| 1. |
- Alfaro-Cuello, M., et al.
(författare)
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A Deep View into the Nucleus of the Sagittarius Dwarf Spheroidal Galaxy with MUSE. II. Kinematic Characterization of the Stellar Populations
- 2020
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 892:1
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Tidskriftsartikel (refereegranskat)abstract
- The Sagittarius dwarf spheroidal galaxy is in an advanced stage of disruption but still hosts its nuclear star cluster (NSC), M54, at its center. In this paper, we present a detailed kinematic characterization of the three stellar populations present in M54: Young metal-rich (YMR); intermediate-age metal-rich (IMR); and old metal-poor (OMP), based on the spectra of 6500 individual M54 member stars extracted from a large Multi-Unit Spectroscopic Explorer (MUSE)/Very Large Telescope data set. We find that the OMP population is slightly flattened with a low amount of rotation (0.8 km s-1) and with a velocity dispersion that follows a Plummer profile. The YMR population displays a high amount of rotation (5 km s-1) and a high degree of flattening, with a lower and flat velocity dispersion profile. The IMR population shows a high but flat velocity dispersion profile, with some degree of rotation (2 km s-1). We complement our MUSE data with information from Gaia DR2 and confirm that the stars from the OMP and YMR populations are comoving in 3D space, suggesting that they are dynamically bound. While dynamical evolutionary effects (e.g., energy equipartition) are able to explain the differences in velocity dispersion between the stellar populations, the strong differences in rotation indicate different formation paths for the populations, as supported by an N-body simulation tailored to emulate the YMR-OMP system. This study provides additional evidence for the M54 formation scenario proposed in our previous work, where this NSC formed via GC accretion (OMP) and in situ formation from gas accretion in a rotationally supported disk (YMR).
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| 2. |
- Hollyhead, Katherine, et al.
(författare)
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Kron 3 : a fourth intermediate age cluster in the SMC with evidence of multiple populations
- 2018
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 476:1, s. 114-121
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of a spectroscopic study of the intermediate age (approximate to 6.5 Gyr) massive cluster Kron 3 in the Small Magellanic Cloud. We measure CN and CH band strengths (at similar or equal to 3839 and 4300 angstrom, respectively) using VLT FORS2 spectra of 16 cluster members and find a sub-population of five stars enriched in nitrogen. We conclude that this is evidence for multiple populations in Kron 3, the fourth intermediate age cluster, after Lindsay 1, NGC 416 and NGC 339 (ages 6-8 Gyr), to display this phenomenon originally thought to be a unique characteristic of old globular clusters. At approximate to 6.5 Gyr this is one of the youngest clusters with multiple populations, indicating that the mechanism responsible for their onset must operate until a redshift of at least 0.75, much later than the peak of globular cluster formation at redshift similar to 3.
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| 3. |
- Hollyhead, Katherine, et al.
(författare)
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Spectroscopic detection of multiple populations in the similar to 2 Gyr old cluster Hodge 6 in the LMC
- 2019
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 484:4, s. 4718-4725
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Tidskriftsartikel (refereegranskat)abstract
- We report the spectroscopic discovery of abundance spreads (i.e. multiple populations) in the similar to 2 Gyr old cluster in the LMC, Hodge 6. We use low-resolution VLT FORS2 spectra of 15 member stars in the cluster to measure their CN and CH band strengths at similar or equal to 3883 and 4300 angstrom, respectively, as well as [C/Fe] and [N/Fe] abundances. We find a subpopulation of two stars that are enriched in nitrogen, and we conclude that this subpopulation is evidence of multiple populations in Hodge 6. This is the second similar to 2 Gyr old cluster (the first being NGC 1978 in the LMC) to show multiple populations and the first spectroscopic detection of MPs in a cluster of this age. This result is interesting as it hints at a possible relationship between the disappearance of extended main sequence turn-offs in clusters younger than similar to 2 Gyr and the onset of multiple populations at similar to 2 Gyr, which should be explored further.
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| 4. |
- Martocchia, S., et al.
(författare)
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Age as a major factor in the onset of multiple populations in stellar clusters
- 2018
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 473:2, s. 2688-2700
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Martocchia, S., et al.
(författare)
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Leveraging HST with MUSE : II. Na-abundance variations in intermediate age star clusters
- 2020
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 499:1, s. 1200-1211
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Tidskriftsartikel (refereegranskat)abstract
- Ancient (>10 Gyr) globular clusters (GCs) show chemical abundance variations in the form of patterns among certain elements, e.g. N correlates with Na and anticorrelates with O. Recently, N abundance spreads have also been observed in massive star clusters that are significantly younger than old GCs, down to an age of similar to 2Gyr. However, so far N has been the only element found to vary in such young objects. We report here the presence of Na abundance variations in the intermediate age massive star clusters NGC 416 (similar to 6.5Gyr old) and Lindsay 1 (similar to 7.5Gyr old) in the Small Magellanic Cloud, by combining Hubble Space Telescope (HST) and European Southern Observatory Very Large Telescope MUSE observations. Using HST photometry, we were able to construct 'chromosome maps' and separate subpopulations with different N content, in the red giant branch of each cluster. MUSE spectra of individual stars belonging to each population were combined, resulting in high signal-to-noise spectra representative of each population, which were compared to search for mean differences in Na. We find a mean abundance variation of Delta[Na/Fe] = 0.18 +/- 0.04 dex for NGC 416 and Delta[Na/Fe] = 0.24 +/- 0.05 dex for Lindsay 1. In both clusters, we find that the population that is enhanced in N is also enhanced in Na, which is the same pattern to the one observed in ancient GCs. Furthermore, we detect a bimodal distribution of core-helium-burning red clump (RC) giants in the UV colour-magnitude diagram of NGC 416. A comparison of the stacked MUSE spectra of the two RCs shows the same mean Na abundance difference between the two populations. The results reported in this work are a crucial hint that star clusters of a large age range share the same origin: they are the same types of objects, but only separated in age.
