Spectroscopy and Photometry of the Least Massive Type II Globular Clusters: NGC 1261 and NGC 6934*

Abstract Recent work has revealed two classes of globular clusters (GCs), dubbed Type I and Type II. Type II GCs are characterized by both a blue and a red red giant branch composed of stars with different metallicities, often coupled with distinct abundances in the slow neutron-capture elements (s-elements). Here we continue the chemical tagging of Type II GCs by adding the two least massive clusters of this class, NGC 1261 and NGC 6934. Based on both spectroscopy and photometry, we find red stars in NGC 1261 to be slightly enhanced in [Fe/H] by ∼0.1 dex and confirm that red stars of NGC 6934 are enhanced in iron by ∼0.2 dex. Neither NGC 1261 nor NGC 6934 show internal variations in the s-elements, which suggests a GC mass threshold for the occurrence of s-process enrichment. We found a significant correlation between the additional Fe locked in the red stars of Type II GCs and the present-day mass of the cluster. Nevertheless, most Type II GCs retained a small fraction of Fe produced by SNe II, lower than the 2%; NGC 6273, M54, and ω Centauri are remarkable exceptions. In the Appendix, we infer for the first time chemical abundances of lanthanum, assumed as representative of the s-elements, in M54, the GC located in the nucleus of the Sagittarius dwarf galaxy. Red-sequence stars are marginally enhanced in [La/Fe] by 0.10 ± 0.06 dex, in contrast with the large [La/Fe] spread of most Type II GCs. We suggest that different processes are responsible for the enrichment in iron and s-elements in Type II GCs.

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Statistical analysis of Galactic globular cluster type properties

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa901 ◽

2020 ◽

Vol 495 (4) ◽

pp. 3981-3989

Author(s):

M Simioni ◽

A Aparicio ◽

G Piotto

Keyword(s):

Globular Clusters ◽

Principal Component ◽

Slow Neutron ◽

Galactic Centre ◽

Type I ◽

Type Ii ◽

Relative Age ◽

Average Complexity ◽

Chromosome Maps ◽

Cluster Type

ABSTRACT The analysis of pseudo-colour diagrams, the so-called chromosome maps, of Galactic globular clusters (GCs) permits to classify them into type I and type II clusters. Type II GCs are characterized by an above-the-average complexity of their chromosome maps and some of them are known to display star-to-star variations of slow neutron-capture reaction elements including iron. This is at the basis of the hypothesis that type II GCs may have an extragalactic origin and were subsequently accreted by the Milky Way. We performed a principal component analysis to explore possible correlations among various GCs parameters in the light of this new classification. The analysis revealed that cluster type correlates mainly with relative age. The cause of this relation was further investigated finding that more metal-rich type II clusters, also appear to be younger and more distant from the Galactic centre. A depletion of type II clusters for positive values of Galactic coordinate Z was also observed, with no type II clusters detected above Z ∼ 2 kpc. Type II cluster orbits also have larger eccentricities than type I ones.

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Chemical abundances of multiple stellar populations in massive globular clusters

Proceedings of the International Astronomical Union ◽

10.1017/s1743921315009485 ◽

2015 ◽

Vol 12 (S316) ◽

pp. 267-274 ◽

Cited By ~ 1

Author(s):

Anna F. Marino

Keyword(s):

Neutron Capture ◽

Globular Clusters ◽

Milky Way ◽

Stellar Populations ◽

Slow Neutron ◽

Light Elements ◽

Chemical Abundance ◽

Chemical Abundances ◽

Origin And Evolution

AbstractMultiple stellar populations in the Milky Way globular clusters manifest themselves with a large variety. Although chemical abundance variations in light elements, including He, are ubiquitous, the amount of these variations is different in different globulars. Stellar populations with distinct Fe, C+N+O and slow-neutron capture elements have been now detected in some globular clusters, whose number will likely increase. All these chemical features correspond to specific photometric patterns. I review the chemical+photometric features of the multiple stellar populations in globular clusters and discuss how the interpretation of data is being more and more challenging. Very excitingly, the origin and evolution of globular clusters is being a complex puzzle to compose.

