In the context of the ESO-VLT Multi-Instrument Kinematic Survey (MIKiS) of Galactic globular clusters (GGCs), we present the line-of-sight rotation curve and velocity dispersion profile of M5 (NGC ...5904), as determined from the radial velocity of more than 800 individual stars observed out to 700″ (∼5 half-mass radii) from the center. We found one of the cleanest and most coherent rotation patterns ever observed for globular clusters, with a very stable rotation axis (having constant position angle of 145° at all surveyed radii) and a well-defined rotation curve. The density distribution turns out to be flattened in the direction perpendicular to the rotation axis, with a maximum ellipticity of ∼0.15. The rotation velocity peak (∼3 km s−1 in projection) is observed at ∼0.6 half-mass radii, and its ratio with respect to the central velocity dispersion (∼0.3–0.4 at 4 projected half-mass radii) indicates that ordered motions play a significant dynamical role. This result strengthens the growing empirical evidence of the kinematic complexity of GGCs and motivates the need of fundamental investigations of the role of angular momentum in collisional stellar dynamics.
Abstract
We present the chemical analysis of 49 giant stars of the globular cluster NGC 2419, using medium resolution spectra collected with the multi-object spectrograph DEIMOS@Keck. Previous ...analysis of this cluster revealed a large dispersion in the line strength of the infrared Ca ii triplet, suggesting an intrinsic star-to-star scatter in its Fe or Ca content. From our analysis, we assess that all the investigated stars share the same Fe/H, Ca/Fe and Ti/Fe abundance ratios, while a large spread in Mg and K abundances is detected. The distribution of Mg/Fe is bimodal, with ∼40 per cent of the observed targets having subsolar Mg/Fe, down to Mg/Fe ∼ −1 dex, a level of Mg deficiency never observed before in globular clusters. It is found that the large dispersion in Mg abundances is likely the main origin of the observed dispersion of the Ca ii triplet lines strengths (that can be erroneously interpreted in terms of Fe or Ca abundance scatter) because Mg plays a relevant role in the atmosphere of giant stars as an electron donor. A strong depletion in the Mg abundance leads to an increase of the line strength of the Ca ii triplet, due to the variation in the electronic pressure, at a constant Fe and Ca abundance. Finally, we detect an anti-correlation between Mg and K abundances, not easily explainable within the framework of the current nucleosynthesis models.
We present new determinations of the iron abundance for 220 stars belonging to the stellar system Terzan 5 in the Galactic bulge. The spectra have been acquired with FLAMES at the Very Large ...Telescope of the European Southern Observatory and DEIMOS at the Keck II Telescope. This is by far the largest spectroscopic sample of stars ever observed in this stellar system. From this data set, a subsample of targets with spectra unaffected by TiO bands was extracted and statistically decontaminated from field stars. Once combined with 34 additional stars previously published by our group, a total sample of 135 member stars covering the entire radial extent of the system has been used to determine the metallicity distribution function of Terzan 5. The iron distribution clearly shows three peaks: a super-solar component at Fe/H Asymptotically = to 0.25 dex, accounting for ~29% of the sample, a dominant sub-solar population at Fe/H Asymptotically = to -0.30 dex, corresponding to ~62% of the total, and a minor (6%) metal-poor component at Fe/H Asymptotically = to -0.8 dex. Such a broad, multi-modal metallicity distribution demonstrates that Terzan 5 is not a genuine globular cluster but the remnant of a much more complex stellar system.
ABSTRACT We used a sample of 25 Galactic globular clusters to empirically measure the parameter A+ recently introduced by Alessandrini et al., and defined as the area enclosed between the cumulative ...radial distribution of blue straggler stars (BSSs) and that of a reference population. Based on N-body simulations, this parameter is expected to efficiently measure the level of BSS central segregation. Observationally, for a proper cluster-to-cluster comparison we use , i.e., the value of the parameter determined out to the half-mass radius in each system. We find that nicely correlates with the position of the minimum of the BSS normalized radial distribution and with the cluster central relaxation time. This demonstrates that it is a sensitive indicator of the cluster dynamical age as traced by the spatial segregation of the BSS population. In the context of the "stellar system dynamical clock," this parameter provides a new clock-hand, which is easier to determine observationally and allows a more robust measure of the cluster dynamical age.
Abstract
We present a detailed analysis of the kinematic properties of the multiple populations (MPs) in the low-mass Galactic globular cluster (GC) NGC 6362 based on a sample of about 500 member ...stars for which radial velocities (RVs), and Fe and Na abundances have been homogeneously derived. At distances from the cluster center larger than about 0.5
r
h
, we find that first-generation (FG–Na-poor) and second-generation (SG–Na-rich) stars show hints of different line-of-sight velocity dispersion profiles, with FG stars being dynamically hotter. This is the first time that differences in the velocity dispersion of MPs are detected using only RVs. While kinematic differences between MPs in GCs are usually described in terms of anisotropy differences driven by the different radial distributions, this explanation hardly seems viable for NGC 6362, where SG and FG stars are spatially mixed. We demonstrate that the observed difference in the velocity dispersion profiles can be accounted for by the effect of binary stars. In fact, thanks to our multi-epoch RV measurements, we find that the binary fraction is significantly larger in the FG sample (
f
∼ 14%) than in the SG population (
f
< 1%), and we show that such a difference can inflate the velocity dispersion of FG with respect to SG by the observed amount in the relevant radial range. Our results nicely match the predictions of state-of-the art
N
-body simulations of the co-evolution of MPs in GCs that include the effects of binaries.
