Search

Using spectra obtained with the VLT/FORS2 and Gemini-S/GMOS-S instruments, we have investigated carbon, nitrogen, and sodium abundances in a sample of red giant members of the Small Magellanic Cloud star cluster Kron 3. The metallicity and luminosity of the cluster are comparable to those of Galactic globular clusters but it is notably younger (age ≈ 6.5 Gyr). We have measured the strengths of the CN and CH molecular bands, finding a bimodal CN band-strength distribution and a CH/CN anticorrelation. Application of spectrum synthesis techniques reveals that the difference in the mean [N/Fe] and [C/Fe] values for the CN-strong and CN-weak stars are Δ <[N/Fe]> = 0.63 ± 0.16 dex and Δ <[C/Fe]> = −0.01 ± 0.07 dex after applying corrections for evolutionary mixing. We have also measured sodium abundances from the Na D lines finding an observed range in [Na/Fe] of ∼0.6 dex that correlates positively with the [N/Fe] values and a Δ <[Na/Fe]> = 0.12 ± 0.12 dex. While the statistical significance of the sodium abundance difference is not high, the observed correlation between the Na and N abundances supports its existence. The outcome represents the first star-by-star demonstration of correlated abundance variations involving sodium in an intermediate-age star cluster. The results add to existing photometric and spectroscopic indications of the presence of multiple populations in intermediate-age clusters with masses in excess of ∼105 M⊙. It confirms that the mechanism(s) responsible for the multiple populations in ancient globular clusters cannot solely be an early cosmological effect applying only in old clusters. [ABSTRACT FROM AUTHOR]

The origin of multiple stellar populations in globular clusters (GCs) is one of the greatest mysteries of modern stellar astrophysics. N-body simulations suggest that the present-day dynamics of GC stars can constrain the events that occurred at high redshift and led to the formation of multiple populations. Here, we combine multiband photometry from the Hubble Space Telescope (HST) and ground-based facilities with HST and Gaia Data Release 2 proper motions to investigate the spatial distributions and the motions in the plane of the sky of multiple populations in the Type II GCs NGC 5139 (ω Centauri) and NGC 6656 (M22). We first analyzed stellar populations with different metallicities. Fe-poor and Fe-rich stars in M22 share similar spatial distributions and rotation patterns and exhibit similar isotropic motions. Similarly, the two main populations with different iron abundance in ω Centauri share similar ellipticities and rotation patterns. When different radial regions are analyzed, we find that the rotation amplitude decreases from the center toward the external regions. Fe-poor and Fe-rich stars of ω Centauri are radially anisotropic in the central region and show similar degrees of anisotropy. We also investigate the stellar populations with different light-element abundances and find that their N-rich stars exhibit higher ellipticity than N-poor stars. In ω Centauri both stellar groups are radially anisotropic. Interestingly, N-rich, Fe-rich stars exhibit different rotation patterns than N-poor stars with similar metallicities. The stellar populations with different nitrogen of M22 exhibit similar rotation patterns and isotropic motions. We discuss these findings in the context of the formation of multiple populations. [ABSTRACT FROM AUTHOR]

Observed mass-to-light ratios (M / L) of metal-rich globular clusters (GCs) disagree with theoretical predictions. This discrepancy is of fundamental importance since stellar population models provide the stellar masses that underpin most of extragalactic astronomy, near and far. We have derived radial velocities for 1622 stars located in the centres of 59 Milky Way GCs – 12 of which have no previous kinematic information – using integral-field unit data from the WAGGS project. Using N -body models, we determine dynamical masses and M / L V for the studied clusters. Our sample includes NGC 6528 and NGC 6553, which extend the metallicity range of GCs with measured M / L up to [Fe/H] ∼ −0.1 dex. We find that metal-rich clusters have M / L V more than two times lower than what is predicted by simple stellar population models. This confirms that the discrepant M / L –[Fe/H] relation remains a serious concern. We explore how our findings relate to previous observations, and the potential causes for the divergence, which we conclude is most likely due to dynamical effects. [ABSTRACT FROM AUTHOR]

