Wind bubbles within H ii regions around slowly moving stars

Abstract

Interstellar bubbles around O stars are driven by a combination of the star's wind and ionizing radiation output. The wind contribution is uncertain because the boundary between the wind and interstellar medium is difficult to observe. Mid-infrared observations (e.g., of the H ii region RCW 120) show arcs of dust emission around O stars, contained well within the H ii region bubble. These arcs could indicate the edge of an asymmetric stellar wind bubble, distorted by density gradients and/or stellar motion. We present twodimensional, radiation-hydrodynamics simulations investigating the evolution of wind bubbles and H ii regions around massive stars moving through a dense (n H = 3000 cm −3 ), uniform medium with velocities ranging from 4 to 16 km s −1 . The H ii region morphology is strongly affected by stellar motion, as expected, but the wind bubble is also very aspherical from birth, even for the lowest space velocity considered. Wind bubbles do not fill their H ii regions (we find filling factors of 10-20 per cent), at least for a main sequence star with mass M ∼ 30 M . Furthermore, even for supersonic velocities the wind bow shock does not significantly trap the ionization front. X-ray emission from the wind bubble is soft, faint, and comes mainly from the turbulent mixing layer between the wind bubble and the H ii region. The wind bubble radiates <1 per cent of its energy in X-rays; it loses most of its energy by turbulent mixing with cooler photoionized gas. Comparison of the simulations with the H ii region RCW 120 shows that its dynamical age is 0.4 Myr and that stellar motion 4 km s −1 is allowed, implying that the ionizing source is unlikely to be a runaway star but more likely formed in situ. The region's youth, and apparent isolation from other O or B stars, makes it very interesting for studies of massive star formation and of initial mass functions.

Systematic search for very-high-energy gamma-ray emission from bow shocks of runaway stars

“…X-ray observations with XMM-Newton of AE Aurigae (HIP 24575) revealed for the first time significant emission, but its nature (very hot thermal or non-thermal) could not be firmly determined (López-Santiago et al 2012). In the case of ζ Ophiuchi, Toalá et al (2016) detected diffuse emission in the vicinity of this candidate, which they attribute to a plasma with a temperature of 2 · 10 6 K, in agreement with predictions of high plasma temperatures caused by instabilities mixing material between the shocked wind and the photo-ionized gas at the wake of the shock (Mackey et al 2015). Recently, (Toalá et al 2017) showed that the X-ray emission close to AE Aurigae is point-like and unrelated to the bow shock.…”

Detecting stellar-wind bubbles through infrared arcs in H ii regions

“…The H  region and wind bubble can become distorted if the star is moving (Weaver et al 1977;Raga et al 1997;Meyer et al 2014), and if the star moves supersonically through the H  region then the wind bubble drives a bow shock in the direction of motion (Baranov et al 1971;Weaver et al 1977;Arthur & Hoare 2006;Zhu et al 2015). In Mackey et al (2015, hereafter Paper I), we showed that even slowly moving stars (space velocity, v = 4 km s −1 ) produce very asymmetric wind bubbles in dense regions.…”

An astrosphere around the blue supergiant κ Cas: possible explanation of its filamentary structure

“…Also, the shape of astrospheres of hot (runaway) stars might be affected by photoevaporation flows from nearby regions of enhanced density (cloudlets) caused by ultraviolet emission of these stars (e.g. Mackey et al 2015;Gvaramadze et al 2017). Fig.…”