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Title: THE PECULIAR CHEMICAL INVENTORY OF NGC 2419: AN EXTREME OUTER HALO 'GLOBULAR CLUSTER'

Abstract

NGC 2419 is a massive outer halo Galactic globular cluster (GC) whose stars have previously been shown to have somewhat peculiar abundance patterns. We have observed seven luminous giants that are members of NGC 2419 with Keck/HIRES at reasonable signal-to-noise ratio. One of these giants is very peculiar, with an extremely low [Mg/Fe] and high [K/Fe] but normal abundances of most other elements. The abundance pattern does not match the nucleosynthetic yields of any supernova model. The other six stars show abundance ratios typical of inner halo Galactic GCs, represented here by a sample of giants in the nearby GC M30. Although our measurements show that NGC 2419 is unusual in some respects, its bulk properties do not provide compelling evidence for a difference between inner and outer halo GCs.

Authors:
;  [1];  [2]
  1. Palomar Observatory, Mail Stop 249-17, California Institute of Technology, Pasadena, CA 91125 (United States)
  2. Brion Technologies Inc., 4211 Burton Drive, Santa Clara, CA 95054 (United States)
Publication Date:
OSTI Identifier:
21587389
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 740; Journal Issue: 2; Other Information: DOI: 10.1088/0004-637X/740/2/60; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABUNDANCE; GALAXIES; STAR CLUSTERS; STARS

