Dispersal of molecular clouds by ionizing radiation

Abstract

Feedback from massive stars is believed to be a key element in the evolution of molecular clouds. We use high‐resolution 3D smoothed particle hydrodynamics simulations to explore the dynamical effects of a single O7 star‐emitting ionizing photons at 1049 s−1 and located at the centre of a molecular cloud with mass 104 M⊙ and radius 6.4 pc; we also perform comparison simulations in which the ionizing star is removed. The initial internal structure of the cloud is characterized by its fractal dimension, which we vary between D=2.0 and 2.8, and the standard deviation of the approximately log‐normal initial density PDF, which is σ10 = 0.38 for all clouds. (i) As regards star formation, in the short term ionizing feedback is positive, in the sense that star formation occurs much more quickly (than in the comparison simulations), in gas that is compressed by the high pressure of the ionized gas. However, in the long term ionizing feedback is negative, in the sense that most of the cloud is dispersed with an outflow rate of up to ∼10−2 M⊙yr−1, on a time‐scale comparable with the sound‐crossing time for the ionized gas (∼1−2 Myr ), and triggered star formation is therefore limited to a few per cent of the cloud's mass. We will describe in greater detail the statistics of the triggered star formation in a companion paper. (ii) As regards the morphology of the ionization fronts (IFs) bounding the H ii 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 ionized gas blows out through a few large holes between these shells; we term these H ii 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 protruding into the H ii region, whilst the ionized gas blows out through a large number of small holes between the pillars; we term these H ii regions pillar dominated. (iii) As regards the injection of bulk kinetic energy, by ∼1 Myr, the expansion of the H ii region has delivered a mass‐weighted rms velocity of ∼6 km s−1; this represents less than 0.1 per cent of the total energy radiated by the O7 star.

The Milky Way Project and Atlasgal: The Distribution and Physical Properties of Cold Clumps Near Infrared Bubbles

“…The importance of bubbles was highlighted in recent work by and Lee et al (2012), who find that the feedback energy from expanding bubbles in star-forming complexes is a major driver of turbulence in the ISM of the inner Milky Way, possibly more important than that from supernovae. Both the triggering and quenching of star formation by massive stellar feedback are observed in smoothed particle hydrodynamics simulations by Dale et al (2005Dale et al ( , 2007 and Walch et al (2012).…”

The formation and destruction of molecular clouds and galactic star formation

“…Dale et al (2012Dale et al ( , 2013 did extensive three-dimensional simulations of star cluster formation with ionization or stellar wind feedback and showed that the effects of photoionization and stellar winds are limited in quenching the star formation in massive molecular clouds (see also Walch et al 2012). Diaz-Miller et al (1998) calculated the steady-state structures of HII regions and pointed out that the photodissociation of hydrogen molecules due to far-ultraviolet (FUV) photons is much more important than photoionization due to UV photons for the destruction of molecular clouds.…”

Structure distribution and turbulence in self-consistently supernova-driven ISM of multiphase magnetized galactic discs

“…Since treating precisely the dense gas and the supernova correlation implies resolving not only the star formation well but also following the detailed star trajectories, this constitutes a very difficult task, that renders prescriptions like the one we are using unavoidable. More generally, other sources of feedback such as HII radiation and stellar winds should be considered as well (Walch et al 2012;Dale et al 2013Dale et al , 2014Geen et al 2015Geen et al , 2016.…”