We present the results of the analysis of deep photometric data for a sample of three Galactic globular clusters (NGC5466, NGC6218 and NGC 6981) with the aim of estimating their degree of mass ...segregation and testing the predictions of analytic dynamical models. The adopted data set, composed of both Hubble Space Telescope and ground-based data, reaches the low-mass end of the mass functions of these clusters from the centre up to their tidal radii allowing us to derive the radial distribution of stars with different masses. All the analysed clusters show evidence of mass segregation with the most massive stars being more concentrated than the low-mass ones. The structures of NGC5466 and NGC6981 are well reproduced by multimass dynamical models adopting a lowered Maxwellian distribution function and the prescription for mass segregation given by Gunn & Griffin. Instead, NGC6218 appears to be more mass segregated than model predictions. By applying the same technique to mock observations derived from snapshots selected from suitable N-body simulations, we show that the deviation from the behaviour predicted by these models depends on the particular stage of dynamical evolution regardless of initial conditions.
Abstract
As part of the ESO-VLT Multi-Instrument Kinematic Survey of Galactic globular clusters (GCs), we present a detailed investigation of the internal kinematics of NGC 5986. The analysis is ...based on about 300 individual radial velocities of stars located at various distances from the cluster center, up to 300″ (about four half-mass radii). Our analysis reveals the presence of a solid-body rotation extending from the cluster center to the outermost regions probed by the data, and a velocity dispersion profile initially declining with the distance from the cluster’s center, but flattening and staying constant at ∼5 km s
−1
for distances larger than about one half-mass radius. This is the first GC for which evidence of the joint presence of solid-body rotation and flattening in the outer velocity dispersion profile has been found. The combination of these two kinematical features provides a unique opportunity to shed light on fundamental aspects of GC dynamics and to probe the extent to which internal relaxation, star escape, angular momentum transport and loss, and the interaction with the Galaxy tidal field can affect a cluster’s dynamical evolution and determine its current kinematical properties. We present the results of a series of
N
-body simulations illustrating the possible dynamical paths leading to kinematic features like those observed in this cluster and the fundamental dynamical processes that underpin them.
The structural and dynamical properties of star clusters are generally derived by means of the comparison between steady-state analytic models and the available observables. With the aim of studying ...the biases of this approach, we fitted different analytic models to simulated observations obtained from a suite of direct N-body simulations of star clusters in different stages of their evolution and under different levels of tidal stress to derive mass, mass function and degree of anisotropy. We find that masses can be under/overestimated up to 50 per cent depending on the degree of relaxation reached by the cluster, the available range of observed masses and distances of radial velocity measures from the cluster centre and the strength of the tidal field. The mass function slope appears to be better constrainable and less sensitive to model inadequacies unless strongly dynamically evolved clusters and a non-optimal location of the measured luminosity function are considered. The degree and the characteristics of the anisotropy developed in the N-body simulations are not adequately reproduced by popular analytic models and can be detected only if accurate proper motions are available. We show how to reduce the uncertainties in the mass, mass function and anisotropy estimation and provide predictions for the improvements expected when Gaia proper motions will be available in the near future.
Massive globular clusters (GCs) contain at least two generations of stars with slightly different ages and clearly distinct light element abundances. The Na-O anticorrelation is the best studied ...chemical signature of multiple stellar generations. Instead, low-mass clusters usually appear to be chemically homogeneous. We are investigating low-mass GCs to understand what the lower mass limit is where multiple populations can form, mainly using the Na and O abundance distribution. We used VLT/FLAMES spectra of giants in the low-mass, metal-poor GC Terzan 8 that belongs to the Sagittarius dwarf galaxy to determine abundances of Fe, O, Na, α-, Fe-peak, and neutron-capture elements in six stars observed with UVES and 14 observed with GIRAFFE. The average metallicity is Fe/H = −2.27 ± 0.03 (rms = 0.08), based on the six high-resolution UVES spectra. Only one star, observed with GIRAFFE, shows an enhanced abundance of Na and we tentatively assign it to the second generation. In this cluster, unlike what happens in more massive GCs, the second generation seems to represent at most a minority fraction. We discuss the implications of our findings, comparing Terzan 8 with the other Sgr dSph GCs, and to GCs and field stars in the Large Magellanic Cloud, Fornax, and in other dwarfs galaxies.
