Abstract
The role of feedback from massive stars is believed to be a key element in the evolution of molecular clouds. We use high-resolution 3D SPH simulations to explore the dynamical effects of a single O7 star located at the centre of a molecular cloud with mass 10^4M_sun and radius 6.4pc. The initial internal structure of the cloud is characterised by its fractal dimension, D=2.0 - 2.8, and its log-normal density PDF. (i) As regards star formation, in the short term ionising feedback is positive, in the sense that star formation occurs much more quickly in gas that is compressed by the high pressure of the ionised gas. However, in the long term ionising feedback is negative, in the sense that most of the cloud is dispersed with an outflow rate of up to ~0.01M_sun/yr, on a timescale comparable with the sound-crossing time for the ionised gas (~1-2Myr), and triggered star formation is therefore limited to a few percent of the cloud's mass. (ii) As regards the morphology of the ionisation fronts (IFs) bounding the HII region and the systematics of outflowing gas, we distinguish two regimes. For low D<=2.2, the initial cloud is dominated by large-scale structures, so the neutral gas tends to be swept up into a few extended coherent shells, and the ionised gas blows out through a few large holes between these shells; we term these HII regions "shell-dominated". Conversely, for high D>=2.6, the initial cloud is dominated by small-scale structures, and these are quickly overrun by the advancing IF, thereby producing neutral pillars whilst the ionised gas blows out through a large number of small holes between the pillars; we term these HII regions "pillar-dominated". (iii) As regards the injection of bulk kinetic energy, by ~1Myr, the expansion of the HII region has delivered a rms velocity of ~6km/s; this represents less than 0.1% of the total energy radiated by the O7 star.
Highlights
Infrared shells and bubbles (Churchwell et al 2006; Churchwell 2008) are ubiquitous in the Galaxy and are often associated with Hii regions, i.e. localised regions in which the Lyman continuum radiation (Eγ > 13.6 eV) from one or more young, massive stars ionises the gas and heats it to TI ∼ 104 K
For high D 2.6, the initial cloud is dominated by small-scale structures, and these are quickly overrun by the advancing ionisation fronts (IFs), thereby producing neutral pillars protruding into the Hii region, whilst the ionised gas blows out through a large number of small holes between the pillars; we term these Hii regions pillar-dominated. (iii) As regards the injection of bulk kinetic energy, by ∼ 1 Myr, the expansion of the Hii has delivered a mass-weighted root-mean-square velocity of ∼ 6 km s−1; this represents less than 0.1% of the total energy radiated by the O7 star
We have used high-resolution 3D SPH simulations to explore the effect of a single O7 star emitting photons at 1049 s−1 and located at the centre of a molecular cloud with mass
Summary
Infrared shells and bubbles (Churchwell et al 2006; Churchwell 2008) are ubiquitous in the Galaxy and are often associated with Hii regions, i.e. localised regions in which the Lyman continuum radiation (Eγ > 13.6 eV) from one or more young, massive stars ionises the gas and heats it to TI ∼ 104 K. In more massive clouds disruption by ionisation plays a less important role (Krumholz & Matzner 2009; Murray et al 2010; Fall et al 2010) These conclusions have recently been confirmed in 3D SPH simulations by Dale et al (2012), who study star-cluster forming clouds with 104 to 106 M ; their lower mass clouds are comparable with the ones we investigate in the current study. They find that massive clouds resist dispersal by ionising feedback, because it is quenched by accretion flows.
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