Friday, August 15, 2014

Astrospheres of Evolving Massive Stars

O-type stars are amongst the most massive and most luminous stars. Isolated O-type stars that move independently through the interstellar medium have a significant influence on their surroundings from the strong stellar winds and ionizing radiation they emit. ζ Ophiuchi is a typical example of an isolated O-type star. It has ~20 times the mass and ~100,000 times the luminosity of the Sun. ζ Ophiuchi moves through the interstellar medium at ~26.5 km/s, generating a bow shock where its strong stellar wind meets the interstellar medium. In the upstream direction, the separation between the star and its bow shock, also know as the standoff distance, is ~5 trillion km, or half a light year. The enormous amount of ionization radiation emitted by ζ Ophiuchi ionizes the surrounding interstellar medium out to a radius of ~30 light years. This is ~60 times larger than the standoff distance and shows that the influence of ζ Ophiuchi extends far beyond its own stellar wind.

Figure 1: An overview of the different types of stars as well as their size and the colour with which they shine.

Massive O-type stars like ζ Ophiuchi live fast and die young. Although such stars are exceedingly rare, their immense luminosities make them easy to detect. When an O-type star begins to exhaust hydrogen in its core, it swells and transforms from a hot blue supergiant to a cooler red supergiant. All of that occurs on a timescale of only ~0.01 to 0.02 million years. Once it becomes a red supergiant, the star stops emitting ionizing radiation and its escape velocity drops dramatically. As a consequence, its stellar wind becomes slower and denser. The stellar wind during its blue supergiant phase is ~400 km/s (fast wind), while the stellar wind during its red supergiant phase slows to ~15 km/s (slow wind). Since the bow shock’s dynamical timescale of ~0.01 to 0.1 million years is much longer than the star’s evolution, a new bow shock forms around the slow wind within the relic bow shock from the fast wind. As a result, for a brief period of time, two bow shocks can exist around the star.

Figure 2: 2D simulations of the circumstellar medium at different times (indicated) for an O-type star’s evolution from blue supergiant (left-most panel) to red supergiant to the pre-supernova stage (right-most panel). The strengthening red supergiant stellar wind expands into the relic bow shock from the blue supergiant phase, creating a short-lived double bow shock. Jonathan Mackey et al. (2014).

Jonathan Mackey et al. (2014), “Effects of stellar evolution and ionizing radiation on the environments of massive stars”, arXiv:1407.8396 [astro-ph.GA]