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Globular clusters exhibit abundance variations, defining `multiple populations', which have prompted a protracted search for their origin. Properties requiring explanation include: the high fraction of polluted stars ($\sim 40{-}90$~percent, correlated with cluster mass), the absence of pollution in young clusters and the lower pollution rate with binarity and distance from the cluster centre. We present a novel mechanism for late delivery of pollutants into stars via accretion of sub-stellar companions. In this scenario, stars move through a medium polluted with AGB and massive star ejecta, accreting material to produce companions with typical mass ratio $q\sim 0.1$. These companions undergo eccentricity excitation due to dynamical perturbations by passing stars, culminating in a merger with their host star. The accretion of the companion alters surface abundances via injected pollutant. Alongside other self-enrichment models, the companion accretion model can explain the dilution of pollutant and correlation with intra-cluster location. The model also explains the ubiquity and discreteness of the populations and correlations of enrichment rates with cluster mass, cluster age and stellar binarity. Abundance variations in some clusters can be broadly reproduced using AGB and massive binary ejecta abundances from the literature. In other clusters, some high companion mass ratios ($q\gtrsim 1$) are required. In these cases, the available mass budget necessitates a variable degree of mixing of the polluted material with the primary star, deviations from model ejecta abundances or mixing of internal burning products. We highlight the avenues of further investigation which are required to explore some of the key processes invoked in this model., Comment: 29 pages, 20 figures, accepted for publication in MNRAS

A recent survey of the inner $0.35\times0.35$pc of the NGC 2024 star forming region revealed two distinct millimetre continuum disc populations that appear to be spatially segregated by the boundary of a dense cloud. The eastern (and more embedded) population is $\sim0.2-0.5$Myr old, with an ALMA mm continuum disc detection rate of about $45\,$per cent. However this drops to only $\sim15$per cent in the 1Myr western population. When presenting this result, van Terwisga et al. (2020) suggested that the two main UV sources, IRS 1 (a B0.5V star in the western region) and IRS 2b (an O8V star in the eastern region, but embedded) have both been evaporating the discs in the depleted western population. In this paper we report the firm discovery in archival HST data of 4 proplyds and 4 further candidate proplyds in NGC 2024, confirming that external photoevaporation of discs is occurring. However, the locations of these proplyds changes the picture. Only three of them are in the depleted western population and their evaporation is dominated by IRS 1, with no obvious impact from IRS 2b. The other 5 proplyds are in the younger eastern region and being evaporated by IRS 2b. We propose that both populations are subject to significant external photoevaporation, which happens throughout the region wherever discs are not sufficiently shielded by the interstellar medium. The external photoevaporation and severe depletion of mm grains in the 0.2-0.5Myr eastern part of NGC 2024 would be in competition even with very early planet formation., Comment: 14 pages, Accepted for publication in MNRAS

Tidal encounters in star clusters perturb discs around young protostars. In Cuello et al. (2019a, Paper I) we detailed the dynamical signatures of a stellar flyby in both gas and dust. Flybys produce warped discs, spirals with evolving pitch angles, increasing accretion rates, and disc truncation. Here we present the corresponding observational signatures of these features in optical/near-infrared scattered light and (sub-) millimeter continuum and CO line emission. Using representative prograde and retrograde encounters for direct comparison, we post-process hydrodynamical simulations with radiative transfer methods to generate a catalogue of multi-wavelength observations. This provides a reference to identify flybys in recent near-infrared and sub-millimetre observations (e.g., RW Aur, AS 205, HV Tau & DO Tau, FU Ori, V2775 Ori, and Z CMa)., 11 pages, 5 figures, accepted for publication in MNRAS

Protoplanetary discs (PPDs) in the Orion Nebula Cluster (ONC) are irradiated by UV fields from the massive star $��^1$C. This drives thermal winds, inducing mass loss rates of up to $\dot{M}_\mathrm{wind}\sim 10^{-7}\,M_\odot$/yr in the `proplyds' (ionised PPDs) close to the centre. For the mean age of the ONC and reasonable initial PPD masses, such mass loss rates imply that discs should have been dispersed. However, ~80% of stars still exhibit a NIR excess, suggesting that significant circumstellar mass remains. This `proplyd lifetime problem' has persisted since the discovery of photoevaporating discs in the core of the ONC by O'Dell & Wen (1994). In this work, we demonstrate how an extended period of star formation can solve this problem. Coupling N-body calculations and a viscous disc evolution model, we obtain high disc fractions at the present day. This is partly due to the migration of older stars outwards, and younger stars inwards such that the most strongly irradiated PPDs are also the youngest. We show how the disc mass distribution can be used to test the recent claims in the literature for multiple stellar populations in the ONC. Our model also explains the recent finding that host mass and PPD mass are only weakly correlated, in contrast with other regions of similar age. We conclude that the status of the ONC as the archetype for understanding the influence of environment on planet formation is undeserved; the complex star formation history (involving star formation episodes within ~0.8 Myr of the present day) results in confusing signatures in the PPD population., 16 pages, 11 figures, accepted for publication in MNRAS

