The distribution of stellar masses that form together, the initial mass function (IMF), is one of the most important astrophysical distribution functions. The determination of the IMF is a very difficult problem because stellar masses cannot be measured directly and because observations usually cannot assess all stars in a population requiring elaborate bias corrections. Nevertheless, impressive advances have been achieved during the last decade, such that the shape of the IMF is reasonably well understood from low-mass brown dwarfs (BDs) to very massive stars. The case can be made for a rather universal form that can be well approximated by a two-part power-law function in the stellar regime. However, there exists a possible hint for a systematic variation with metallicity. From very elaborate observational surveys a picture is emerging according to which the binary properties of very-low-mass stars (VLMSs) and BDs may be fundamentally different from those of late-type stars implying the probable existence of a discontinuity in the IMF, but the surveys also appear to suggest the number of BDs per star to be independent of the physical conditions of current Galactic star formation. Star-burst clusters and thus globular cluster may, however, have a much larger abundance of BDs. Very recent advances have allowed the measurement of the physical upper stellar mass limit, which also appears to be disconcertingly robust to variations in metallicity. Furthermore, it now appears that star clusters are formed in a rather organised fashion from lowto high stellar masses, such that the most-massive stars just forming terminate further star-formation within the particular cluster. Populations formed from many star clusters, composite populations, would then have steeper IMFs (fewer massive stars per low-mass star) than the simple populations in the constituent clusters. A near invariant star-cluster mass function implies the maximal cluster mass to correlate with the galaxy-wide star-formation rate. This then leads to the result that the composite-stellar IMFs vary in dependence of galaxy type, with potentially dramatic implications for theories of galaxy formation and evolution.The simple and composite IMF 5 30 Dor cluster (R136) in the LMC, NGC 3603 in the MW, and the Arches cluster near the Galactic centre. The 30 Dor star-burst cluster (

The origin of brown dwarfs (BDs) is still an unsolved mystery. While the standard model describes the formation of BDs and stars in a similar way recent data on the multiplicity properties of stars and BDs show them to have different binary distribution functions. Here we show that proper treatment of these uncovers a discontinuity of the multiplicity-corrected mass distribution in the very-low-mass star (VLMS) and BD mass regime. A continuous IMF can be discarded with extremely high confidence. This suggests that VLMSs and BDs on the one hand, and stars on the other, are two correlated but disjoint populations with different dynamical histories. The analysis presented here suggests that about one BD forms per five stars and that the BD-star binary fraction is about 2%-3% among stellar systems.

The dynamical stability of disk galaxies is sensitive to whether their anomalous rotation curves are caused by dark matter halos or Milgromian dynamics (MOND). We investigate this by setting up a MOND model of M33. We first simulate it in isolation for 6 Gyr, starting from an initial good match to the rotation curve (RC). Too large a bar and bulge form when the gas is too hot, but this is avoided by reducing the gas temperature. A strong bar still forms in 1 Gyr, but rapidly weakens and becomes consistent with the observed weak bar. Previous work showed this to be challenging in Newtonian models with a live dark matter halo, which developed strong bars. The bar pattern speed implies a realistic corotation radius of 3 kpc. However, the RC still rises too steeply, and the central line-of-sight velocity dispersion (LOSVD) is too high. We then add a constant external acceleration field of 8.4 × 10−12 m s−2 at 30° to the disk as a first-order estimate for the gravity exerted by M31. This suppresses buildup of material at the center, causing the RC to rise more slowly and reducing the central LOSVD. Overall, this simulation bears good resemblance to several global properties of M33, and highlights the importance of including even a weak external field on the stability and evolution of disk galaxies. Further simulations with a time-varying external field, modeling the full orbit of M33, will be needed to confirm its resemblance to observations.

Most stars are born in clusters and the resulting gravitational interactions between cluster members may significantly affect the evolution of circumstellar disks and therefore the formation of planets and brown dwarfs (BDs). Recent findings suggest that tidal perturbations of typical circumstellar disks due to close encounters may inhibit rather than trigger disk fragmentation and so would seem to rule out planet formation by external tidal stimuli. However, the disk models in these calculations were restricted to disk radii of 40 AU and disk masses below 0.1 M . Here, we show that even modest encounters can trigger fragmentation around 100 AU in the sorts of massive (∼0.5 M ), extended ( 100 AU) disks that are observed around young stars. Tidal perturbation alone can do this; no disk-disk collision is required. We also show that very low mass binary systems can form through the interaction of objects in the disk. In our computations, otherwise non-fragmenting massive disks, once perturbed, fragment into several objects between about 0.01 and 0.1 M , i.e., over the whole BD 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 BDs and very massive planets which, interestingly, leads to a mass distribution consistent with the canonical substellar initial mass function. As a minor outcome, a possible explanation for the origin of misaligned extrasolar planetary systems is discussed.

