Abstract
We present the first results of the Multi-Instrument Kinematic Survey of Galactic Globular Clusters (GGCs), a project aimed at exploring the internal kinematics of a representative sample of ...GGCs from the radial velocity of individual stars, covering the entire radial extension of each system. This is achieved by exploiting the formidable combination of multi-object and integral field unit spectroscopic facilities of the ESO Very Large Telescope. As a first step, here we discuss the results obtained for 11 clusters from high and medium resolution spectra acquired through a combination of FLAMES and KMOS observations. We provide the first kinematical characterization of NGC 1261 and NGC 6496. In all the surveyed systems, the velocity dispersion profile declines at increasing radii, in agreement with the expectation from the King model that best fits the density/luminosity profile. In the majority of the surveyed systems, we find evidence of rotation within a few half-mass radii from the center. These results are in general overall agreement with the predictions of recent theoretical studies, suggesting that the detected signals could be the relic of significant internal rotation set at the epoch of the cluster’s formation.
The Galactic bulge is dominated by an old, metal-rich stellar population. The possible presence and the amount of a young (a few gigayears old) minor component is one of the major issues debated in ...the literature. Recently, the bulge stellar system Terzan 5 was found to harbor three sub-populations with iron content varying by more than one order of magnitude (from 0.2 up to two times the solar value), with chemical abundance patterns strikingly similar to those observed in bulge field stars. Here we report on the detection of two distinct main-sequence turnoff points in Terzan 5, providing the age of the two main stellar populations: 12 Gyr for the (dominant) sub-solar component and 4.5 Gyr for the component at super-solar metallicity. This discovery classifies Terzan 5 as a site in the Galactic bulge where multiple bursts of star formation occurred, thus suggesting a quite massive progenitor possibly resembling the giant clumps observed in star-forming galaxies at high redshifts. This connection opens a new route of investigation into the formation process and evolution of spheroids and their stellar content.
ABSTRACT
We used a combination of optical and near-UV
Hubble Space Telescope
photometry and FLAMES/ESO-VLT high-resolution spectroscopy to characterize the stellar content of the old and massive ...globular cluster (GC) NGC 121 in the Small Magellanic Cloud (SMC). We report on the detection of multiple stellar populations, the first case in the SMC stellar cluster system. This result enforces the emerging scenario in which the presence of multiple stellar populations is a distinctive-feature of old and massive GCs regardless of the environment, as far as the light-element distribution is concerned. We find that second-generation (SG) stars are more centrally concentrated than first-generation (FG) ones. More interestingly, at odds with what is typically observed in Galactic GCs, we find that NGC 121 is the only cluster so far to be dominated by FG stars that account for more than 65% of the total cluster mass. In the framework where GCs were born with 90%–95% of FG stars, this observational finding would suggest that either NGC 121 experienced a milder stellar mass-loss with respect to Galactic GCs or it formed a smaller fraction of SG stars.
We present a detailed study of the composition of 20 M giants in the Galactic center with 15 of them confirmed to be in the nuclear star cluster. As a control sample we have also observed 7 M giants ...in the Milky Way disk with similar stellar parameters. All 27 stars are observed using the NIRSPEC spectrograph on the KECK II telescope in the K-band at a resolving power of R = 23,000. We report the first silicon abundance trends versus Fe/H for stars in the Galactic center. While finding a disk/bulge-like trend at subsolar metallicities, we find that Si/Fe is enhanced at supersolar metallicities. We speculate on possible enrichment scenarios to explain such a trend. However, the sample size is modest and the result needs to be confirmed by additional measurements of silicon and other -elements. We also derive a new distribution of Fe/H and find the most metal-rich stars at Fe/H = +0.5 dex, confirming our earlier conclusions that the Galactic center hosts no stars with extreme chemical compositions.
