The implications of clustered star formation for (proto)planetary systems and habitability

Star formation is spatially clustered across a range of environments, from dense stellar clusters to unbound associations. As a result, radiative or dynamical interactions with neighbouring stars disrupt (proto)planetary systems and limit their radii, leaving a lasting impact on their potential habitability. In the solar neighbourhood, we find that the vast majority of stars form in unbound associations, such that the interaction of (proto)planetary systems with neighbouring stars is limited to the densest sub-regions. However, the fraction of star formation occurring in compact clusters was considerably higher in the past, peaking at ~50% in the young Milky Way at redshift z~2. These results demonstrate that the large-scale star formation environment affects the demographics of planetary systems and the occupation of the habitable zone. We show that planet formation is governed by multi-scale physics, in which Mpc-scale events such as galaxy mergers affect the AU-scale properties of (proto)planetary systems

The dynamical evolution of molecular clouds near the Galactic Centre -- III. Tidally--induced star formation in protocluster clouds

As part of a series of papers aimed at understanding the evolution of the Milky Way's Central Molecular Zone (CMZ), we present hydrodynamical simulations of turbulent molecular clouds orbiting in an accurate model of the gravitational potential extant there. We consider two sets of model clouds differing in the energy content of their velocity fields. In the first, self--virialised set, the turbulent kinetic energies are chosen to be close in magnitude to the clouds' self--gravitational potential energies. Comparison with isolated clouds evolving without an external potential shows that the self--virialised clouds are unable to withstand the compressive tidal field of the CMZ and rapidly collapse, forming stars much faster and reaching gas exhaustion after a small fraction of a Galactocentric orbit. In the second, tidally--virialised, set of simulations, the clouds' turbulent kinetic energies are in equilibrium with the external tidal field. These models are better supported against the field and the stronger turbulence suppresses star formation. Our results strongly support the inference that anomalously low star formation rates in the CMZ are due primarily to high velocity dispersions in the molecular gas. The clouds follow open, eccentric orbits oscillating in all three spatial coordinates. We examine the consequences of the orbital dynamics, particularly pericentre passage, by performing companion simulations of clouds on circular orbits. The increased tidal forces at pericentre produce transient accelerations in star formation rates of at most a factor of 2.7. Our results demonstrate that modelling star formation in galactic centres requires the inclusion of tidal forces

Star formation rates on global and cloud scales within the Galactic Centre

The environment within the inner few hundred parsecs of the Milky Way, known as the "Central Molecular Zone" (CMZ), harbours densities and pressures orders of magnitude higher than the Galactic Disc; akin to that at the peak of cosmic star formation (Kruijssen & Longmore 2013). Previous studies have shown that current theoretical star-formation models under-predict the observed level of star-formation (SF) in the CMZ by an order of magnitude given the large reservoir of dense gas it contains. Here we explore potential reasons for this apparent dearth of star formation activity

Dynamical cluster disruption and its implications for multiple population models in the E-MOSAICS simulations

© 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. Several models have been advanced to explain the multiple stellar populations observed in globular clusters (GCs). Most models necessitate a large initial population of unenriched stars that provide the pollution for an enriched population, and which are subsequently lost from the cluster. This scenario generally requires clusters to lose > 90 per cent of their birth mass. We use a suite of 25 cosmological zoom-in simulations of present-day Milky Way mass galaxies from the E-MOSAICS project to study whether dynamical disruption by evaporation and tidal shocking provides the necessary mass-loss. We find that GCs with present-day masses M > 105M⊙were only 2-4 times more massive at birth, in conflict with the requirements of the proposed models. This factor correlates weakly with metallicity, gas pressure at birth, or galactocentric radius, but increases towards lower GC masses. To reconcile our results with observational data, either an unphysically steep cluster mass-size relation must be assumed, or the initial enriched fractions must be similar to their present values. We provide the required relation between the initial enriched fraction and cluster mass. Dynamical cluster mass-loss cannot reproduce the high observed enriched fractions nor their trend with cluster mass