Many stars form in regions of enhanced stellar density, wherein the influence of stellar neighbours can have a strong influence on a protoplanetary disc (PPD) population. In particular, far ultraviolet (FUV) flux from massive stars drives thermal winds from the outer edge of PPDs, accelerating disc destruction. In this work, we present a novel technique for constraining the dynamical history of a star forming environment using PPD properties in a strongly FUV irradiated environment. Applying recent models for FUV induced mass loss rates to the PPD population of Cygnus OB2, we constrain how long ago primordial gas was expelled from the region; $ 0.5$ Myr ago if the Shakura & Sunyaev $\alpha$-viscosity parameter is $\alpha = 10^{-2}$ (corresponding to a viscous timescale of $\tau_\mathrm{visc} \approx 0.5$ Myr for a disc of scale radius $40$ au around a $1\, M_\odot$ star). This value of $\alpha$ is effectively an upper limit, since it assumes efficient extinction of FUV photons throughout the embedded phase. With this gas expulsion timescale we are able to produce a full dynamical model that fits kinematic and morphological data as well as disc fractions. We suggest Cygnus OB2 was originally composed of distinct massive clumps or filaments, each with a stellar mass $\sim 10^4 \, M_\odot$. Finally we predict that in regions of efficient FUV induced mass loss, disc mass $M_\mathrm{disc}$ as a function of stellar host mass $m_\mathrm{star}$ follows a power law with $M_\mathrm{disc} \propto m_\mathrm{star}^\beta$, where $\beta \gtrsim 2.7$ (depending on disc initial conditions and FUV exposure). This is steeper than observed correlations in regions of moderate FUV flux ($1 < \beta, Comment: 20 pages, 17 figures, accepted by MNRAS

The stellar birth environment can significantly shorten protoplanetary disc (PPD) lifetimes due to the influence of stellar feedback mechanisms. The degree to which these mechanisms suppress the time and mass available for planet formation is dependent on the local far-ultraviolet (FUV) field strength, stellar density, and ISM properties. In this work, we present the first theoretical framework quantifying the distribution of PPD dispersal time-scales as a function of parameters that describe the galactic environment. We calculate the probability density function for FUV flux and stellar density in the solar neighbourhood. In agreement with previous studies, we find that external photoevaporation is the dominant environment-related factor influencing local stellar populations after the embedded phase. Applying our general prescription to the Central Molecular Zone of the Milky Way (i.e. the central ~250 pc), we predict that 90% of PPDs in the region are destroyed within 1 Myr of the dispersal of the parent molecular cloud. Even in such dense environments, we find that external photoevaporation is the dominant disc depletion mechanism over dynamical encounters between stars. PPDs around low-mass stars are particularly sensitive to FUV-induced mass loss, due to a shallower gravitational potential. For stars of mass ~1 $M_\odot$, the solar neighbourhood lies at approximately the highest gas surface density for which PPD dispersal is still relatively unaffected by external FUV photons, with a median PPD dispersal timescale of ~4 Myr. We highlight the key questions to be addressed to further contextualise the significance of the local galactic environment for planet formation., Comment: 22 pages, 12 figures; accepted for publication in MNRAS

The majority of stars form in a clustered environment. This has an impact on the evolution of surrounding protoplanetary discs (PPDs) due to either photoevaporation or tidal truncation. Consequently, the development of planets depends on formation environment. Here we present the first thorough investigation of tidally induced angular momentum loss in PPDs in the distant regime, partly motivated by claims in the literature for the importance of distant encounters in disc evolution. We employ both theoretical predictions and dynamical/hydrodynamical simulations in 2D and 3D. Our theoretical analysis is based on that of Ostriker (1994) and leads us to conclude that in the limit that the closest approach distance $x_{min} \gg r$, the radius of a particle ring, the fractional change in angular momentum scales as $(x_{min}/r)^{-5}$. This asymptotic limit ensures that the cumulative effect of distant encounters is minor in terms of its influence on disc evolution. The angular momentum transfer is dominated by the $m=2$ Lindblad resonance for closer encounters and by the $m=1$, $\omega = 0$ Lindblad resonance at large $x_{min}/r$. We contextualise these results by comparing expected angular momentum loss for the outer edge of a PPD due to distant and close encounters. Contrary to the suggestions of previous works we do not find that distant encounters contribute significantly to angular momentum loss in PPDs. We define an upper limit for closest approach distance where interactions are significant as a function of arbitrary host to perturber mass ratio $M_2/M_1$., Comment: 14 pages, 11 figures