The mean surface density of companions in a stellar-dynamical context

Applying the mean surface density of companions, Sigma(r), to the dynamical evolution of star clusters is an interesting approach to quantifying structural changes in a cluster. It has the advantage that the entire density structure, ranging from the closest binary separations, over the core-halo structure through to the density distribution in moving groups that originate from clusters, can be analysed coherently as one function of the stellar separations r. This contribution assesses the evolution of Sigma(r) for clusters with different initial densities and binary populations. The changes in the binary, cluster and halo branches as the clusters evolve are documented using direct N-body calculations, and are correlated with the cluster core and half-mass radius. The location of breaks in the slope of Sigma(r) and the possible occurrence of a binary gap can be used to infer dynamical cluster properties.Comment: 12 pages including 7 figures, accepted for publication in A&

The mass-metallicity relation of tidal dwarf galaxies

Dwarf galaxies generally follow a mass-metallicity (MZ) relation, where more massive objects retain a larger fraction of heavy elements. Young tidal dwarf galaxies (TDGs), born in the tidal tails produced by interacting gas-rich galaxies, have been thought to not follow the MZ relation, because they inherit the metallicity of the more massive parent galaxies. We present chemical evolution models to investigate if TDGs that formed at very high redshifts, where the metallicity of their parent galaxy was very low, can produce the observed MZ relation. Assuming that galaxy interactions were more frequent in the denser high-redshift universe, TDGs could constitute an important contribution to the dwarf galaxy population. The survey of chemical evolution models of TDGs presented here captures for the first time an initial mass function (IMF) of stars that is dependent on both the star formation rate and the gas metallicity via the integrated galactic IMF (IGIMF) theory. As TDGs form in the tidal debris of interacting galaxies, the pre-enrichment of the gas, an underlying pre-existing stellar population, infall, and mass dependent outflows are considered. The models of young TDGs that are created in strongly pre-enriched tidal arms with a pre-existing stellar population can explain the measured abundance ratios of observed TDGs. The same chemical evolution models for TDGs, that form out of gas with initially very low metallicity, naturally build up the observed MZ relation. The modelled chemical composition of ancient TDGs is therefore consistent with the observed MZ relation of satellite galaxies.Comment: 7 pages, 3 figures, MNRAS accepte

On the origin of the distribution of binary-star periods

Pre-main sequence and main-sequence binary systems are observed to have periods, P, ranging from one day to 10^(10) days and eccentricities, e, ranging from 0 to 1. We pose the problem if stellar-dynamical interactions in very young and compact star clusters may broaden an initially narrow period distribution to the observed width. N-body computations of extremely compact clusters containing 100 and 1000 stars initially in equilibrium and in cold collapse are preformed. In all cases the assumed initial period distribution is uniform in the narrow range 4.5 < log10(P) < 5.5 (P in days) which straddles the maximum in the observed period distribution of late-type Galactic-field dwarf systems. None of the models lead to the necessary broadening of the period distribution, despite our adopted extreme conditions that favour binary--binary interactions. Stellar-dynamical interactions in embedded clusters thus cannot, under any circumstances, widen the period distribution sufficiently. The wide range of orbital periods of very young and old binary systems is therefore a result of cloud fragmentation and immediate subsequent magneto-hydrodynamical processes operating within the multiple proto-stellar system.Comment: 11 pages, 4 figures, ApJ, in pres

Limits on the primordial stellar multiplicity

Most stars - especially young stars - are observed to be in multiple systems. Dynamical evolution is unable to pair stars efficiently, which leads to the conclusion that star-forming cores must usually fragment into \geq 2 stars. However, the dynamical decay of systems with \geq 3 or 4 stars would result in a large single-star population that is not seen in the young stellar population. Additionally, ejections would produce a significant population of hard binaries that are not observed. This leads to a strong constraint on star formation theories that cores must typically produce only 2 or 3 stars. This conclusion is in sharp disagreement with the results of currently available numerical simulations that follow the fragmentation of molecular cores and typically predict the formation of 5--10 seeds per core. In addition, open cluster remnants may account for the majority of observed highly hierarchical higher-order multiple systems in the field.Comment: A&A in press, 5 pages (no figures