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The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters – XIX. A chemical tagging of the multiple stellar populations over the chromosome maps

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz1415 ◽

2019 ◽

Vol 487 (3) ◽

pp. 3815-3844 ◽

Cited By ~ 33

Author(s):

A F Marino ◽

A P Milone ◽

A Renzini ◽

F D’Antona ◽

J Anderson ◽

...

Keyword(s):

Globular Clusters ◽

Hubble Space Telescope ◽

Empirical Relation ◽

Light Element ◽

Stellar Populations ◽

Type I ◽

Type Ii ◽

Chemical Abundances ◽

Space Telescope ◽

Galactic Globular Clusters

Abstract The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters (GCs) has investigated GCs and their stellar populations. In previous papers of this series we have introduced a pseudo two-colour diagram, or ‘chromosome map’ (ChM) that maximizes the separation between the multiple populations. We have identified two main classes of GCs: Type I, including ∼83 per cent of the objects, and Type II clusters. Both classes host two main groups of stars, referred to in this series as first (1G) and second generation (2G). Type II clusters host more complex ChMs, exhibiting two or more parallel sequences of 1G and 2G stars. We exploit spectroscopic elemental abundances from the literature to assign the chemical composition to the distinct populations as identified on the ChMs of 29 GCs. We find that stars in different regions of the ChM have different compositions: 1G stars share the same light-element content as field stars, while 2G stars are enhanced in N and Na and depleted in O. Stars with enhanced Al, as well as stars with depleted Mg, populate the extreme regions of the ChM. We investigate the intriguing colour spread among 1G stars observed in many Type I GCs, and find no evidence for internal variations in light elements among these stars, whereas either a ∼0.1 dex iron spread or a variation in He among 1G stars remains to be verified. In the attempt of analysing the global properties of the multiple-population phenomenon, we have constructed a universal ChM, which highlights that, though very variegate, the phenomenon has some common pattern among all the analysed GCs. The universal ChM reveals a tight connection with Na abundances, for which we have provided an empirical relation. The additional ChM sequences observed in Type II GCs are enhanced in metallicity and, in some cases, s-process elements. Omega Centauri can be classified as an extreme Type II GC, with a ChM displaying three main extended ‘streams’, each with its own variations in chemical abundances. One of the most noticeable differences is found between the lower and upper streams, with the latter, associated with higher He, being also shifted towards higher Fe and lower Li abundances. We publicly release the ChMs.

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A stellar census in globular clusters with MUSE

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936508 ◽

2020 ◽

Vol 635 ◽

pp. A114 ◽

Cited By ~ 3

Author(s):

Tim-Oliver Husser ◽

Marilyn Latour ◽

Jarle Brinchmann ◽

Stefan Dreizler ◽

Benjamin Giesers ◽

...

Keyword(s):

Globular Clusters ◽

Near Infrared ◽

Quadratic Function ◽

Absolute Magnitude ◽

Type I ◽

Type Ii ◽

Single Star ◽

Link Type ◽

Different Populations ◽

Red Giant

Aims. We use the spectra of more than 30 000 red giant branch (RGB) stars in 25 globular clusters (GC), obtained within the MUSE survey of Galactic globular clusters, to calibrate the Ca II triplet metallicity relation and derive metallicities for all individual stars. We investigate the overall metallicity distributions as well as those of the different populations within each cluster. Methods. The Ca II triplet in the near-infrared at 8498, 8542, and 8662 Å is visible in stars with spectral types between F and M and can be used to determine their metallicities. In this work, we calibrate the relation using average cluster metallicities from literature and MUSE spectra, and extend it below the horizontal branch – a cutoff that has traditionally been made to avoid a non-linear relation – using a quadratic function. In addition to the classic relation based on V − VHB, we also present calibrations based on absolute magnitude and luminosity. The obtained relations were used to calculate metallicities for all the stars in the sample and to derive metallicity distributions for different populations within a cluster, which have been separated using so-called “chromosome maps” based on HST photometry. Results. We show that despite the relatively low spectral resolution of MUSE (R = 1900 − 3700) we can derive single star metallicities with a mean statistical intra-cluster uncertainty of ∼0.12 dex. We present metallicity distributions for the RGB stars in 25 GCs and investigate the different metallicities of the populations P3 (and higher) in so-called metal-complex or Type II clusters, finding metallicity variations in all of them. We also detected unexpected metallicity variations in the Type I cluster NGC 2808 and confirm the Type II status of NGC 7078.