Citation Formats

Cohen, Judith G., Kirby, Evan N., and Huang Wenjin, E-mail: jlc@astro.caltech.edu, E-mail: enk@astro.caltech.edu, E-mail: wenjin.huang@brion.com. THE PECULIAR CHEMICAL INVENTORY OF NGC 2419: AN EXTREME OUTER HALO 'GLOBULAR CLUSTER'. United States: N. p., 2011. Web. doi:10.1088/0004-637X/740/2/60; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA).
Cohen, Judith G., Kirby, Evan N., & Huang Wenjin, E-mail: jlc@astro.caltech.edu, E-mail: enk@astro.caltech.edu, E-mail: wenjin.huang@brion.com. THE PECULIAR CHEMICAL INVENTORY OF NGC 2419: AN EXTREME OUTER HALO 'GLOBULAR CLUSTER'. United States. doi:10.1088/0004-637X/740/2/60; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA).
Cohen, Judith G., Kirby, Evan N., and Huang Wenjin, E-mail: jlc@astro.caltech.edu, E-mail: enk@astro.caltech.edu, E-mail: wenjin.huang@brion.com. 2011. "THE PECULIAR CHEMICAL INVENTORY OF NGC 2419: AN EXTREME OUTER HALO 'GLOBULAR CLUSTER'". United States. doi:10.1088/0004-637X/740/2/60; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA).
@article{osti_21587389,
title = {THE PECULIAR CHEMICAL INVENTORY OF NGC 2419: AN EXTREME OUTER HALO 'GLOBULAR CLUSTER'},
author = {Cohen, Judith G. and Kirby, Evan N. and Huang Wenjin, E-mail: jlc@astro.caltech.edu, E-mail: enk@astro.caltech.edu, E-mail: wenjin.huang@brion.com},
abstractNote = {NGC 2419 is a massive outer halo Galactic globular cluster (GC) whose stars have previously been shown to have somewhat peculiar abundance patterns. We have observed seven luminous giants that are members of NGC 2419 with Keck/HIRES at reasonable signal-to-noise ratio. One of these giants is very peculiar, with an extremely low [Mg/Fe] and high [K/Fe] but normal abundances of most other elements. The abundance pattern does not match the nucleosynthetic yields of any supernova model. The other six stars show abundance ratios typical of inner halo Galactic GCs, represented here by a sample of giants in the nearby GC M30. Although our measurements show that NGC 2419 is unusual in some respects, its bulk properties do not provide compelling evidence for a difference between inner and outer halo GCs.},
doi = {10.1088/0004-637X/740/2/60; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA)},
journal = {Astrophysical Journal},
number = 2,
volume = 740,
place = {United States},
year = 2011,
month =
}
  • We present new Keck/HIRES observations of six red giants in the globular cluster (GC) NGC 2419. Although the cluster is among the most distant and most luminous in the Milky Way, it was considered chemically ordinary until very recently. Our previous work showed that the near-infrared Ca II triplet line strength varied more than expected for a chemically homogeneous cluster, and that at least one star had unusual abundances of Mg and K. Here, we confirm that NGC 2419 harbors a population of stars, comprising about one-third of its mass, that is depleted in Mg by a factor of eightmore » and enhanced in K by a factor of six with respect to the Mg-normal population. Although the majority, Mg-normal population appears to have a chemical abundance pattern indistinguishable from ordinary, inner-halo GCs, the Mg-poor population exhibits dispersions of several elements. The abundances of K and Sc are strongly anti-correlated with Mg, and some other elements (Si and Ca among others) are weakly anti-correlated with Mg. These abundance patterns suggest that the different populations of NGC 2419 sample the ejecta of diverse supernovae in addition to asymptotic giant branch ejecta. However, the abundances of Fe-peak elements except Sc show no star-to-star variation. We find no nucleosynthetic source that satisfactorily explains all of the abundance variations in this cluster. Because NGC 2419 appears like no other GC, we reiterate our previous suggestion that it is not a GC at all, but rather the core of an accreted dwarf galaxy.« less
  • Two independent studies recently uncovered two distinct populations among giants in the distant, massive globular cluster (GC) NGC 2419. One of these populations has normal magnesium (Mg) and potassium (K) abundances for halo stars: enhanced Mg and roughly solar K. The other population has extremely depleted Mg and very enhanced K. To better anchor the peculiar NGC 2419 chemical composition, we have investigated the behavior of K in a few red giant branch stars in NGC 6752, NGC 6121, NGC 1904, and {omega} Cen. To verify that the high K abundances are intrinsic and not due to some atmospheric featuresmore » in giants, we also derived K abundances in less evolved turn-off and subgiant stars of clusters 47 Tuc, NGC 6752, NGC 6397, and NGC 7099. We normalized the K abundance as a function of the cluster metallicity using 21 field stars analyzed in a homogeneous manner. For all GCs of our sample, the stars lie in the K-Mg abundance plane on the same locus occupied by the Mg-normal population in NGC 2419 and by field stars. This holds for both giants and less-evolved stars. At present, NGC 2419 seems unique among GCs.« less
  • Color-magnitude diagrams (CMDs) derived from Hubble Space Telescope (HST) Advanced Camera for Surveys F606W, F814W photometry of six globular clusters (GCs) are presented. The six GCs form two loose groupings in Galactocentric distance (R{sub GC}): IC 4499, NGC 6426, and Ruprecht 106 at {approx}15-20 kpc and NGC 7006, Palomar 15, and Pyxis at {approx}40 kpc. The CMDs allow the ages to be estimated from the main-sequence turnoff in every case. In addition, the age of Palomar 5 (R{sub GC} {approx} 18 kpc) is estimated using archival HST Wide Field Planetary Camera 2 V, I photometry. The age analysis reveals themore » following: IC 4499, Ruprecht 106, and Pyxis are 1-2 Gyr younger than inner halo GCs with similar metallicities; NGC 7006 and Palomar 5 are marginally younger than their inner halo counterparts; NGC 6426 and Palomar 15, the two most metal-poor GCs in the sample, are coeval with all the other metal-poor GCs within the uncertainties. Combined with our previous efforts, the current sample provides strong evidence that the Galactic GC age-metallicity relation consists of two distinct branches. One suggests a rapid chemical enrichment in the inner Galaxy while the other suggests prolonged GC formation in the outer halo. The latter is consistent with the outer halo GCs forming in dwarf galaxies and later being accreted by the Milky Way.« less
  • We use CFHT/MegaCam data to search for outer halo star clusters in M33 as part of the Pan-Andromeda Archaeological Survey. This work extends previous studies out to a projected radius of 50 kpc and covers over 40 deg{sup 2}. We find only one new unambiguous star cluster in addition to the five previously known in the M33 outer halo (10 kpc {<=} r {<=} 50 kpc). Although we identify 2440 cluster candidates of various degrees of confidence from our objective image search procedure, almost all of these are likely background contaminants, mostly faint unresolved galaxies. We measure the luminosity, color,more » and structural parameters of the new cluster in addition to the five previously known outer halo clusters. At a projected radius of 22 kpc, the new cluster is slightly smaller, fainter, and redder than all but one of the other outer halo clusters, and has g' {approx} 19.9, (g' - i') {approx} 0.6, concentration parameter c {approx} 1.0, a core radius r{sub c} {approx} 3.5 pc, and a half-light radius r{sub h} {approx} 5.5 pc. For M33 to have so few outer halo clusters compared to M31 suggests either tidal stripping of M33's outer halo clusters by M31, or a very different, much calmer accretion history of M33.« less
  • The KPNO video camera system has been used to determine a color-magnitude diagram for the distant halo globular cluster Pal 14. In common with other outer halo systems, this cluster possesses a predominantly red horizontal branch. If M/sub v/ = +0.6 for the horizontal branch stars, then (m--M)/sub 0/ = 19.4, and the cluster lies 70 kpc from the galactic center and 50 kpc above the galactic plane.