We report the first high spectral resolution study of 17 M giants kinematically confirmed to lie within a few parsecs of the Galactic center, using spectroscopy from Keck/NIRSPEC and a new line list ...for the infrared K band. We consider their luminosities and kinematics, which classify these stars as members of the older stellar population and the central cluster. We find a median metallicity of and a large spread from approximately −0.3 to (quartiles). We find that the highest metallicities are , with most of the stars being at or below the solar iron abundance. The abundances and the abundance distribution strongly resemble those of the Galactic bulge rather than the disk or halo; in our small sample we find no statistical evidence for a dependence of velocity dispersion on metallicity.
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.
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.
Very strong Sc i lines have recently been found in cool M giants in the Nuclear Star Cluster (NSC) in the Galactic center. Interpreting these as anomalously high scandium abundances in the Galactic ...center would imply a unique enhancement signature and chemical evolution history for NSCs, and a potential test for models of chemical enrichment in these objects. We present high resolution K-band spectra (NIRSPEC/Keck II) of cool M giants situated in the solar neighborhood and compare them with spectra of M giants in the NSC. We clearly identify strong Sc i lines in our solar neighborhood sample as well as in the NSC sample. The strong Sc i lines in M giants are therefore not unique to stars in the NSC and we argue that the strong lines are a property of the line formation process that currently escapes accurate theoretical modeling. We further conclude that for giant stars with effective temperatures below approximately 3800 K these Sc i lines should not be used for deriving the scandium abundances in any astrophysical environment until we better understand how these lines are formed. We also discuss the lines of vanadium, titanium, and yttrium identified in the spectra, which demonstrate a similar striking increase in strength below 3500 K effective temperature.
Abstract
Similarities in the chemical composition of two of the closest Milky Way satellites, namely, the Large Magellanic Cloud (LMC) and the Sagittarius (Sgr) dwarf galaxy, have been proposed in ...the literature, suggesting similar chemical enrichment histories between the two galaxies. This proposition, however, rests on different abundance analyses, which likely introduce various systematics that hamper a fair comparison among the different data sets. In order to bypass this issue (and highlight real similarities and differences between their abundance patterns), we present a homogeneous chemical analysis of 30 giant stars in the LMC, 14 giant stars in Sgr, and 14 giants in the Milky Way, based on high-resolution spectra taken with the spectrograph UVES-FLAMES. The LMC and Sgr stars, in the considered metallicity range (Fe/H > −1.1 dex), show very similar abundance ratios for almost all the elements, with differences only in the heavy
s
-process elements Ba, La, and Nd, suggesting a different contribution by asymptotic giant branch stars. On the other hand, the two galaxies have chemical patterns clearly different from those measured in the Galactic stars, especially for the elements produced by massive stars. This finding suggests that the massive stars contributed less to the chemical enrichment of these galaxies with respect to the Milky Way. The derived abundances support similar chemical enrichment histories for the LMC and Sgr.
We report abundance analysis for 30 M giant stars in two inner Galactic bulge fields at (l, b) = (0degrees, -1degrees.75) and at (l, b) = (1degrees, -2degrees.65), based on R = 25,000 infrared ...spectroscopy (1.5-1.8 mu m) using NIRSPEC at the Keck II telescope. We find iron abundances of left angle bracketFe/Hright angle bracket = -0.16 + or - 0.03 dex with a 1sigma dispersion of 0.12 + or - 0.02 and left angle bracketFe/H right angle bracket = -0.21 + or - 0.02 dex, with a 1sigma dispersion of 0.09 + or - 0.016 for the (l, b) = (0degrees, -1degrees.75) and (l, b) = (1degrees, -2degrees.65) fields, respectively. In agreement with all prior studies, we find enhanced alpha /Fe of +0.3 dex. We confirm the lack of any major vertical abundance or composition gradient in the innermost ~600 pc between Baade's Window, at (l, b) = (1degrees, -3degrees.9), and ~150 pc from the Galactic plane. We also confirm that the known enhancement of alpha elements observed between 500 and 1000 pc from the nucleus is also present over the volume of the inner bulge and may therefore be presumed to be characteristic of all bulge/bar stars within 1 kpc of the Galactic center.
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