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Detection of multiple stellar populations in extragalactic massive clusters with JWST

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936252 ◽

2019 ◽

Vol 629 ◽

pp. A40 ◽

Cited By ~ 3

Author(s):

M. Salaris ◽

S. Cassisi ◽

A. Mucciarelli ◽

D. Nardiello

Keyword(s):

Globular Clusters ◽

High Redshift ◽

Major Change ◽

Stellar Populations ◽

Magellanic Clouds ◽

Chemical Abundances ◽

Multiple Populations ◽

Stellar Spectra ◽

Red Giant ◽

Massive Clusters

The discovery of multiple stellar populations (multiple in the sense of inhomogeneous chemical abundances, with specific patterns of variations of a few light elements) in Galactic globular clusters and massive intermediate-age and old clusters in the Magellanic Clouds, both through spectroscopy and photometry, has led to a major change in our views about the formation of these objects. To date, none of the proposed scenarios are able to quantitatively explain all the chemical patterns observed in individual clusters. An extension of the study of multiple populations to resolved extragalactic massive clusters beyond the Magellanic Clouds would therefore be welcome, as it would enable the investigation and characterisation of the presence of multiple populations in different environments and age ranges. To this purpose, the James Webb Space Telescope (JWST) can potentially play a major role. On the one hand, the JWST promises direct observations of proto-globular cluster candidates at high redshift, and on the other hand, it can potentially push the sample of resolved clusters with detected multiple populations to larger distances. In this paper we address this second goal. Using theoretical stellar spectra and stellar evolution models, we investigated the effect of multiple population chemical patterns on synthetic magnitudes in the JWST infrared NIRCam filters. We identified the colours (F150W − F460M), (F115W − F460M) and pseudocolours CF150W, F460M, F115W = (F150W − F460M)−(F460M − F115W), CF150W, F277W, F115W = (F150W − F277W)−(F277W − F115W), as diagnostics able to reveal the presence of multiple populations along the red giant branches of old and intermediate-age clusters. Using the available online simulator for the NIRCam detector, we estimated that multiple populations can be potentially detected – depending on the exposure times, exact filter combination used, and extent of the abundance variations and the cluster [Fe/H] – out to a distance of ∼5 Mpc (approximately the distance to the M83 group).

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The accreted nuclear clusters of the Milky Way

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3407 ◽

2020 ◽

Vol 500 (2) ◽

pp. 2514-2524

Author(s):

Joel Pfeffer ◽

Carmela Lardo ◽

Nate Bastian ◽

Sara Saracino ◽

Sebastian Kamann

Keyword(s):

Globular Clusters ◽

Milky Way ◽

Dwarf Galaxy ◽

Star Clusters ◽

Host Galaxy ◽

The Galaxy ◽

Nuclear Clusters ◽

The Common ◽

Massive Clusters ◽

Peculiar Case

ABSTRACT A number of the massive clusters in the halo, bulge, and disc of the Galaxy are not genuine globular clusters (GCs) but instead are different beasts altogether. They are the remnant nuclear star clusters (NSCs) of ancient galaxies since accreted by the Milky Way. While some clusters are readily identifiable as NSCs and can be readily traced back to their host galaxy (e.g. M54 and the Sagittarius Dwarf galaxy), others have proven more elusive. Here, we combine a number of independent constraints, focusing on their internal abundances and overall kinematics, to find NSCs accreted by the Galaxy and trace them to their accretion event. We find that the true NSCs accreted by the Galaxy are: M54 from the Sagittarius Dwarf, ω Centari from Gaia-Enceladus/Sausage, NGC 6273 from Kraken, and (potentially) NGC 6934 from the Helmi Streams. These NSCs are prime candidates for searches of intermediate-mass black holes (BHs) within star clusters, given the common occurrence of galaxies hosting both NSCs and central massive BHs. No NSC appears to be associated with Sequoia or other minor accretion events. Other claimed NSCs are shown not to be such. We also discuss the peculiar case of Terzan 5, which may represent a unique case of a cluster–cluster merger.

