Christian I. Johnson

Space Telescope Science Institute

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R. Michael Rich

University of California, Los Angeles

C. A. Pilachowski

Indiana University Bloomington

Andrea Kunder

Saint Martin's University

Andreas Koch

Heidelberg University

W. I. Clarkson

University of Michigan–Dearborn

Mario Mateo

University of Michigan–Ann Arbor

Nelson Caldwell

Center for Astrophysics Harvard & Smithsonian

Iulia T. Simion

Shanghai Normal University

John I. Bailey

University of California, Santa Barbara

Matthew G. Walker

Carnegie Mellon University

Cooperative Institutions

Max Planck Institute for Astronomy

Max Planck Society

Space Telescope Science Institute

Center for Astrophysics Harvard & Smithsonian

Centre National de la Recherche Scientifique

California Institute of Technology

Massachusetts Institute of Technology

Harvard University

University of Cambridge

National Institute for Astrophysics

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The Most Metal-poor Stars in Omega Centauri (NGC 5139)*

The Astronomical Journal (2020)

Christian I. Johnson A. K. Dupree Mario Mateo John I. Bailey Edward W. Olszewski

Abstract The most massive and complex globular clusters in the Galaxy are thought to have originated as the nuclear cores of now tidally disrupted dwarf galaxies, but the connection between globular clusters and dwarf galaxies is tenuous with the M54/Sagittarius system representing the only unambiguous link. The globular cluster Omega Centauri ( ω Cen) is more massive and chemically diverse than M 54, and is thought to have been the nuclear star cluster of either the Sequoia or Gaia-Enceladus galaxy. Local Group dwarf galaxies with masses equivalent to these systems often host significant populations of very metal-poor stars ([Fe/H] < −2.5), and one might expect to find such objects in ω Cen. Using high-resolution spectra from Magellan-M2FS, we detected 11 stars in a targeted sample of 395 that have [Fe/H] ranging from −2.30 to −2.52. These are the most metal-poor stars discovered in the cluster, and are five times more metal-poor than ω Cen’s dominant population. However, these stars are not so metal-poor as to be unambiguously linked to a dwarf galaxy origin. The cluster’s metal-poor tail appears to contain two populations near [Fe/H] ∼ −2.1 and −2.4, which are very centrally concentrated but do not exhibit any peculiar kinematic signatures. Several possible origins for these stars are discussed.

Dwarf spheroidal galaxy

Sagittarius

Blue straggler

10.3847/1538-3881/ab8819

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Citations (9)

A Large Sample Fe and Al Abundance Study in the Globular Cluster Omega Centauri

Bulletin of the American Astronomical Society (2007)

Christian I. Johnson C. A. Pilachowski Jennifer Simmerer D. R. Schwenk

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DETAILED ABUNDANCES FOR A LARGE SAMPLE OF GIANT STARS IN THE GLOBULAR CLUSTER 47 TUCANAE (NGC 104)

The Astrophysical Journal (2013)

M. J. Cordero C. A. Pilachowski Christian I. Johnson I. McDonald A. A. Zijlstra

47 Tuc is an ideal target to study chemical evolution and GC formation in massive more metal-rich GCs since is the closest, massive GC. We present chemical abundances for O, Na, Al, Si, Ca, Ti, Fe, Ni, La, and Eu in 164 red giant branch (RGB) stars in the massive globular cluster 47 Tuc using spectra obtained with both the Hydra multi-fiber spectrograph at the Blanco 4-m telescope and the FLAMES multi-object spectrograph at the Very Large Telescope. We find an average [Fe/H]=--0.79$\pm$0.09 dex, consistent with literature values, as well as over-abundances of alpha-elements ($[\alpha/\mbox{Fe}]\sim0.3$ dex). The n-capture process elements indicate that 47 Tuc is r-process dominated ([Eu/La]=+0.24), and the light elements O, Na, and Al exhibit star-to-star variations. The Na-O anti-correlation, a signature typically seen in Galactic globular clusters, is present in 47 Tuc, and extends to include a small number of stars with [O/Fe] $\sim$\,--0.5. Additionally, the [O/Na] ratios of our sample reveal that the cluster stars can be separated into three distinct populations. A KS-test demonstrates that the O-poor/Na-rich stars are more centrally concentrated than the O-rich/Na-poor stars. The observed number and radial distribution of 47 Tuc's stellar populations, as distinguished by their light element composition, agrees closely with the results obtained from photometric data. We do not find evidence supporting a strong Na-Al correlation in 47 Tuc, which is consistent with current models of AGB nucleosynthesis yields.