We use recently published measurements of the kinematics, surface brightness and stellar mass-to-light ratio of the globular cluster NGC 2419 to examine the possibility that this Galactic halo ...satellite is embedded in a low-mass dark matter halo. NGC 2419 is a promising target for such a study, since its extreme Galactocentric distance and large mass would have greatly facilitated the retention of dark matter. A Markov chain Monte Carlo approach is used to investigate composite dynamical models containing a stellar and a dark matter component. We find that it is unlikely that a significant amount of dark matter ( 6 per cent of the luminous mass inside the tidal limit of the cluster) can be present if the stars follow an anisotropic Michie model and the dark matter a double power-law model. However, we find that more general models, derived using a new technique we have developed to compute non-parametric solutions to the spherical Jeans equation, suggest the presence of a significant dark matter fraction (approximately twice the stellar mass). Thus, the presence of a dark matter halo around NGC 2419 cannot be fully ruled out at present, yet any dark matter within the 10 arcmin visible extent of the cluster must be highly concentrated and cannot exceed 1.1 × 106 M (99 per cent confidence), in stark contrast to expectations for a plausible progenitor halo of this structure.
Our FLAMES survey of Na-O anticorrelation in globular clusters (GCs) is extended to NGC 4833, a metal-poor GC with a long blue tail on the horizontal branch (HB). We present the abundance analysis ...for a large sample of 78 red giants based on UVES and GIRAFFE spectra acquired at the ESO-VLT. We derived abundances of Na, O, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Ba, La, and Nd. This is the first extensive study of this cluster from high resolution spectroscopy. On the scale of our survey, the metallicity of NGC 4833 is Fe/H = −2.015 ± 0.004 ± 0.084 dex (rms = 0.014 dex) from 12 stars observed with UVES, where the first error is from statistics and the second one refers to the systematic effects. The iron abundance in NGC 4833 is homogeneous at better than 6%. On the other hand, the light elements involved in proton-capture reactions at high temperature show the large star-to-star variations observed in almost all GCs studied so far. The Na-O anticorrelation in NGC 4833 is quite extended, as expected from the high temperatures reached by stars on the HB, and NGC 4833 contains a conspicuous fraction of stars with extreme O/Na ratios. More striking is the finding that large star-to-star variations are also seen for Mg, which spans a range of more than 0.5 dex in this GC. Depletions in Mg are correlated to the abundances of O and anti-correlated with Na, Al, and Si abundances. This pattern suggests the action of nuclear processing at unusually high temperatures, producing the extreme chemistry observed in the stellar generations of NGC 4833. These extreme changes are also seen in giants of the much more massive GCs M 54 and ω Cen, and our conclusion is that NGC 4833 has probably lost a conspicuous fraction of its original mass due to bulge shocking, as also indicated by its orbit.
Binary fraction and orbital characteristics provide indications on the conditions of star formation, as they shed light on the environment they were born in. Multiple systems are more common in low ...density environments than in higher density environments. In the current debate about the formation of globular clusters and their multiple populations, studying the binary incidence in the populations they host offers a crucial piece of information on the environment of their birth and their subsequent dynamical evolution. Through a multiyear observational campaign using FLAMES at VLT, we monitored the radial velocity of 968 red-giant-branch stars located around the half-light radii in a sample of ten Galactic globular clusters. We found a total of 21 radial velocity variables identified as bona fide binary stars, for a binary fraction of 2.2% ± 0.5%. When separating the sample into first generation and second generation stars, we find a binary fraction of 4.9% ± 1.3% and 1.2% ± 0.4%, respectively. Through simulations that take possible sources of bias into account in detecting radial velocity variations in the two populations, we show that the difference is significant and only marginally affected by these effects. This kind of different binary fraction strongly suggests different conditions in the environment of formation and evolution of first and second generations stars, with the latter being born in a much denser environment. Our result hence strongly supports the idea that the second generation forms in a dense subsystem at the center of the loosely distributed first generation, where (loose) binaries are efficiently destroyed.
We present an observational estimate of the fraction and distribution of dark mass in the innermost region of the two Galactic globular clusters NGC 6218 (M12) and NGC 288. Such an assessment has ...been made by comparing the dynamical and luminous mass profiles derived from an accurate analysis of the most extensive spectroscopic and photometric surveys performed on these stellar systems. We find that non-luminous matter constitutes more than 60 per cent of the total mass in the region probed by our data (R < 1.6 arcmin ∼ r
h) in both clusters. We have carefully analysed the effects of binaries and tidal heating on our estimate and ruled out the possibility that our result is a spurious consequence of these effects. The dark component appears to be more concentrated than the most massive stars suggesting that it is likely composed of dark remnants segregated in the cluster core.
We present the results of a comprehensive analysis of the structure and kinematics of six Galactic globular clusters. By comparing the results of the most extensive photometric and kinematical ...surveys available to date with suitable dynamical models, we determine the stellar and dynamical masses of these stellar systems taking into account the effect of mass segregation, anisotropy, and unresolved binaries. We show that the stellar masses of these clusters are on average smaller than those predicted by canonical integrated stellar evolution models because of the shallower slopes of their mass functions. The derived stellar masses are found to be also systematically smaller than the dynamical masses by ~40%, although the presence of systematics affecting our estimates cannot be excluded. If confirmed, this evidence can be linked to an increased fraction of retained dark remnants or to the presence of a modest amount of dark matter.
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