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| 6. |
- Martocchia, S., et al.
(författare)
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The search for multiple populations in Magellanic Cloud clusters - IV. Coeval multiple stellar populations in the young star cluster NGC 1978
- 2018
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 477:4, s. 4696-4705
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Tidskriftsartikel (refereegranskat)abstract
- We have recently shown that the similar to 2 Gyr old Large Magellanic Cloud star cluster NGC 1978 hosts multiple populations in terms of star-to-star abundance variations in [N/Fe]. These can be seen as a splitting or spread in the subgiant and red giant branches (SGB and RGB) when certain photometric filter combinations are used. Because of its relative youth, NGC 1978 can be used to place stringent limits on whether multiple bursts of star formation have taken place within the cluster, as predicted by some models for the origin of multiple populations. We carry out two distinct analyses to test whether multiple star formation epochs have occurred within NGC 1978. First, we use ultraviolet colour-magnitude diagrams (CMDs) to select stars from the first and second population along the SGB, and then compare their positions in optical CMDs, where the morphology is dominantly controlled by age as opposed to multiple population effects. We find that the two populations are indistinguishable, with age differences of 1 +/- 20 Myr between them. This is in tension with predictions from the asymptotic giant branch scenario for the origin of multiple populations. Second, we estimate the broadness of the main-sequence turn-off (MSTO) of NGC 1978 and we report that it is consistent with the observational errors. We find an upper limit of similar to 65 Myr on the age spread in the MSTO of NGC 1978. This finding is in conflict with the age spread scenario as origin of the extended MSTO in intermediate-age clusters, while it fully supports predictions from the stellar rotation model.
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| 7. |
- Saracino, S., et al.
(författare)
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Leveraging HST with MUSE - I. Sodium abundance variations within the 2-Gyr-old cluster NGC 1978
- 2020
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 498:3, s. 4472-4480
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Tidskriftsartikel (refereegranskat)abstract
- Nearly all of the well-studied ancient globular clusters (GCs), in the Milky Way and in nearby galaxies, show star-to-star variations in specific elements (e.g. He, C, N, O, Na, and Al), known as 'multiple populations' (MPs). However, MPs are not restricted to ancient clusters, with massive clusters down to similar to 2 Gyr showing signs of chemical variations. This suggests that young and old clusters share the same formation mechanism but most of the work to date on younger clusters has focused on N variations. Initial studies even suggested that younger clusters may not host spreads in other elements beyond N (e.g. Na), calling into question whether these abundance variations share the same origin as in the older GCs. In this work, we combine Hubble Space Telescope (HST) photometry with Very Large Telescope (VLT)/Multi-Unit Spectroscopic Explorer (MUSE) spectroscopy of a large sample of red giant branch (RGB) stars (338) in the Large Magellanic Cloud cluster NGC 1978, the youngest globular to date with reported MPs in the form of N spreads. By combining the spectra of individual RGB stars into N-normal and N-enhanced samples, based on the 'chromosome map' derived from HST, we search for mean abundance variations. Based on the NaD line, we find a Na difference of Delta[Na/Fe] = 0.07 +/- 0.01 between the populations. While this difference is smaller than typically found in ancient GCs (which may suggest a correlation with age), this result further confirms that the MP phenomenon is the same, regardless of cluster age and host galaxy. As such, these young clusters offer some of the strictest tests for theories on the origin of MPs.
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| 8. |
- Koch, A., et al.
(författare)
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An intermediate-band photometric study of the "globular cluster" NGC 2419
- 2013
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Ingår i: ; , s. 67-70
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Konferensbidrag (refereegranskat)abstract
- NGC 2419 is one of the remotest star clusters in the Milky Way halo and its exact nature is yet unclear. While it has traits reminiscent of a globular cluster, its large radius and suggestions of an abundance spread have fueled the discussion about its origin in an extragalactic environment, possibly the remnant of the accretion of a dwarf galaxy. Here, we present first results from deep intermediate-band photometry of NGC 2419, which enables us to search for chemical (light element) abundance variations, metallicity spreads, and thus multiple stellar populations through well calibrated Strömgren indices.
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| 9. |
- San Roman, I., et al.
(författare)
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The Gaia-ESO Survey : Detailed abundances in the metal-poor globular cluster NGC 4372
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 579
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Tidskriftsartikel (refereegranskat)abstract
- We present the abundance analysis for a sample of 7 red giant branch stars in the metal-poor globular cluster NGC 4372 based on UVES spectra acquired as part of the Gaia-ESO Survey. This is the first extensive study of this cluster from high-resolution spectroscopy. We derive abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, Fe, Cr, Ni, Y, Ba, and La. We find a metallicity of [Fe/H] = -2.19 +/- 0.03 and find no evidence of any metallicity spread. This metallicity makes NGC 4372 one of the most metal-poor Galactic globular clusters. We also find an a-enhancement typical of halo globular clusters at this metallicity. Significant spreads are observed in the abundances of light elements. In particular, we find a Na-O anticorrelation. Abundances of O are relatively high compared with other globular clusters. This could indicate that NGC 4372 was formed in an environment with high O for its metallicity. A Mg-Al spread is also present that spans a range of more than 0.5 dex in Al abundances. Na is correlated with Al and Mg abundances at a lower significance level. This pattern suggests that the Mg-Al burning cycle is active. This behavior can also be seen in giant stars of other massive, metal-poor clusters. A relation between light and heavy s-process elements has been identified.
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