Red-giant branch

Giant star

Horizontal branch

10.1088/0004-637x/780/1/94

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NGC 7789: AN OPEN CLUSTER CASE STUDY

The Astronomical Journal (2014)

Jamie Overbeek Eileen D. Friel Heather R. Jacobson Christian I. Johnson C. A. Pilachowski

ABSTRACT We have obtained high-resolution spectra of 32 giants in the open cluster NGC 7789 using the Wisconsin–Indiana–Yale–NOAO Hydra spectrograph. We explore differences in atmospheric parameters and elemental abundances caused by the use of the linelist developed for the Gaia-ESO Survey (GES) compared to one based on Arcturus used in our previous work. [Fe/H] values decrease when using the GES linelist instead of the Arcturus-based linelist; these differences are probably driven by systematically lower (∼−0.1 dex) GES surface gravities. Using the GES linelist we determine abundances for 10 elements—Fe, Mg, Si, Ca, Ti, Na, Ni, Zr, Ba, and La. We find the clusterʼs average metallicity [Fe/H] = 0.03 ± 0.07 dex, in good agreement with literature values, and a lower [Mg/Fe] abundance than has been reported before for this cluster (0.11 ± 0.05 dex). We also find the neutron-capture element barium to be highly enhanced—[Ba/Fe] = +0.48 ± 0.08—and disparate from cluster measurements of neutron-capture elements La and Zr (−0.08 ± 0.05 and 0.08 ± 0.08, respectively). This is in accordance with recent discoveries of supersolar Ba enhancement in young clusters along with more modest enhancement of other neutron-capture elements formed in similar environments.

10.1088/0004-6256/149/1/15

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A Wide View of the Galactic Globular Cluster NGC 2808: Red Giant and Horizontal Branch Star Spatial Distributions

The Astronomical Journal (2023)

Christian I. Johnson A. Calamida Justin A. Kader I. Ferraro C. A. Pilachowski

Abstract Wide-field and deep DECam multiband photometry, combined with HST data for the core of the Galactic globular cluster NGC 2808, allowed us to study the distribution of various stellar subpopulations and stars in different evolutionary phases out to the cluster tidal radius. We used the C ugi = ( u − g ) − ( g − i ) index to identify three chemically distinct subpopulations along the red giant branch and compared their spatial distributions. The most light-element-enriched subpopulation (P3) is more centrally concentrated; however, it shows a more extended distribution in the external regions of the cluster compared to the primordial (P1) and intermediate (P2) composition populations. Furthermore, the P3 subpopulation centroid is off-center relative to those of the P1 and P2 groups. We also analyzed the spatial distribution of horizontal branch stars and found that the relative fraction of red horizontal branch stars increases for radial distances larger than ≈1.′5, while that of the blue and hotter stars decreases. These new observations, combined with literature spectroscopic measurements, suggest that the red horizontal branch stars are the progeny of all the stellar subpopulations in NGC 2808, i.e., primordial and light-element enhanced, while the blue stars are possibly the result of a combination of the “hot-flasher” and the “helium-enhanced” scenarios. A similar distribution of different red giant branch subpopulations and horizontal branch stars was also found for the most massive Galactic globular cluster, ω Cen, based on combined DECam and HST data, which suggests that the two may share a similar origin.