The 'chromosome map' diagram (ChM) proved a successful tool to identify and characterize multiple populations (MPs) in 59 Galactic globular clusters (GCs). Here, we construct ChMs for 11 GCs of both Magellanic Clouds (MCs) and with different ages to compare MPs in Galactic and extragalactic environments, and explore whether this phenomenon is universal through 'place' and 'time'. MPs are detected in five clusters. The fractions of 1G stars, ranging from ∼50 per cent to >80 per cent, are significantly higher than those observed in Galactic GCs with similar present-day masses. By considering both Galactic and MC clusters, the fraction of 1G stars exhibits: (i) a strong anticorrelation with the present-day mass, and (ii) with the present-day mass of 2G stars; (iii) a mild anticorrelation with 1G present-day mass. All Galactic clusters without MPs have initial masses smaller than ∼1.5 · 105 M⊙ but a mass threshold governing the occurrence of MPs seems challenged by massive simple-population MC GCs; (iv) Milky Way clusters with large perigalactic distances typically host larger fractions of 1G stars, but the difference disappears when we use initial cluster masses. These facts are consistent with a scenario where the stars lost by GCs mostly belong to the 1G. By exploiting recent work based on Gaia , half of the known Type II GCs appear clustered in a distinct region of the integral of motions space, thus suggesting a common progenitor galaxy. Except for these Type II GCs, we do not find any significant difference in the MPs between clusters associated with different progenitors. [ABSTRACT FROM AUTHOR]

We introduce the southern stellar stream spectroscopy survey (S 5), an on-going program to map the kinematics and chemistry of stellar streams in the southern hemisphere. The initial focus of S 5 has been spectroscopic observations of recently identified streams within the footprint of the dark energy survey (DES), with the eventual goal of surveying streams across the entire southern sky. Stellar streams are composed of material that has been tidally striped from dwarf galaxies and globular clusters and hence are excellent dynamical probes of the gravitational potential of the Milky Way, as well as providing a detailed snapshot of its accretion history. Observing with the 3.9 m Anglo-Australian Telescope's 2-degree-Field fibre positioner and AAOmega spectrograph, and combining the precise photometry of DES DR1 with the superb proper motions from Gaia DR2, allows us to conduct an efficient spectroscopic survey to map these stellar streams. So far S 5 has mapped nine DES streams and three streams outside of DES; the former are the first spectroscopic observations of these recently discovered streams. In addition to the stream survey, we use spare fibres to undertake a Milky Way halo survey and a low-redshift galaxy survey. This paper presents an overview of the S 5 program, describing the scientific motivation for the survey, target selection, observation strategy, data reduction, and survey validation. Finally, we describe early science results on stellar streams and Milky Way halo stars drawn from the survey. Updates on S 5, including future public data releases, can be found at http://s5collab.github.io. [ABSTRACT FROM AUTHOR]

The origin of the star-to-star abundance variations found for the light elements in Galactic globular clusters (GGCs) is not well understood, which is a significant problem for stellar astrophysics. While the light element abundance variations are very common in globular clusters, they are comparatively rare in the Galactic halo field population. However, little is known regarding the occurrence of the abundance anomalies in other environments such as that of dwarf spheroidal (dSph) galaxies. Consequently, we have investigated the anticorrelation and bimodality of CH- and CN-band strengths, which are markers of the abundance variations in GGCs, in the spectra of red giants in the Sculptor dwarf spheroidal galaxy. Using spectra at the Na D lines, informed by similar spectra for five GGCs (NGC 288, 1851, 6752, 6809, and 7099), we have also searched for any correlation between CN and Na in the Sculptor red giant sample. Our results indicate that variations analogous to those seen in GGCs are not present in our Sculptor sample. Instead, we find a weak positive correlation between CH and CN and no correlation between Na and CN. We also reveal a deficiency in [Na/Fe] for the Sculptor stars relative to the values in GGCs, a result that is consistent with the previous work for dSph galaxies. The outcomes reinforce the apparent need for a high stellar density environment to produce the light element abundance variations. [ABSTRACT FROM AUTHOR]

Although the globular clusters in the Milky Way have been studied for a long time, a significant fraction of them lack homogeneous metallicity and radial velocity measurements. In an earlier paper we presented the first part of a project to obtain metallicities and radial velocities of Galactic globular clusters from multiobject spectroscopy of their member stars using the ESO Very Large Telescope. In this paper we add metallicities and radial velocities for a new sample of 28 globular clusters, including in particular globular clusters in the MW halo and the Galactic bulge. Together with our previous results, this study brings the number of globular clusters with homogeneous measurements to ∼69% of those listed in the Harris' catalogue. As in our previous work, we have used the Ca II triplet lines to derive metallicities and radial velocities. For most of the clusters in this study, this is the first analysis based on spectroscopy of individual member stars. The metallicities derived from the Ca II triplet are then compared to the results of our parallel study based on spectral fitting in the optical region and the implications for different calibrations of the Ca II triplet line strengths are discussed. We also comment on some interesting clusters and investigate the presence of an abundance spread in the globular clusters here. A hint of a possible intrinsic spread is found for NGC 6256, which therefore appears to be a good candidate for further study. [ABSTRACT FROM AUTHOR]

