Hügel 71, Bonn 53121, Germany
² South African Astronomical Observatory, PO Box 9, 7935 Observatory, South Africa
e-mail: shazrene@saao.ac.za

Received: 1 September 2011
Accepted: 14 January 2012

Abstract

Betelgeuse, the bright, cool red supergiant in Orion, is moving supersonically relative to the local interstellar medium. The star emits a powerful stellar wind that collides with this medium, forming a cometary structure, a bow shock, pointing in the direction of motion. We present the first 3D hydrodynamic simulations of the formation and evolution of Betelgeuse’s bow shock. The models include realistic low-temperature cooling and cover a range of plausible interstellar medium densities of 0.3–1.9 cm^-3 and stellar velocities of 28–73 km s^-1. We show that the flow dynamics and morphology of the bow shock differ substantially because of the growth of Rayleigh-Taylor or Kelvin-Helmholtz instabilities. The former dominate the models with slow stellar velocities resulting in a clumpy bow shock substructure, whereas the latter produce a smoother, more layered substructure in the fast models. If the mass in the bow shock shell is low, as seems to be implied by the AKARI luminosities (~3 × 10^-3 M_⊙), then Betelgeuse’s bow shock is very young and is unlikely to have reached a steady state. The circular nature of the bow shock shell is consistent with this conclusion. Thus, our results suggest that Betelgeuse only entered the red supergiant phase recently.