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, 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 on to 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.

Context. The Milky Way (MW) and Andromeda (M 31) galaxies possess rotating planes of satellites. Their formation has not been explained satisfactorily yet. It was suggested that the MW and M 31 satellites are ancient tidal dwarf galaxies, which could explain their configuration. This suggestion gained support by an analytic backward calculation of the relative MW-M 31 orbit in the MOND modified dynamics paradigm by Zhao et al. (2013) implying their close flyby 7-11 Gyr ago. Aims. Here we explore the Local Group history in MOND in more detail using a simplified first-ever self-consistent simulation. We note the features induced by the encounter in the simulation and identify their possible real counterparts. Methods. The initial conditions were set to eventually roughly reproduce the observed MW and M 31 masses, effective radii, separation, relative velocity and disk inclinations. We used the publicly available adaptive-mesh-refinement code Phantom of RAMSES. Results. Matter was transferred from the MW to M 31 along a tidal tail in the simulation. The encounter induced formation of several structures resembling the peculiarities of the Local Group. Most notably: 1) A rotating planar structure formed around M 31 from the transferred material. It had a size similar to the observed satellite plane and was oriented edge-on to the simulated MW, just as the real one. 2) The same structure also resembled the tidal features observed around M 31 by its size and morphology. 3) A warp in the MW developed with an amplitude and orientation similar to that observed. 4) A cloud of particles formed around the simulated MW, with the extent of the actual MW satellite system. The encounter did not end by merging in a Hubble time. The simulated stellar disks also thickened as a result of the encounter. Conclusions. The simulation demonstrated that MOND can possibly explain many peculiarities of the Local Group, which should be verified by future more elaborate simulations. The simulation moreover showed that tidal features observed in galaxies, usually interpreted as merger remnants, could have been formed by matter exchange during non-merging galactic flybys in some cases.

We present the adaptable MUESLI code for investigating dynamics and erosion processes of globular clusters (GCs) in galaxies. MUESLI follows the orbits of individual clusters and applies internal and external dissolution processes to them. Orbit integration is based on the self-consistent field method in combination with a timetransformed leapfrog scheme, allowing us to handle velocity-dependent forces like triaxial dynamical friction.In a first application, the erosion of globular cluster systems (GCSs) in elliptical galaxies is investigated. Observations show that massive ellipticals have rich, radially extended GCSs, while some compact dwarf ellipticals contain no GCs at all.For several representative examples, spanning the full mass scale of observed elliptical galaxies, we quantify the influence of radial anisotropy, galactic density profiles, SMBHs, and dynamical friction on the GC erosion rate. We find that GC number density profiles are centrally flattened in less than a Hubble time, naturally explaining observed cored GC distributions. The erosion rate depends primarily on a galaxy's mass, half-mass radius and radial anisotropy. The fraction of eroded GCs is nearly 100% in compact, M 32 like galaxies and lowest in extended and massive galaxies. Finally, we uncover the existence of a violent tidal disruption dominated phase which is important for the rapid build-up of halo stars.

When bars form within galaxy formation simulations in the standard cosmological context, dynamical friction with dark matter (DM) causes them to rotate rather slowly. However, almost all observed galactic bars are fast in terms of the ratio between corotation radius and bar length. Here, we explicitly display an 8σ tension between the observed distribution of this ratio and that in the EAGLE simulation at redshift 0. We also compare the evolution of Newtonian galactic discs embedded in DM haloes to their evolution in three extended gravity theories: Milgromian Dynamics (MOND), a model of non-local gravity, and a scalar-tensor-vector gravity theory (MOG). Although our models start with the same initial baryonic distribution and rotation curve, the long-term evolution is different. The bar instability happens more violently in MOND compared to the other models. There are some common features between the extended gravity models, in particular the negligible role played by dynamical friction − which plays a key role in the DM model. Partly for this reason, all extended gravity models predict weaker bars and faster bar pattern speeds compared to the DM case. Although the absence of strong bars in our idealized, isolated extended gravity simulations is in tension with observations, they reproduce the strong observational preference for ‘fast’ bar pattern speeds, which we could not do with DM. We confirm previous findings that apparently ‘ultrafast’ bars can be due to bar-spiral arm alignment leading to an overestimated bar length, especially in extended gravity scenarios where the bar is already fast.