We started a photometric survey using the WFC3/UVIS instrument onboard the Hubble Space Telescope to search for multiple populations within Magellanic Cloud star clusters at various ages. In this ...paper, we introduce this survey. As first results of this programme, we also present multiband photometric observations of NGC 121 in different filters taken with the WFC3/UVIS and ACS/WFC instruments. We analyse the colour-magnitude diagram (CMD) of NGC 121, which is the only 'classical' globular cluster within the Small Magellanic Cloud. Thereby, we use the pseudo-colour C sub( F336W,F438W,F343N) = (F336W - F438W) - (F438W - F343N) to separate populations with different C and N abundances. We show that the red giant branch splits up in two distinct populations when using this colour combination. NGC 121 thus appears to be similar to Galactic globular clusters in hosting multiple populations. The fraction of enriched stars (N rich, C poor) in NGC 121 is about 32 per cent plus or minus 3 per cent, which is lower than the median fraction found in Milky Way globular clusters. The enriched population seems to be more centrally concentrated compared to the primordial one. These results are consistent with the recent results by Dalessandro et al. The morphology of the horizontal branch in a CMD using the optical filters F555W and F814W is best produced by a population with a spread in helium of ...Y = 0.025 plus or minus 0.005. (ProQuest: ... denotes formulae/symbols omitted.)
It is now well established that globular clusters (GCs) exhibit star-to-star light-element abundance variations (known as multiple populations, MPs). Such chemical anomalies have been found in ...(nearly) all the ancient GCs (more than 10 Gyr old) of our Galaxy and its close companions, but so far no model for the origin of MPs is able to reproduce all the relevant observations. To gain new insights into this phenomenon, we have undertaken a photometric Hubble Space Telescope survey to study clusters with masses comparable to that of old GCs, where MPs have been identified, but with significantly younger ages. Nine clusters in the Magellanic Clouds with ages between similar to 1.5 and 11 Gyr have been targeted in this survey. We confirm the presence of MPs in all clusters older than 6 Gyr and we add NGC 1978 to the group of clusters for which MPs have been identified. With an age of similar to 2 Gyr, NGC 1978 is the youngest cluster known to host chemical abundance spreads found to date. We do not detect evident star-to-star variations for slightly younger massive clusters (similar to 1.7 Gyr), thus pointing towards an unexpected age dependence for the onset of MPs. This discovery suggests that the formation of MPs is not restricted to the early Universe and that GCs and young massive clusters share common formation and evolutionary processes.
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.
We used an appropriate combination of high-resolution Hubble Space Telescope observations and wide-field, ground-based data to derive the radial stellar density profiles of 26 Galactic globular ...clusters from resolved star counts (which can be all freely downloaded on-line). With respect to surface brightness (SB) profiles (which can be biased by the presence of sparse, bright stars), star counts are considered to be the most robust and reliable tool to derive cluster structural parameters. For each system, a detailed comparison with both King and Wilson models has been performed and the most relevant best-fit parameters have been obtained. This collection of data represents the largest homogeneous catalog collected so far of star count profiles and structural parameters derived therefrom. The analysis of the data of our catalog has shown that (1) the presence of the central cusps previously detected in the SB profiles of NGC 1851, M13, and M62 is not confirmed; (2) the majority of clusters in our sample are fit equally well by the King and the Wilson models; (3) we confirm the known relationship between cluster size (as measured by the effective radius) and galactocentric distance; (4) the ratio between the core and the effective radii shows a bimodal distribution, with a peak at ~0.3 for about 80% of the clusters and a secondary peak at ~0.6 for the remaining 20%. Interestingly, the main peak turns out to be in agreement with that expected from simulations of cluster dynamical evolution and the ratio between these two radii correlates well with an empirical dynamical-age indicator recently defined from the observed shape of blue straggler star radial distribution, thus suggesting that no exotic mechanisms of energy generation are needed in the cores of the analyzed clusters.
We have collected UVES-FLAMES high-resolution spectra for a sample of 6 asymptotic giant branch (AGB) and 13 red giant branch (RGB) stars in the Galactic globular cluster (GC) M62 (NGC 6266). Here we ...present the detailed abundance analysis of iron, titanium, and light elements (O, Na, Mg, and Al). For the majority (five out of six) of the AGB targets, we find that the abundances of both iron and titanium determined from neutral lines are significantly underestimated with respect to those obtained from ionized features, the latter being, instead, in agreement with those measured for the RGB targets. This is similar to recent findings in other clusters and may suggest the presence of nonlocal thermodynamic equilibrium (NLTE) effects. In the O-Na, Al-Mg, and Na-Al planes, the RGB stars show the typical correlations observed for GC stars. Instead, all the AGB targets are clumped in the regions where first-generation stars are expected to lie, similar to what was recently found for the AGB population of NGC 6752. While the sodium and aluminum abundances could be underestimated as a consequence of the NLTE bias affecting iron and titanium, the oxygen line used does not suffer from the same effects, and the lack of O-poor AGB stars therefore is a solid result. We can thus conclude that none of the investigated AGB stars belongs to the second stellar generation of M62. We also find an RGB star with extremely high sodium abundance (Na/Fe = +1.08 dex).
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