A comprehensive chemical abundance study of the outer halo globular cluster M 75

Context: M 75 is a relatively young Globular Cluster (GC) found at 15 kpc from the Galactic centre at the transition region between the inner and outer Milky Way halos. Aims: Our aims are to perform a comprehensive abundance study of a variety of chemical elements in this GC such as to investigate its chemical enrichment history in terms of early star formation, and to search for any multiple populations. Methods: We have obtained high resolution spectroscopy with the MIKE instrument at the Magellan telescope for 16 red giant stars. Their membership within the GC is confirmed from radial velocity measurements. Our chemical abundance analysis is performed via equivalent width measurements and spectral synthesis, assuming local thermodynamic equilibrium (LTE). Results: We present the first comprehensive abundance study of M 75 to date. The cluster is metal-rich ([Fe/H]=-1.16+/-0.02 dex, [alpha/Fe]=+0.30+/-0.02 dex), and shows a marginal spread in [Fe/H] of 0.07 dex, typical of most GCs of similar luminosity. A moderately extended O-Na anticorrelation is clearly visible, likely showing three generations of stars, formed on a short timescale. Additionally the two most Na-rich stars are also Ba-enhanced by 0.4 and 0.6 dex, respectively, indicative of pollution by lower mass (M ~ 4-5 M_Sun) Asymptotic Giant Branch (AGB) stars. The overall n-capture element pattern is compatible with predominant r-process enrichment, which is rarely the case in GCs of such a high metallicity.Comment: 15 pages, 11 figures, 7 tables, accepted for publication in A&

Precise abundance analysis of the outer halo globular cluster M 75

Globular clusters (GCs) are the oldest stellar systems in the Milky Way. Long time considered as simple stellar populations, nowadays we recognize their complex star formation history through precise abundance analysis of a variety of chemical elements in individual cluster members. Although we do not necessarily see clues for multiple populations in all GC colour-magnitude diagrams, all GCs present significant spreads and certain anticorrelations between their light and alpha element abundances. Furthermore, the heavy element abundances in individual stars of the primordial generation and their comparison to halo field stars and dwarf galaxies could provide us with valuable information about the very first stars that could have formed in GCs. M75 is a unique outer halo (galactocentric distance of ~15 kpc) GC with a peculiar Horizontal Branch morphology. Here we present the first abundance measurements of 16 individual red giants from high resolution spectroscopy. The cluster is metal rich ([Fe/H] = -1.17 +/- 0.02), alpha-enhanced, and shows a marginal spread in [Fe/H] of 0.07 dex, typical of most GCs of similar luminosity. The O-Na anticorrelation is clearly visible, showing at least two generations of stars, formed on a short timescale. We also discuss r- and s-process element abundances in the context of the earliest cluster enrichment phases.Comment: 3 pages, FIRST STARS IV conference proceedin

The Galactic evolution of sulphur as traced by globular clusters

Sulphur is an important, volatile alpha element but its role in the Galactic chemical evolution is still uncertain. We derive the S abundances in RGB stars in three Galactic globular clusters (GC) that cover a wide metallicity range (-2.3<[Fe/H]<-1.2), namely M4, M22, and M30. The halo field stars show a large scatter in the [S/Fe] ratio in this metallicity span, which is inconsistent with canonical chemical evolution models. To date, very few measurements of [S/Fe] exist for stars in GCs, which are good tracers of the chemical enrichment of their environment. However, some light and alpha elements show star-to-star variations within individual GCs and it is yet unclear whether sulphur also varies between GC stars. We used the the infrared spectrograph CRIRES to obtain high-resolution (R~50000), high signal-to-noise (SNR~200 per px) spectra in the region of the S I multiplet 3 at 1045 nm for 15 GC stars selected from the literature (6 stars in M4, 6 stars in M22 and 3 stars in M30). Multiplet 3 is better suited for S abundance derivation than the more commonly used lines of multiplet 1 at 920 nm, since its lines are not blended by telluric absorption or other stellar features at low metallicity. We used spectral synthesis to derive the [S/Fe] ratio of the stars assuming local thermodynamic equilibrium (LTE). We find mean [S/Fe] = 0.58 +/- 0.01 +/- 0.20 dex (statistical and systematic error) for M4, [S/Fe] = 0.57+/-0.01+/-0.19 dex for M22, and [S/Fe] = 0.55+/-0.02+/-0.16 dex for M30. The negative NLTE corrections are estimated to be in the order of the systematic uncertainties. With the tentative exception of two stars with measured high S abundances, we conclude that sulphur behaves like a typical alpha element in the studied Galactic GCs, showing enhanced abundances with respect to the solar value at metallicities below [Fe/H] = -1.0 dex without a considerable spread.Comment: 9 pages, 7 figures, accepted for publication in A&