The varying mass distribution of molecular clouds across M83

The work of Adamo et al. showed that the mass distributions of young massive stellar clusters were truncated above a maximum-mass scale in the nearby galaxy M83 and that this truncation mass varies with the galactocentric radius. Here, we present a cloud-based analysis of Atacama Large Millimeter/submillimeter Array CO(1 → 0) observations of M83 to search for such a truncation mass in the molecular cloud population. We identify a population of 873 molecular clouds in M83 that is largely similar to those found in the Milky Way and Local Group galaxies, though clouds in the centre of the galaxy show high surface densities and enhanced turbulence, as is common for clouds in high-density nuclear environments. Like the young massive clusters, we find a maximum-mass scale for the molecular clouds which decreases radially in the galaxy. We find that the most young massive cluster tracks the most massive molecular cloud with the cluster mass being 10−2 times that of the most massive molecular cloud. Outside the nuclear region of M83 (Rg > 0.5 kpc), there is no evidence for changing internal conditions in the population of molecular clouds, with the average internal pressures, densities and free-fall times remaining constant for the cloud population over the galaxy. This result is consistent with the bound cluster formation efficiency depending only on the large-scale properties of the interstellar medium rather than the internal conditions of individual clouds

Towards a multi-tracer timeline of star formation in the LMC -- I.\ Deriving the lifetimes of H\,{\sc i} clouds

The time-scales associated with the various stages of the star formation process remain poorly constrained. This includes the earliest phases of star formation, during which molecular clouds condense out of the atomic interstellar medium. We present the first in a series of papers with the ultimate goal of compiling the first multi-tracer timeline of star formation, through a comprehensive set of evolutionary phases from atomic gas clouds to unembedded young stellar populations. In this paper, we present an empirical determination of the lifetime of atomic clouds using the Uncertainty Principle for Star Formation formalism, based on the de-correlation of H$\alpha$ and H\,{\sc i} emission as a function of spatial scale. We find an atomic gas cloud lifetime of 48$\substack{+13\\-8}$\,Myr. This timescale is consistent with the predicted average atomic cloud lifetime in the LMC (based on galactic dynamics) that is dominated by the gravitational collapse of the mid-plane ISM. We also determine the overlap time-scale for which both H\,{\sc i} and H$\alpha$ emission are present to be very short ($t_{over}<1.7$\,Myr), consistent with zero, indicating that there is a near-to-complete phase change of the gas to a molecular form in an intermediary stage between H\,{\sc i} clouds and H\,{\sc ii} regions. We utilise the time-scales derived in this work to place empirically determined limits on the time-scale of molecular cloud formation. By performing the same analysis with and without the 30 Doradus region included, we find that the most extreme star forming environment in the LMC has little effect on the measured average atomic gas cloud lifetime. By measuring the lifetime of the atomic gas clouds, we place strong constraints on the physics that drives the formation of molecular clouds and establish a solid foundation for the development of a multi-tracer timeline of star formation in the LMC

Star formation rates and efficiencies in the Galactic Centre

The inner few hundred parsecs of the Milky Way harbours gas densities, pressures, velocity dispersions, an interstellar radiation field and a cosmic ray ionisation rate orders of magnitude higher than the disc; akin to the environment found in star-forming galaxies at high-redshift. Previous studies have shown that this region is forming stars at a rate per unit mass of dense gas which is at least an order of magnitude lower than in the disc, potentially violating theoretical predictions. We show that all observational star formation rate diagnostics - both direct counting of young stellar objects and integrated light measurements - are in agreement within a factor two, hence the low star formation rate is not the result of the systematic uncertainties that affect any one method. As these methods trace the star formation over different timescales, from $0.1 - 5$ Myr, we conclude that the star formation rate has been constant to within a factor of a few within this time period. We investigate the progression of star formation within gravitationally bound clouds on $\sim$ parsec scales and find $1 - 4$ per cent of the cloud masses are converted into stars per free-fall time, consistent with a subset of the considered "volumetric" star formation models. However, discriminating between these models is obstructed by the current uncertainties on the input observables and, most importantly and urgently, by their dependence on ill-constrained free parameters. The lack of empirical constraints on these parameters therefore represents a key challenge in the further verification or falsification of current star formation theories

Which feedback mechanisms dominate in the high-pressure environment of the Central Molecular Zone?

