Detecting stellar-wind bubbles through infrared arcs in HII regions

Abstract

Mid-infrared arcs of dust emission are often seen near ionizing stars within H ii regions. A possible explanations for these arcs is that they could show the outer edges of asymmetric stellar wind bubbles. We use two-dimensional, radiation-hydrodynamics simulations of wind bubbles within H ii regions around individual stars to predict the infrared emission properties of the dust within the H ii region. We assume that dust and gas are dynamically well-coupled and that dust properties (composition, size distribution) are the same in the H ii region as outside it, and that the wind bubble contains no dust. We post-process the simulations to make synthetic intensity maps at infrared wavebands using the torus code. We find that the outer edge of a wind bubble emits brightly at 24 μm through starlight absorbed by dust grains and re-radiated thermally in the infrared. This produces a bright arc of emission for slowly moving stars that have asymmetric wind bubbles, even for cases where there is no bow shock or any corresponding feature in tracers of gas emission. The 24 μm intensity decreases exponentially from the arc with increasing distance from the star because the dust temperature decreases with distance. The size distribution and composition of the dust grains has quantitative but not qualitative effects on our results. Despite the simplifications of our model, we find good qualitative agreement with observations of the H ii region RCW 120, and can provide physical explanations for any quantitative differences. Our model produces an infrared arc with the same shape and size as the arc around CD −38°11636 in RCW 120, and with comparable brightness. This suggests that infrared arcs around O stars in H ii regions may be revealing the extent of stellar wind bubbles, although we have not excluded other explanations.