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Chemical Abundances of Seven Outer Halo M31 Globular Clusters from the Pan-Andromeda Archaeological Survey

Proceedings of the International Astronomical Union ◽

10.1017/s1743921316011200 ◽

2016 ◽

Vol 11 (S321) ◽

pp. 10-12

Author(s):

Charli M. Sakari

Keyword(s):

Globular Clusters ◽

Milky Way ◽

Dwarf Galaxy ◽

Large Magellanic Cloud ◽

Archaeological Survey ◽

Chemical Abundances ◽

Light Spectra ◽

Sigma Level ◽

Stellar Streams ◽

Detailed Chemical

AbstractObservations of stellar streams in M31’s outer halo suggest that M31 is actively accreting several dwarf galaxies and their globular clusters (GCs). Detailed abundances can chemically link clusters to their birth environments, establishing whether or not a GC has been accreted from a satellite dwarf galaxy. This talk presents the detailed chemical abundances of seven M31 outer halo GCs (with projected distances from M31 greater than 30 kpc), as derived from high-resolution integrated-light spectra taken with the Hobby Eberly Telescope. Five of these clusters were recently discovered in the Pan-Andromeda Archaeological Survey (PAndAS)—this talk presents the first determinations of integrated Fe, Na, Mg, Ca, Ti, Ni, Ba, and Eu abundances for these clusters. Four of the target clusters (PA06, PA53, PA54, and PA56) are metal-poor ([Fe/H] < -1.5), α-enhanced (though they are possibly less alpha-enhanced than Milky Way stars at the 1 sigma level), and show signs of star-to-star Na and Mg variations. The other three GCs (H10, H23, and PA17) are more metal-rich, with metallicities ranging from [Fe/H] = -1.4 to -0.9. While H23 is chemically similar to Milky Way field stars, Milky Way GCs, and other M31 clusters, H10 and PA17 have moderately-low [Ca/Fe], compared to Milky Way field stars and clusters. Additionally, PA17’s high [Mg/Ca] and [Ba/Eu] ratios are distinct from Milky Way stars, and are in better agreement with the stars and clusters in the Large Magellanic Cloud (LMC). None of the clusters studied here can be conclusively linked to any of the identified streams from PAndAS; however, based on their locations, kinematics, metallicities, and detailed abundances, the most metal-rich PAndAS clusters H23 and PA17 may be associated with the progenitor of the Giant Stellar Stream, H10 may be associated with the SW Cloud, and PA53 and PA56 may be associated with the Eastern Cloud.

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Neutron-capture elements in dwarf galaxies

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936125 ◽

2019 ◽

Vol 631 ◽

pp. A171 ◽

Cited By ~ 16

Author(s):

Á. Skúladóttir ◽

C. J. Hansen ◽

S. Salvadori ◽

A. Choplin

Keyword(s):

Neutron Capture ◽

Milky Way ◽

Dwarf Galaxies ◽

Core Collapse ◽

Chemical Abundances ◽

Relative Contribution ◽

History Of ◽

Red Giant ◽

Process Element ◽

Giant Branch Stars

The heavy elements (Z > 30) are created in neutron (n)-capture processes that are predicted to happen at vastly different nucleosynthetic sites. To study these processes in an environment different from the Milky Way, we targeted the n-capture elements in red giant branch stars in the Sculptor dwarf spheroidal galaxy. Using ESO VLT/FLAMES spectra, we measured the chemical abundances of Y, Ba, La, Nd, and Eu in 98 stars covering the metalliticy range −2.4 < [Fe/H] < −0.9. This is the first paper in a series about the n-capture elements in dwarf galaxies, and here we focus on the relative and absolute timescales of the slow (s)- and rapid (r)-processes in Sculptor. From the abundances of the s-process element Ba and the r-process element Eu, it is clear that the r-process enrichment occurred throughout the entire chemical evolution history of Sculptor. Furthermore, there is no evidence for the r-process to be significantly delayed in time relative to core-collapse supernovae. Neutron star mergers are therefore unlikely the dominant (or only) nucleosynthetic site of the r-process. However, the products of the s-process only become apparent at [Fe/H] ≈ −2 in Sculptor, and the s-process becomes the dominant source of Ba at [Fe/H] ≳ −2. We tested the use of [Y/Mg] and [Ba/Mg] as chemical clocks in Sculptor. Similarly to what is observed in the Milky Way, [Y/Mg] and [Ba/Mg] increase towards younger ages. However, there is an offset in the trends, where the abundance ratios of [Y/Mg] in Sculptor are significantly lower than those of the Milky Way at any given age. This is most likely caused by metallicity dependence of yields from the s-process, as well as by a different relative contribution of the s-process to core-collapse supernovae in these galaxies. Comparisons of our results with data of the Milky Way and the Fornax dwarf spheroidal galaxy furthermore show that these chemical clocks depend on both metallicity and environment.

