Ionizing feedback from massive stars in massive clusters III: Disruption of partially unbound clouds

We extend our previous SPH parameter study of the effects of photoionization from O-stars on star-forming clouds to include initially unbound clouds. We generate a set of model clouds in the mass range $10^{4}-10^{6}$M$_{odot}$ with initial virial ratios $E_{rm kin}/E_{rm pot}$=2.3, allow them to form stars, and study the impact of the photoionizing radiation produced by the massive stars. We find that, on the 3Myr timescale before supernovae are expected to begin detonating, the fractions of mass expelled by ionizing feedback is a very strong function of the cloud escape velocities. High-mass clouds are largely unaffected dynamically, while lower-mass clouds have large fractions of their gas reserves expelled on this timescale. However, the fractions of stellar mass unbound are modest and significant portions of the unbound stars are so only because the clouds themselves are initially partially unbound. We find that ionization is much more able to create well-cleared bubbles in the unbound clouds, owing to their intrinsic expansion, but that the presence of such bubbles does not necessarily indicate that a given cloud has been strongly influenced by feedback. We also find, in common with the bound clouds from our earlier work, that many of the systems simulated here are highly porous to photons and supernova ejecta, and that most of them will likely survive their first supernova explosions.

Ionization--induced star formation IV: Triggering in bound clusters

We present a detailed study of star formation occurring in bound star--forming clouds under the influence of internal ionizing feedback from massive stars across a spectrum of cloud properties. We infer which objects are triggered by comparing our feedback simulations with control simulations in which no feedback was present. We find feedback always results in a lower star--formation efficiency and usually but not always results in a larger number of stars or clusters. Cluster mass functions are not strongly affected by feedback, but stellar mass functions are biased towards lower masses. Ionization also affects the geometrical distribution of stars in ways that are robust against projection effects, but may make the stellar associations more or less subclustered depending on the background cloud environment. We observe a prominent pillar in one simulation which is the remains of an accretion flow feeding the central ionizing cluster of its host cloud and suggest that this may be a general formation mechanism for pillars such as those observed in M16. We find that the association of stars with structures in the gas such as shells or pillars is a good but by no means foolproof indication that those stars have been triggered and we conclude overall that it is very difficult to deduce which objects have been induced to form and which formed spontaneously simply from observing the system at a single time.

Massive stars in massive clusters II: Disruption of bound clusters by photoionization

We present an SPH parameter study of the dynamical effect of photoionization from O--type stars on star--forming clouds of a range of masses and sizes during the time window before supernovae explode. Our model clouds all have the same degree of turbulent support initially, the ratio of turbulent kinetic energy to gravitational potential energy being set to $E_{rm kin}/|E_{rm pot}|$=0.7. We allow the clouds to form stars and study the dynamical effects of the ionizing radiation from the massive stars or clusters born within them. We find that dense filamentary structures and accretion flows limit the quantities of gas that can be ionized, particularly in the higher density clusters. More importantly, the higher escape velocities in our more massive (10$^{6}$M$_{odot}$) clouds prevent the HII regions from sweeping up and expelling significant quantities of gas, so that the most massive clouds are largely dynamically unaffected by ionizing feedback. However, feedback has a profound effect on the lower--density 10$^{4}$ and 10$^{5}$M$_{odot}$ clouds in our study, creating vast evacuated bubbles and expelling tens of percent of the neutral gas in the 3Myr timescale before the first supernovae are expected to detonate, resulting in clouds highly porous to both photons and supernova ejecta.

Red giant stellar collisions in the Galactic Centre

We show that collisions with stellar--mass black holes can partially explain the absence of bright giant stars in the Galactic Centre, first noted by Genzel et al, 1996. We show that the missing objects are low--mass giants and AGB stars in the range 1-3 M$_{odot}$. Using detailed stellar evolution calculations, we find that to prevent these objects from evolving to become visible in the depleted K bands, we require that they suffer collisions on the red giant branch, and we calculate the fractional envelope mass losses required. Using a combination of Smoothed Particle Hydrodynamic calculations, restricted three--body analysis and Monte Carlo simulations, we compute the expected collision rates between giants and black holes, and between giants and main--sequence stars in the Galactic Centre. We show that collisions can plausibly explain the missing giants in the $10.5<K<12$ band. However, depleting the brighter ($K<10.5$) objects out to the required radius would require a large population of black hole impactors which would in turn deplete the $10.5<K<12$ giants in a region much larger than is observed. We conclude that collisions with stellar--mass black holes cannot account for the depletion of the very brightest giants, and we use our results to place limits on the population of stellar--mass black holes in the Galactic Centre.

The effect of stellar winds on the formation of a protocluster

We present SPH simulations of protoclusters including the effects of winds from massive stars. Using a particle-injection method, we investigate the effect of structure close to the wind sources and the short-timescale influence of winds on protoclusters. Structures such as disks and gaseous filaments have a strong collimating effect. By a different technique of injecting momentum from point sources, we compare the large-scale, long-term effects of isotropic and intrinsically-collimated winds and find them to be similar. Both types of wind dramatically slow the global star formation process, but the timescale on which they expel significant mass from the cluster is rather long (approaching 10 freefall times). Clusters may then experience rapid star formation early in their lifetimes, before switching to a mode where gas is gradually expelled, while star formation proceeds much more slowly. This complicates conclusions regarding slow star formation derived from measuring the star-formation efficiency per freefall time. Estimates of the efficacy of winds in dispersing clusters derived simply from the total wind momentum flux may not be very reliable. This is due to material being expelled from deep within stellar potential wells, often to velocities well in excess of the cluster escape velocity, and also to the loss of momentum flux through holes in the gas distribution. Winds have little effect on the accretion--driven stellar IMF except at the very high-mass end, where they serve to prevent some of the most massive objects accreting. We also find that the morphology of the gas, the rapid motions of the wind sources and the action of accretion flows prevent the formation of bubble-like structures. This may make it difficult to discern the influence of winds on very young clusters.