We present a second set of results from a wide-field photometric survey of the environs of Milky Way globular clusters. The clusters studied are NGC 1261, NGC 1851 and NGC 5824: all have data from the Dark Energy Camera on the Blanco 4 m telescope. NGC 5824 also has data from the Magellan Clay telescope with MegaCam. We confirm the existence of a large diffuse stellar envelope surrounding NGC 1851 of size at least 240 pc in radius. The radial density profile of the envelope follows a power-law decline with index γ =-1.5 ± 0.2 and the projected shape is slightly elliptical. For NGC 5824, there is no strong detection of a diffuse stellar envelope, but we find the cluster is remarkably extended and is similar in size (at least 230 pc in radius) to the envelope of NGC 1851. A stellar envelope is also revealed around NGC 1261. However, it is notably smaller in size with radius ∼105 pc. The radial density profile of the envelope is also much steeper with γ =-3.8 ± 0.2. We discuss the possible nature of the diffuse stellar envelopes, but are unable to draw definitive conclusions based on the current data. NGC 1851, and potentially NGC 5824, could be stripped dwarf galaxy nuclei, akin to the cases of ω Cen, M54 and M2. On the other hand, the different characteristics of the NGC 1261 envelope suggest that it may be the product of dynamical evolution of the cluster. [ABSTRACT FROM AUTHOR]

We present the results of a wide-field imaging survey of the periphery of the Milky Way globular cluster NGC 7089 (M2). Data were obtained with MegaCam on the Magellan Clay Telescope and the Dark Energy Camera on the Blanco Telescope.We find that M2 is embedded in a diffuse stellar envelope extending to a radial distance of at least ~60 arcmin (~210 pc) - five times the nominal tidal radius of the cluster. The envelope appears nearly circular in shape, has a radial density decline well described by a power law of index γ =-2.2 ± 0.2, and contains approximately 1.6 per cent of the luminosity of the entire system. While the origin of the envelope cannot be robustly identified using the presently available data, the fact that M2 also hosts stellar populations exhibiting a broad dispersion in the abundances of both iron and a variety of neutron capture elements suggests that this object might plausibly constitute the stripped nucleus of a dwarf galaxy that was long ago accreted and destroyed by the Milky Way. [ABSTRACT FROM AUTHOR]

Context. Globular clusters trace the formation and evolution of the Milky Way and surrounding galaxies, and outline their chemical enrichment history. To accomplish these tasks it is important to have large samples of clusters with homogeneous data and analysis to derive kinematics, chemical abundances, ages and locations. Aims. We obtain homogeneous metallicities and α-element enhancement for 51 Galactic bulge, disc, and halo globular clusters that are among the most distant and/or highly reddened in the Galaxy's globular cluster system. We also provide membership selection based on stellar radial velocities and atmospheric parameters. The implications of our results are discussed. Methods. We observed R ~ 2000 spectra in the wavelength interval 456-586 nm for over 800 red giant stars in 51 Galactic globular clusters. We applied full spectrum fitting with the code ETOILE together with libraries of observed and synthetic spectra. We compared the mean abundances of all clusters with previous work and with field stars. We used the relation between mean metallicity and horizontal branch morphology defined by all clusters to select outliers for discussion. Results. [Fe/H], [Mg/Fe], and [α/Fe] were derived in a consistent way for almost one-third of all Galactic globular clusters. We find our metallicities are comparable to those derived from high-resolution data to within σ = 0.08 dex over the interval -2:5 < [Fe/H] < 0.0. Furthermore, a comparison of previous metallicity scales with our values yields σ < 0:16 dex. We also find that the distribution of [Mg/Fe] and [α/Fe] with [Fe/H] for the 51 clusters follows the general trend exhibited by field stars. It is the first time that the following clusters have been included in a large sample of homogeneous stellar spectroscopic observations and metallicity derivation: BH 176, Djorg 2, Pal 10, NGC 6426, Lynga 7, and Terzan 8. In particular, only photometric metallicities were available previously for the first three clusters, and the available metallicity for NGC 6426 was based on integrated spectroscopy and photometry. Two other clusters, HP 1 and NGC 6558, are confirmed as candidates for the oldest globular clusters in the Milky Way. Conclusions. Stellar spectroscopy in the visible at R ~ 2000 for a large sample of globular clusters is a robust and effcient way to trace the chemical evolution of the host galaxy and to detect interesting objects for follow-up at higher resolution and with forthcoming giant telescopes. The technique used here can also be applied to globular cluster systems in nearby galaxies with current instruments and to distant galaxies with the advent of ELTs. [ABSTRACT FROM AUTHOR]

