We model the dynamical evolution of star forming regions with a wide range of initial properties. We follow the evolution of the regions substructure using the Q-parameter, we search for dynamical mass segregation using the Lambda_MSR technique, and
we also quantify the evolution of local density around stars as a function of mass using the Sigma_LDR method. The amount of dynamical mass segregation measured by Lambda_MSR is generally only significant for subvirial and virialised, substructured regions - which usually evolve to form bound clusters. The Sigma_LDR method shows that massive stars attain higher local densities than the median value in all regions, even those that are supervirial and evolve to form (unbound) associations. We also introduce the Q-Sigma_LDR plot, which describes the evolution of spatial structure as a function of mass-weighted local density in a star forming region. Initially dense (>1000 stars pc^{-2}), bound regions always have Q >1, Sigma_LDR > 2 after 5Myr, whereas dense unbound regions always have Q < 1, Sigma_LDR > 2 after 5Myr. Less dense regions (<100 stars pc^{-2}) do not usually exhibit Sigma_LDR > 2 values, and if relatively high local density around massive stars arises purely from dynamics, then the Q-Sigma_LDR plot can be used to estimate the initial density of a star forming region.
Whether or not the rich star cluster population in the Large Magellanic Cloud (LMC) is affected by significant disruption during the first few x 10^8 yr of its evolution is an open question and the subject of significant current debate. Here, we revi
sit the problem, adopting a homogeneous data set of broad-band imaging observations. We base our analysis mainly on two sets of self-consistently determined LMC cluster ages and masses, one using standard modelling and one which takes into account the effects of stochasticity in the clusters stellar mass functions. On their own, the results based on any of the three complementary analysis approaches applied here are merely indicative of the physical conditions governing the cluster population. However, the combination of our results from all three different diagnostics leaves little room for any conclusion other than that the optically selected LMC star cluster population exhibits no compelling evidence of significant disruption -- for clusters with masses, M_cl, of log(M_cl/M_sun) >= 3.0-3.5 -- between the age ranges of [3-10] Myr and [30-100] Myr, either infant mortality or otherwise. In fact, there is no evidence of any destruction beyond that expected from simple models just including stellar dynamics and stellar evolution for ages up to 1 Gyr. It seems, therefore, that the difference in environmental conditions in the Magellanic Clouds on the one hand and significantly more massive galaxies on the other may be the key to understanding the apparent variations in cluster disruption behaviour at early times.
Binary properties are usually expressed (for good observational reasons) as a function of primary mass. It has been found that the distribution of companion masses -- the mass ratio distribution -- is different for different primary masses. We argue
that system mass is the more fundamental physical parameter to use. We show that if system masses are drawn from a log-normal mass function, then the different observed mass ratio distributions as a function of primary mass, from M-dwarfs to A-stars, are all consistent with a universal, flat, system mass ratio distribution. We also show that the brown dwarf mass ratio distribution is not drawn from the same flat distribution, suggesting that the process which decides upon mass ratios is very different in brown dwarfs and stars.
Observations and theory suggest that star clusters can form in a subvirial (cool) state and are highly substructured. Such initial conditions have been proposed to explain the level of mass segregation in clusters through dynamics, and have also been
successful in explaining the origin of trapezium-like systems. In this paper we investigate, using N-body simulations, whether such a dynamical scenario is consistent with the observed binary properties in the Orion Nebula Cluster (ONC). We find that several different primordial binary populations are consistent with the overall fraction and separation distribution of visual binaries in the ONC (in the range 67 - 670 au), and that these binary systems are heavily processed. The substructured, cool-collapse scenario requires a primordial binary fraction approaching 100 per cent. We find that the most important factor in processing the primordial binaries is the initial level of substructure; a highly substructured cluster processes up to 20 per cent more systems than a less substructured cluster because of localised pockets of high stellar density in the substructure. Binaries are processed in the substructure before the cluster reaches its densest phase, suggesting that even clusters remaining in virial equilibrium or undergoing supervirial expansion would dynamically alter their primordial binary population. Therefore even some expanding associations may not preserve their primordial binary population.
Recent discoveries of strongly misaligned transiting exoplanets pose a challenge to the established planet formation theory which assumes planetary systems to form and evolve in isolation. However, the fact that the majority of stars actually do form
in star clusters raises the question how isolated forming planetary systems really are. Besides radiative and tidal forces the presence of dense gas aggregates in star-forming regions are potential sources for perturbations to protoplanetary discs or systems. Here we show that subsequent capture of gas from large extended accretion envelopes onto a passing star with a typical circumstellar disc can tilt the disc plane to retrograde orientation, naturally explaining the formation of strongly inclined planetary systems. Furthermore, the inner disc regions may become denser, and thus more prone to speedy coagulation and planet formation. Pre-existing planetary systems are compressed by gas inflows leading to a natural occurrence of close-in misaligned hot Jupiters and short-period eccentric planets. The likelihood of such events mainly depends on the gas content of the cluster and is thus expected to be highest in the youngest star clusters.
