Abstract
ABSTRACT In this paper, we make predictions for the behaviour of wind bubbles around young massive stars using analytic theory. We do this in order to determine why there is a discrepancy between theoretical models that predict that winds should play a secondary role to photoionization in the dynamics of H iiregions, and observations of young H iiregions that seem to suggest a driving role for winds. In particular, regions such as M42 in Orion have neutral hydrogen shells, suggesting that the ionizing radiation is trapped closer to the star. We first derive formulae for wind bubble evolution in non-uniform density fields, focusing on singular isothermal sphere density fields with a power-law index of -2. We find that a classical ‘Weaver’-like expansion velocity becomes constant in such a density distribution. We then calculate the structure of the photoionized shell around such wind bubbles, and determine at what point the mass in the shell cannot absorb all of the ionizing photons emitted by the star, causing an ‘overflow’ of ionizing radiation. We also estimate perturbations from cooling, gravity, magnetic fields and instabilities, all of which we argue are secondary effects for the conditions studied here. Our wind-driven model provides a consistent explanation for the behaviour of M42 to within the errors given by observational studies. We find that in relatively denser molecular cloud environments around single young stellar sources, champagne flows are unlikely until the wind shell breaks up due to turbulence or clumping in the cloud.
Highlights
IntroductionLyman continuum photons leave the star and photoionise the gas around them in a region bounded by an ionisation front, which separates this photoionised gas and the neutral gas outside
Ionising radiation from stars plays an important role in the interstellar medium
We assume for the purposes of this work that the wind bubble is expanding rapidly into a ω = 2 density field, since this is similar to what we find in simulations of young massive stars in clouds similar to this region (Geen et al 2021)
Summary
Lyman continuum photons leave the star and photoionise the gas around them in a region bounded by an ionisation front, which separates this photoionised gas and the neutral gas outside. These volumes of photoionised gas are called H regions. Since the photoionised gas is warmer (∼ 104 K) than the neutral gas outside (10-1000 K), this causes thermal expansion of the H region This expansion reduces the density of the photoionised gas, lowering its recombination rate and allowing ionising photons to reach larger radii before being absorbed, and the ionisation front moves outwards as the H region expands. The thermal expansion produces a leading shock wave that accelerates and condenses the surrounding gas into a dense shell of partially or wholly neutral
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