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