We report the identification of extended tidal debris potentially associated with the globular cluster NGC 3201, using the RAdial Velocity Experiment (RAVE) catalogue. We find the debris stars are located at a distance range of 1-7 kpc based on the forthcoming RAVE distance estimates. The derived space velocities and integrals of motion show interesting connections to NGC 3201, modulo uncertainties in the proper motions. Three stars, which are among the four most likely candidates for NGC 3201 tidal debris, are separated by 80° on the sky yet are well matched by the 12 Gyr, [Fe/H] = -1.5 isochrone appropriate for the cluster. This is the first time tidal debris around this cluster has been reported over such a large spatial extent, with implications for the cluster's origin and dynamical evolution. [ABSTRACT FROM AUTHOR]

In this paper, we present the results of a search for members of the globular cluster Palomar 5 and its associated tidal tails. The analysis has been performed using intermediate and low-resolution spectroscopy with the AAOmega spectrograph on the Anglo-Australian Telescope. Based on kinematics, line strength and photometric information, we identify 39 new red giant branch stars along ~20° of the tails, a larger angular extent than has been previously studied. We also recover eight previously known tidal tail members. Within the cluster, we find seven new red giant and one blue horizontal branch members and confirm a further 12 known red giant members. In total, we provide velocity data for 67 stars in the cluster and the tidal tails. Using a maximum likelihood technique, we derive a radial velocity for Pal 5 of -57.4 ± 0.3 km s-1 and a velocity dispersion of 1.2 ± 0.3 km s-1. We confirm and extend the linear velocity gradient along the tails of 1.0 ± 0.1 km s-1 deg-1, with an associated intrinsic velocity dispersion of 2.1 ± 0.4 km s-1. Neither the velocity gradient nor the dispersion change in any significant way with angular distance from the cluster, although there is some indication that the gradient may be smaller at greater angular distances in the trailing tail. Our results verify the tails as kinematically cold structures and will allow further constraints to be placed on the orbit of Pal 5, ultimately permitting a greater understanding of the shape and extent of the Galaxy's dark matter halo. [ABSTRACT FROM AUTHOR]

Context. We have observed almost one third of the globular clusters in the Milky Way, targeting distant and/or highly reddened objects, as well as a few reference clusters. A large sample of red giant stars was observed with FORS2 at VLT/ESO at R ~ 2000. The method for deriving stellar parameters is presented with application to six reference clusters. Aims. We aim at deriving the stellar parameters effective temperature, gravity, metallicity, and alpha-element enhancement as well as the radial velocity to confirm the membership of individual stars in each cluster. We analyse the spectra collected for the reference globular clusters NGC 6528 ([Fe/H] ~ -0.1), NGC 6553 ([Fe/H] ~ -0.2), M 71 ([Fe/H] ~ -0.8), NGC 6558 ([Fe/H] ~ -1.0), NGC 6426 ([Fe/H] ~ -2.1), and Terzan 8 ([Fe/H] ~ -2.2). They cover the full range of globular cluster metallicities, and are located in the bulge, disc, and halo. Methods. Full spectrum-fitting techniques were applied by comparing each target spectrum with a stellar library in the optical region at 4560-5860 Å. We employed the library of observed spectra MILES, and the Coelho synthetic library. We validated the method by recovering the known atmospheric parameters for 49 well-studied stars that cover a wide range in the parameter space. We adopted as final stellar parameters (effective temperatures, gravities, metallicities) the average of results using the observed and synthetic spectral libraries. Results. We identified 4 member stars in NGC 6528, 13 in NGC 6553, 10 inM71, 5 in NGC 6558, 5 in NGC 6426, and 12 in Terzan 8. Radial velocities, Teff, log(g), [Fe/H], and alpha-element enhancements were derived. We derived (vhelio)= -242±11 km s-1, [Fe/H] = -2.39 ± 0.04, [Mg/Fe] = 0.38 ± 0.06 for NGC 6426 from spectroscopy for the first time. Conclusions. The method proved to be reliable for red giant stars observed with resolution R ~ 2000, yielding results compatible with high-resolution spectroscopy. The derived a-element abundances show [a/Fe] vs. [Fe/H] consistent with that of field stars at the same metallicities. [ABSTRACT FROM AUTHOR]