Kinematics of outer halo globular clusters: M 75 and NGC 6426

Globular clusters (GCs) and their dynamic interactions with the Galactic components provide an important insight into the structure and formation of the early Milky Way. Here, we present a kinematic study of two outer halo GCs based on a combination of VLT/FORS2, VLT/FLAMES, and Magellan/MIKE low- and high-resolution spectroscopy of 32 and 27 member stars, respectively. Although both clusters are located at Galactocentric distances of 15 kpc, they have otherwise very different properties. M 75 is a luminous and metal-rich system at [Fe/H] = $-1.2$ dex, a value that we confirm from the calcium triplet region. This GC shows mild evidence for rotation with an amplitude of A$_{\rm rot}\sim$5 km s$^{-1}$. One of the most metal-poor GCs in the Milky Way (at [FeII/H] = $-2.3$ dex), NGC 6426 exhibits marginal evidence of internal rotation at the 2 km s$^{-1}$ level. Both objects have velocity dispersions that are consistent with their luminosity. Although limited by small-number statistics, the resulting limits on their $A_{\rm rot}/\sigma_0$ ratios suggest that M 75 is a slow rotator driven by internal dynamics rather than being effected by the weak Galactic tides at its large distances. Here, M 75 ($A_{\rm rot}/\sigma=0.31$) is fully consistent with the properties of other, younger halo clusters. At $A_{\rm rot}/\sigma_0=0.8\pm0.4$, NGC 6426 appears to have a remarkably ordered internal motion for its low metallicity, but the large uncertainty does not allow for an unambiguous categorization as a fast rotator. An accretion origin of M 75 cannot be excluded, based on the eccentric orbit, which we derived from the recent data release 2 of Gaia, and considering its younger age.Comment: 9 pages, 9 figures, accepted for publication in Astronomy & Astrophysic

Chemical abundances and kinematic properties of Galactic globular clusters

High-resolution spectroscopic studies of globular clusters (GC) have been a major break-through in our understanding of these ancient stellar systems. The observations carried out in the last decade revealed a very heterogeneous population of objects – not only have GCs undergone a complex star formation history resulting in large chemical inhomogeneities common amongst all of them, but there are also non-negligible cluster-to-cluster differences in both chemistry and kinematics that are not yet fully understood. In this thesis, we present the first ever high-resolution studies of two poorly known GCs M75 and NGC4372. M75 is a massive, relatively metal-rich, outer halo GC with extremely broad horizontal branch. We found that, besides the typical light-element variations, it shows a marginal spread in metallicity and likely has a small s-process rich population. Surprisingly for its metallicity, the majority of the analysed stars lack s-process enrichment, which hints to a formation in an environment that built metals very quickly. On the other hand, NGC4372 is one of the most metal-poor GCs in the Galaxy. It is found in the inner halo and has experienced multiple disk crossings. The chemical analysis revealed it as a standard representative of the old, metal-poor halo group. More interesting are its structural and kinematic properties as the cluster has an unusually high intrinsic rotation for its metallicity and appears to be rotationally flattened. Furthermore, since GCs trace the chemical properties of their environment, the thesis also includes the first homogeneous study of the Galactic halo evolution of the poorly studied a element sulphur traced by GC stars covering a large metallicity span

Stellar populations and star formation histories of the nuclear star clusters in six nearby galaxies