Horizontal branch

Star (game theory)

Star count

Blue straggler

10.3847/1538-3881/acd3eb

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Characterization of Stellar Sub-Structure in the Galactic Bulge

noao (2014)

Andrea Kunder Roberto De Propris R. Michael Rich Andreas Koch Christian I. Johnson

Characterization

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Radial Velocities of RR Lyrae Stars in and around NGC 6441

The Astronomical Journal (2018)

Andrea Kunder Arthur K. Mills Joseph Edgecomb M. Thomas Levi Schilter

Abstract Detailed elemental abundance patterns of metal-poor ([Fe/H] ∼ −1 dex) stars in the Galactic bulge indicate that a number of them are consistent with globular cluster (GC) stars and may be former members of dissolved GCs. This would indicate that a few per cent of the Galactic bulge was built up from destruction and/or evaporation of GCs. Here, an attempt is made to identify such presumptive stripped stars originating from the massive, inner Galaxy GC NGC 6441 using its rich RR Lyrae variable star (RRL) population. We present radial velocities of 40 RRLs centered on the GC NGC 6441. All 13 of the RRLs observed within the cluster tidal radius have velocities consistent with cluster membership, with an average radial velocity of 24 ± 5 km s −1 and a star-to-star scatter of 11 km s −1 . This includes two new RRLs that were previously not associated with the cluster. Eight RRLs with radial velocities consistent with cluster membership but up to three time the distance from the tidal radius are also reported. These potential extra-tidal RRLs also have exceptionally long periods, which is a curious characteristic of the NGC 6441 RRL population that hosts RRLs with periods longer than seen anywhere else in the Milky Way. As expected of stripped cluster stars, most are inline with the cluster’s orbit. Therefore, either the tidal radius of NGC 6441 is underestimated and/or we are seeing dissolving cluster stars stemming from NGC 6441 that are building up the old spheroidal bulge.

RR Lyrae variable

Variable star

10.3847/1538-3881/aab42d

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Citations (10)

ALPHA ENHANCEMENT AND THE METALLICITY DISTRIBUTION FUNCTION OF PLAUT’S WINDOW

The Astrophysical Journal (2011)

Christian I. Johnson R. Michael Rich J. P. Fulbright E. Valenti Andrew McWilliam

We present Fe, Si, and Ca abundances for 61 giants in Plaut's window (l = −1°, b = −85) and Fe abundances for an additional 31 giants in a second, nearby field (l = 0°, b = −8°) derived from high-resolution (R ≈ 25,000) spectra obtained with the Blanco 4 m telescope and Hydra multifiber spectrograph. The median metallicity of red giant branch (RGB) stars in the Plaut's field is ∼0.4 dex lower than those in Baade's window, and confirms the presence of an iron abundance gradient along the bulge minor axis. The full metallicity range of our (biased) RGB sample spans −1.5 < [Fe/H] < +0.3, which is similar to that found in other bulge fields. We also derive a photometric metallicity distribution function for RGB stars in the (l = −1°, b = −8°.5) field and find very good agreement with the spectroscopic metallicity distribution. The radial velocity (RV) and dispersion data for the bulge RGB stars are in agreement with previous results of the Bulge Radial Velocity Assay survey, and we find evidence for a decreasing velocity dispersion with increasing [Fe/H]. The [α/Fe] enhancement in Plaut field stars is nearly identical to that observed in Baade's window, and suggests that an [α/Fe] gradient does not exist between b = −4° and −8°. Additionally, a subset of our sample (23 stars) appears to be foreground red clump stars that are very metal rich, exhibit small metallicity and RV dispersions, and are enhanced in α elements. While these stars likely belong to the Galactic inner disk population, they exhibit [α/Fe] ratios that are enhanced above the thin and thick disk.

Red-giant branch

Velocity dispersion

10.1088/0004-637x/732/2/108

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Citations (72)

VizieR Online Data Catalog: Spectroscopy of NGC 7789 (Overbeek+, 2015)

VizieR Online Data Catalog (2015)

Jamie Overbeek E. D. Friel Heather R. Jacobson Christian I. Johnson C. A. Pilachowski

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VizieR Online Data Catalog: Radial velocity and photometry in NGC 4372 (Kacharov+, 2014)