The rotationally stabilized VPOS and predicted proper motions of the Milky Way satellite galaxies

The satellite galaxies of the Milky Way (MW) define a vast polar structure (VPOS), a thin plane perpendicular to the MW disc. Proper motion (PM) measurements are now available for all of the 11 brightest, `classical' satellites and allow an updated analysis of the alignment of their orbital poles with this spatial structure. The coherent orbital alignment of 7 to 9 out of 11 satellites demonstrates that the VPOS is a rotationally stabilized structure and not only a pressure-supported, flattened ellipsoid. This allows us to empirically and model independently predict the PMs of almost all satellite galaxies by assuming that the MW satellite galaxies orbit within the VPOS. As a test of our method, the predictions are best met by satellites whose PMs are already well constrained, as expected because more uncertain measurements tend to deviate more from the true values. Improved and new PM measurements will further test these predictions. A strong alignment of the satellite galaxy orbital poles is not expected in dark matter based simulations of galaxy formation. Coherent orbital directions of satellite galaxies are, however, a natural consequence of tidal dwarf galaxies formed together in the debris of a galaxy collision. The orbital poles of the MW satellite galaxies therefore lend further support to tidal scenarios for the origin of the VPOS and are a very significant challenge for the standard LCDM model of cosmology. We also note that the dependence of the MW satellite speeds on Galactocentric distance appear to map an effective potential with a constant velocity of approximately 240 km/s to about 250 kpc. The individual satellite velocities are only mildly radial.Comment: 17 pages, 8 figures, 4 tables, accepted for publication in MNRA

Evidence for the Strong Effect of Gas Removal on the Internal Dynamics of Young Stellar Clusters

We present detailed luminosity profiles of the young massive clusters M82-F, NGC 1569-A, and NGC 1705-1 which show significant departures from equilibrium (King and EFF) profiles. We compare these profiles with those from N-body simulations of clusters which have undergone the rapid removal of a significant fraction of their mass due to gas expulsion. We show that the observations and simulations agree very well with each other suggesting that these young clusters are undergoing violent relaxation and are also losing a significant fraction of their stellar mass. That these clusters are not in equilibrium can explain the discrepant mass-to-light ratios observed in many young clusters with respect to simple stellar population models without resorting to non-standard initial stellar mass functions as claimed for M82-F and NGC 1705-1. We also discuss the effect of rapid gas removal on the complete disruption of a large fraction of young massive clusters (``infant mortality''). Finally we note that even bound clusters may lose >50% of their initial stellar mass due to rapid gas loss (``infant weight-loss'').Comment: 6 pages, 3 figures, MNRAS letters, accepte

A discontinuity in the low-mass initial mass function

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.Comment: 14 pages, 11 figures, uses emulateapj.cls. Minor corrections and 1 reference added after being accepted by the Ap

The Origin of the Arches Stellar Cluster Mass Function

We investigate the time evolution of the mass distribution of pre-stellar cores (PSCs) and their transition to the initial stellar mass function (IMF) in the central parts of a molecular cloud (MC) under the assumption that the coalescence of cores is important. Our aim is to explain the observed shallow IMF in dense stellar clusters such as the Arches cluster. The initial distributions of PSCs at various distances from the MC center are those of gravitationally unstable cores resulting from the gravo-turbulent fragmentation of the MC. As time evolves, there is a competition between the PSCs rates of coalescence and collapse. Whenever the local rate of collapse is larger than the rate of coalescence in a given mass bin, cores are collapsed into stars. With appropriate parameters, we find that the coalescence-collapse model reproduces very well all the observed characteristics of the Arches stellar cluster IMF; Namely, the slopes at high and low mass ends and the peculiar bump observed at ~5-6 M_sol. Our results suggest that today's IMF of the Arches cluster is very similar to the primordial one and is prior to the dynamical effects of mass segregation becoming importantComment: 5 pages, 2 figures, accepted to MNRAS Letter