Most stars are born in clusters and the resulting gravitational interactions between cluster members may significantly affect the evolution of circumstellar discs and therefore the formation of planets and brown dwarfs. Recent findings suggest that t
idal perturbations of typical circumstellar discs due to close encounters may inhibit rather than trigger disc fragmentation and so would seem to rule out planet formation by external tidal stimuli. However, the disc models in these calculations were restricted to disc radii of 40 AU and disc masses below 0.1 M_sun. Here we show that even modest encounters can trigger fragmentation around 100 AU in the sorts of massive (~0.5 M_sun), extended (>=100 AU) discs that are observed around young stars. Tidal perturbation alone can do this, no disc-disc collision is required. We also show that very-low-mass binary systems can form through the interaction of objects in the disc. In our computations, otherwise non-fragmenting massive discs, once perturbed, fragment into several objects between about 0.01 and 0.1 M_sun, i.e., over the whole brown dwarf mass range. Typically these orbit on highly eccentric orbits or are even ejected. While probably not suitable for the formation of Jupiter- or Neptune-type planets, our scenario provides a possible formation mechanism for brown dwarfs and very massive planets which, interestingly, leads to a mass distribution consistent with the canonical substellar IMF. As a minor outcome, a possible explanation for the origin of misaligned extrasolar planetary systems is discussed.
The evolution of star clusters in the Magellanic Clouds has been the subject of significant recent controversy, particularly regarding the importance and length of the earliest, largely mass-independent disruption phase (referred to as infant mortali
ty). Here, we take a fresh approach to the problem, using a large, independent, and homogeneous data set of UBVR imaging observations, from which we obtain the cluster age and mass distributions in both the Large and Small Magelanic Clouds (LMC, SMC) in a self-consistent manner. We conclude that the (optically selected) SMC star cluster population has undergone at most ~30% (1sigma) infant mortality between the age range from about 3-10 Myr, to that of approximately 40-160 Myr. We rule out a 90% cluster mortality rate per decade of age (for the full age range up to 10^9 yr) at a >6sigma level. Using a simple approach, we derive a characteristic cluster disruption time-scale for the cluster population in the LMC that implies that we are observing the INITIAL cluster mass function. Preliminary results suggest that the LMC cluster population may be affected by <10% infant mortality.
Stars form from dense molecular cores, and the mass function of these cores (the CMF) is often found to be similar to the form of the stellar initial mass function (IMF). This suggests that the form of the IMF is the result of the form of the CMF. Ho
wever, most stars are thought to form in binary and multiple systems, therefore the relationship between the IMF and the CMF cannot be trivial. We test two star formation scenarios - one in which all stars form as binary or triple systems, and one in which low-mass stars form in a predominantly single mode. We show that from a log-normal CMF, similar to those observed, and expected on theoretical grounds, the model in which all stars form as multiples gives a better fit to the IMF.
The early evolution of star clusters in the Small Magellanic Cloud (SMC) has been the subject of significant recent controversy, particularly regarding the importance and length of the earliest, largely mass-independent disruption phase (referred to
as infant mortality). Here, we take a fresh approach to the problem, using an independent, homogeneous data set of UBVR imaging observations, from which we obtain the SMCs cluster age and mass distributions in a self-consistent manner. We conclude that the (optically selected) SMC star cluster population has undergone at most ~30 per cent (1 sigma) infant mortality between the age range from about (3-10) Myr, to that of approximately (40-160) Myr. We rule out a 90 per cent cluster mortality rate per decade of age (for the full age range up to 10^9 yr) at a > 6 sigma level. We independently affirm this scenario based on the age distribution of the SMC cluster sample.
Around 4% of O-stars are observed in apparent isolation, with no associated cluster, and no indication of having been ejected from a nearby cluster. We define an isolated O-star as a star > 17.5 M_odot in a cluster with total mass <100 M_odot which c
ontains no other massive (>10 M_odot) stars. We show that the fraction of apparently isolated O-stars is reproduced when stars are sampled (randomly) from a standard initial mass function and a standard cluster mass function of the form N(M) propto M^-2. This result is difficult to reconcile with the idea that there is a fundamental relationship between the mass of a cluster and the mass of the most massive star in that cluster. We suggest that such a relationship is a typical result of star formation in clusters, and that `isolated O-stars are low-mass clusters in which massive stars have been able to form.