The majority of spiral and elliptical galaxies in the Universe host very dense and compact stellar systems at their centres known as nuclear star clusters (NSCs). In this work we study the stellar populations and star formation histories (SFH) of the NSCs of six nearby galaxies with stellar masses ranging between $2$ and $8\times10^9~{\rm M_{\odot}}$ (four late-type spirals and two early-types) with high resolution spectroscopy. Our observations are taken with the X-Shooter spectrograph at the VLT. We make use of an empirical simple stellar population (SSP) model grid to fit composite stellar populations to the data and recover the SFHs of the nuclei. We find that the nuclei of all late-type galaxies experienced a prolonged SFH, while the NSCs of the two early-types are consistent with SSPs. The NSCs in the late-type galaxies sample appear to have formed a significant fraction of their stellar mass already more than $10$ Gyr ago, while the NSCs in the two early-type galaxies are surprisingly younger. Stars younger than $100$ Myr are present in at least two nuclei: NGC 247 and NGC 7793, with some evidence for young star formation in NGC 300's NSC. The NSCs of the spirals NGC 247 and NGC 300 are consistent with prolonged \in situ star formation with a gradual metallicity enrichment from $\sim-1.5$ dex more than $10$ Gyr ago, reaching super-Solar values few hundred Myr ago. NGC 3621 appears to be very metal rich already in the early Universe and NGC 7793 presents us with a very complex SFH, likely dominated by merging of various massive star clusters coming from different environments.Comment: Accepted for publication in MNRA

A Deep View into the Nucleus of the Sagittarius Dwarf Spheroidal Galaxy with MUSE. III. Discrete Multicomponent Population-dynamical Models Based on the Jeans Equations

We present comprehensive multicomponent dynamical models of M54 (NGC 6715), the nuclear star cluster of the Sagittarius (Sgr) dwarf galaxy, which is undergoing a tidal disruption in the Milky Way halo. Previous papers in this series used a large MUSE mosaic data set to identify multiple stellar populations in the system and study their kinematic differences. Here, we use Jeans-based dynamical models that fit the population properties (mean age and metallicity), spatial distributions, and kinematics simultaneously. They provide a solid physical explanation for our previous findings. Population-dynamical models deliver a comprehensive view of the whole system, and allow us to disentangle the different stellar populations. We explore their dynamical interplay and confirm our previous findings about the build-up of Sgr’s nuclear cluster via contributions from globular cluster stars, Sgr inner field stars, and in situ star formation. We explore various parameterizations of the gravitational potential and show the importance of a radially varying mass-to-light ratio for the proper treatment of the mass profile. We find a total dynamical mass within M54's tidal radius (∼75 pc) of 1.60 ± 0.07 × 106 M ⊙ in excellent agreement with N-body simulations. Metal-poor globular cluster stars contribute about 65% of the total mass or 1.04 ± 0.05 × 106 M ⊙. Metal-rich stars can be further divided into young and intermediate-age populations, which contribute 0.32 ± 0.02 × 106 M ⊙ (20%) and 0.24 ± 0.02 × 106 M ⊙ (15%), respectively. Our population-dynamical models successfully distinguish the different stellar populations in Sgr’s nucleus because of their different spatial distributions, ages, metallicities, and kinematic features

Kinematics of stellar substructures in the small magellanic cloud

We present a kinematic analysis of the Small Magellanic Cloud using 3700 spectra extracted from the European Southern Observatory archive. We used data from Gaia and near-infrared photometry to select stellar populations and discard Galactic foreground stars. The sample includes main-sequence red giant branch and red clump stars, observed with Fibre Large Array Multi Wavelength Spectrograph. The spectra have a resolving power lambda/Delta lambda from 6500 to 38 000. We derive radial velocities by employing a full spectrum fitting method using a penalized pixel fitting routine. We obtain a mean radial velocity for the galaxy of 159 +/- 2 km s(-1), with a velocity dispersion of 33 +/- 2 km s(-1). Our velocities agree with literature estimates for similar (young or old) stellar populations. The radial velocity of stars in the Wing and bar-like structures differ as a consequence of the dynamical interaction with the Large Magellanic Cloud. The higher radial velocity of young main-sequence stars in the bar compared to that of supergiants can be attributed to star formation around 40 Myr ago from gas already influenced by tidal stripping. Similarly, young main-sequence stars in the northern part of the bar, resulting from a prominent star forming episode 25 Myr ago, have a higher radial velocity than stars in the southern part. Radial velocity differences between the northern and southern bar overdensities are also traced by giant stars. They are corroborated by studies of the cold gas and proper motion indicating stretching/tidal stripping of the galaxy