Supernovae (SNe) dominate the energy and momentum budget of stellar feedback, but the efficiency with which they couple to the interstellar medium (ISM) depends strongly on how effectively early, pre-SN feedback clears dense gas from star-forming regions. There are observational constraints on the magnitudes and timescales of early stellar feedback in low ISM pressure environments, yet no such constraints exist for more cosmologically typical high ISM pressure environments. In this paper, we determine the mechanisms dominating the expansion of HII regions as a function of size-scale and evolutionary time within the high-pressure (P/k_\rm{B}~$10^{7-8}$K cm$^{-3}$) environment in the inner 100pc of the Milky Way. We calculate the thermal pressure from the warm ionised (P_\rm{HII}; 10$^{4}$K) gas, direct radiation pressure (P_\rm{dir}), and dust processed radiation pressure (P_\rm{IR}). We find that (1) P_\rm{dir} dominates the expansion on small scales and at early times (0.01-0.1pc; $0.1$pc; $>1$Myr); (3) during the first ~1Myr of growth, but not thereafter, either $P_{\rm IR}$ or stellar wind pressure likely make a comparable contribution. Despite the high confining pressure of the environment, natal star-forming gas is efficiently cleared to radii of several pc within ~2Myr, i.e. before the first SNe explode. This `pre-processing' means that subsequent SNe will explode into low density gas, so their energy and momentum will efficiently couple to the ISM. We find the HII regions expand to a radius of 3pc, at which point they have internal pressures equal with the surrounding external pressure. A comparison with HII regions in lower pressure environments shows that the maximum size of all HII regions is set by pressure equilibrium with the ambient ISM

An uncertainty principle for star formation -- V. The influence of dust extinction on star formation rate tracer lifetimes and the inferred molecular cloud lifecycle

Recent observational studies aiming to quantify the molecular cloud lifecycle require the use of known 'reference time-scales' to turn the relative durations of different phases of the star formation process into absolute time-scales. We previously constrained the characteristic emission time-scales of different star formation rate (SFR) tracers, as a function of the SFR surface density and metallicity. However, we omitted the effects of dust extinction. Here, we extend our suite of SFR tracer emission time-scales by accounting for extinction, using synthetic emission maps of a high-resolution hydrodynamical simulation of an isolated, Milky-Way-like disc galaxy. The stellar feedback included in the simulation is inefficient compared to observations, implying that it represents a limiting case in which the duration of embedded star formation (and the corresponding effect of extinction) is overestimated. Across our experiments, we find that extinction mostly decreases the SFR tracer emission time-scale, changing the time-scales by factors of 0.04-1.74, depending on the gas column density. UV filters are more strongly affected than H$\alpha$ filters. We provide the limiting correction factors as a function of the gas column density and flux sensitivity limit for a wide variety of SFR tracers. Applying these factors to observational characterisations of the molecular cloud lifecycle produces changes that broadly fall within the quoted uncertainties, except at high kpc-scale gas surface densities ($\Sigma_{\rm g}\gtrsim20~{\mathrm{M_{\odot}\,pc^{-2}}}$). Under those conditions, correcting for extinction may decrease the measured molecular cloud lifetimes and feedback time-scales, which further strengthens previous conclusions that molecular clouds live for a dynamical time and are dispersed by early, pre-supernova feedback

The origin of the 'blue tilt' of globular cluster populations in the E-MOSAICS simulations

The metal-poor sub-population of globular cluster (GC) systems exhibits a correlation between the GC average colour and luminosity, especially in those systems associated with massive elliptical galaxies. More luminous (more massive) GCs are typically redder and hence more metal-rich. This 'blue tilt' is often interpreted as a mass-metallicity relation stemming from GC self-enrichment, whereby more massive GCs retain a greater fraction of the enriched gas ejected by their evolving stars, fostering the formation of more metal-rich secondary generations. We examine the E-MOSAICS simulations of the formation and evolution of galaxies and their GC populations, and find that their GCs exhibit a colour-luminosity relation similar to that observed in local galaxies, without the need to invoke mass-dependent self-enrichment. We find that the blue tilt is most appropriately interpreted as a dearth of massive, metal-poor GCs: the formation of massive GCs requires high interstellar gas surface densities, conditions that are most commonly fostered by the most massive, and hence most metal rich, galaxies, at the peak epoch of GC formation. The blue tilt is therefore a consequence of the intimate coupling between the small-scale physics of GC formation and the evolving properties of interstellar gas hosted by hierarchically-assembling galaxies