Computing the Dust Distribution in the Bow Shock of a Fast-Moving, Evolved Star

Abstract

We study the hydrodynamical behavior occurring in the turbulent interaction zone of a fast moving red supergiant star, where the circumstellar and interstellar material collide. In this wind-interstellar medium collision, the familiar bow shock, contact discontinuity, and wind termination shock morphology forms, with localized instability development.Our model includes a detailed treatment of dust grains in the stellar wind, and takes into account the drag forces between dust and gas. The dust is treated as pressureless gas components binned per grainsize, for which we use ten representative grainsize bins. Our simulations allow to deduce how dust grains of varying sizes become distributed throughout the circumstellar medium.We show that smaller dust grains (radius < 0.045µm) tend to be strongly bound to the gas and therefore follow the gas density distribution closely, with intricate finestructure due to essentially hydrodynamical instabilities at the windrelated contact discontinuity. Larger grains which are more resistant to drag forces are shown to have their own unique dust distribution, with progressive deviations from the gas morphology. Specifically, small dust grains stay entirely within the zone bound by shocked wind material. The large grains are capable of leaving the shocked wind layer, and can penetrate into the shocked or even unshocked interstellar medium. Depending on how the number of dust grains varies with grainsize, this should leave a clear imprint in infrared observations of bowshocks of red supergiants and other evolved stars.

Asymmetric supernova remnants generated by Galactic, massive runaway stars

“…The works by van Marle et al (2011) and Decin et al (2012) tailor models to Betelgeuse's bow shock and estimate in the context of recent observations (Cox et al 2012) how the drag force on dust grains modifies the evolution of its contact discontinuity. The effects of the mass loss and space velocity on the shape and luminosity of bow shocks around red supergiant stars is investigated in Meyer et al (2014, hereafter Paper I).…”

“…Supernovae showing evidence of interaction with circumstellar structures are commonly denoted as Type IIn and their corresponding lightcurves provide information on the progenitor and properties of its close surroundings (Schlegel 1990;Filippenko 1997;van Marle et al 2010). About 10−100 yr after the explosion, the shock wave collides with the bow shock along the direction of motion of its progenitor, whereas it expands in a cavity of wind material in the opposite direction (Borkowski et al 1992).…”

Dust-enshrouded star near supermassive black hole: predictions for high-eccentricity passages near low-luminosity galactic nuclei

“…It has been explored by various authors (e.g., Zurek et al 1994;Armitage et al 1996;Šubr & Karas 1999;Vilkoviskij & Czerny 2002, and references therein) in the context of repetitive interactions of stars of the nuclear cluster with the material of the accretion disc or a dusty torus in AGN.…”

“…A similar effect is expected to occur in our model as well. One of the differences that distinguishes the core-less cloud from a dust-enshrouded star is the size of the bow shock (e.g., van Marle et al 2011;Araudo et al 2013). In the case of a star, the bow shock is formed by the wind interaction; a powerful wind outflow (as in the case of massive stars) develops the stagnation point radius at a much larger radius than the size of the star.…”

“…MacGregor & Stencel 1992;van Marle et al 2011;Keppens et al 2012). If large portions of the medium are neutral, viscous forces play an important role, while in a fully ionised plasma the efficient mechanism of dust grain charging operates.…”