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Transport Properties of Type II Sodium-Silicon Clathrates

MRS Proceedings ◽

10.1557/proc-0886-f10-02 ◽

2005 ◽

Vol 886 ◽

Author(s):

Matt Beekman ◽

Jan Grkyo ◽

George S. Nolas

Keyword(s):

Thermal Conductivity ◽

Sodium Content ◽

Type I ◽

Type Ii ◽

Conductivity Measurements ◽

Scanning Calorimetry ◽

Resistivity Measurements ◽

Silicon Clathrates ◽

Covalently Bonded ◽

First Time

ABSTRACTWe have synthesized the type II silicon clathrates Na1Si136 and Na8Si136, and report on the electrical and thermal transport in these materials. The crystal structure consists of a covalently bonded silicon framework in which sodium guest atoms are encapsulated inside the silicon host framework. Differential scanning calorimetry measurements show the compounds decompose above 600°C to diamond-structure silicon. Temperature dependant electrical resistivity measurements show the specimens to have an insulating character, with magnitudes that decrease with increasing sodium content. For the first time, thermal conductivity measurements on type II sodium-silicon clathrates are presented. The thermal conductivity is very low for both specimens, and for Na8Si136 exhibits a clear dip in the range from 50 to 70 K. These data suggest that the “rattling” behavior observed in type I clathrates may also be present in type II clathrates.

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Light element discontinuities suggest an early termination of star formation in the globular cluster NGC 6402 (M14)

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz587 ◽

2019 ◽

Vol 485 (3) ◽

pp. 4311-4329 ◽

Cited By ~ 5

Author(s):

Christian I Johnson ◽

Nelson Caldwell ◽

R Michael Rich ◽

Mario Mateo ◽

John I Bailey

Keyword(s):

Globular Clusters ◽

First Generation ◽

Light Element ◽

Chemical Abundances ◽

Intermediate Composition ◽

Chemical Enrichment ◽

The Galaxy ◽

Red Giant ◽

Composition Properties ◽

Delayed Phase

ABSTRACT NGC 6402 is among the most massive globular clusters in the Galaxy, but little is known about its detailed chemical composition. Therefore, we obtained radial velocities and/or chemical abundances of 11 elements for 41 red giant branch stars using high resolution spectra obtained with the Magellan-M2FS instrument. We find NGC 6402 to be only moderately metal-poor with 〈[Fe/H]〉 = −1.13 dex (σ = 0.05 dex) and to have a mean heliocentric radial velocity of −61.1 km s−1 (σ = 8.5 km s−1). In general, NGC 6402 exhibits mean composition properties that are similar to other inner Galaxy clusters, such as [α/Fe] ∼+0.3 dex, [Cr,Ni/Fe] ∼ 0.0 dex, and 〈[La/Eu]〉 = −0.08 dex. Similarly, we find large star-to-star abundance variations for O, Na, Mg, Al, and Si that are indicative of gas that experienced high temperature proton-capture burning. Interestingly, we not only detect three distinct populations but also find large gaps in the [O/Fe], [Na/Fe], and [Al/Fe] distributions that may provide the first direct evidence of delayed formation for intermediate composition stars. A qualitative enrichment model is discussed where clusters form stars through an early ($\lesssim$5–10 Myr) phase, which results in first generation and ‘extreme’ composition stars, and a delayed phase ($\gtrsim$40 Myr), which results in the dilution of processed and pristine gas and the formation of intermediate composition stars. For NGC 6402, the missing intermediate composition stars suggest the delayed phase terminated prematurely, and as a result the cluster may uniquely preserve details of the chemical enrichment process.

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