Context. When they are established with sufficient precision, the ages, metallicities and kinematics of Galactic globular clusters (GGCs) can shed much light on the dynamical and chemical evolution of the Galactic halo and bulge.While the most fundamental way of determining GC abundances is by means of high-resolution spectroscopy, in practice this method is limited to only the brighter stars in the nearest and less reddened objects. This restriction has, over the years, led to the development of a large number of techniques that measure the overall abundance indirectly from parameters that correlate with overall metallicity. One of the most efficient methods is measuring the equivalent width (EW) of the calcium II triplet (CaT) at λ ≈ 8500 Å in red giants, which are corrected for the luminosity and temperature effects using the V magnitude differences from the horizontal branch (HB). Aims. We establish a similar method in the near-infrared (NIR), by combining the power of the differential magnitudes technique with the advantages of NIR photometry to minimize differential reddening effects. Methods. We used the Ks magnitude difference between the star and the reddest part of the HB (RHB) or of the red clump (RC) to generate reduced equivalent widths (rEW) from previously presented datasets. Then we calibrated these rEWagainst three previously reported different metallicity scales; one of which we corrected using high-resolution spectroscopic metallicities. Results. We calculated the calibration relations for the two datasets and the three metallicity scales and found that they are approximately equivalent, with almost negligible differences. We compared our NIR calibrations with the corresponding optical ones, and found them to be equivalent, which shows that the luminosity-corrected rEW using the Ks magnitude is compatible with the one obtained from the V magnitude. We then used the metallicities obtained from the calibration to investigate the internal metallicity distributions of the GCs. Conclusions. We have established that the ([Fe/H]:rEW) relation is independent of the magnitude used for the luminosity correction and find that the calibration relations change only slightly for different metallicity scales. The CaT technique using NIR photometry is thus a powerful tool to derive metallicities. In particular, it can be used to study the internal metallicity spread of a GC. We confirm the presence of at least two metallicity populations in NGC6656 and find that several other GCs present peculiar metallicity distributions. [ABSTRACT FROM AUTHOR]

We have used the high-resolution observations obtained at the Anglo-Australian Telescope with Ultra-High Resolution Facility (R ∼ 100,000) and at Gemini-S with b-HROS (R ∼ 150,000) to determine magnesium isotope ratios for seven ω Cen red giants that cover a range in iron abundance from [Fe/H]=−1.78 to −0.78 dex, and for two red giants in M4 (NGC 6121). The ω Cen stars sample both the "primordial" (i.e., O-rich, Na- and Al-poor) and the "extreme" (O-depleted, Na- and Al-rich) populations in the cluster. The primordial population stars in both ω Cen and M4 show (25Mg, 26Mg)/24Mg isotopic ratios that are consistent with those found for the primordial population in other globular clusters with similar [Fe/H] values. The isotopic ratios for the ω Cen extreme stars are also consistent with those for extreme population stars in other clusters. The results for the extreme population stars studied indicate that the 26Mg/24Mg ratio is highest at intermediate metallicities ([Fe/H] < −1.4 dex), and for the highest [Al/Fe] values. Further, the relative abundance of 26Mg in the extreme population stars is notably higher than that of 25Mg, in contrast to model predictions. The 25Mg/24Mg isotopic ratio in fact does not show any obvious dependence on either [Fe/H] or [Al/Fe] nor, intriguingly, any obvious difference between the primordial and extreme population stars. [ABSTRACT FROM AUTHOR]

The multi-object fiber-fed spectrograph AAOmega at the Anglo-Australian Telescope has been used to establish and measure accurate (⩽1 km s–1) radial velocities for a new sample of members in the outer parts of the stellar system ω Centauri. The new sample more than doubles the number of known members with precise velocities that lie between 25′ and 45′ from the cluster center. Combining this sample with earlier work confirms that the line-of-sight velocity dispersion of ω Cen remains approximately constant at ∼6.5 km s–1 in the outer parts of the cluster, which contain only a small fraction of the total cluster stellar mass. It is argued that the approximately constant velocity dispersion in the outer regions is most likely a consequence of external influences, such as the tidal shock heating that occurs each time ω Cen crosses the Galactic plane. There is therefore no requirement to invoke dark matter or non-standard gravitational theories. [ABSTRACT FROM AUTHOR]