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Stromgren uvby photometry of the peculiar globular cluster NGC 2419

Frank, Matthias J.; Koch, Andreas; Feltzing, Sofia LU ; Kacharov, Nikolay; Wilkinson, Mark I. and Irwin, Mike (2015) In Astronomy & Astrophysics 581.
Abstract
NGC 2419 is a peculiar Galactic globular cluster off set from the others in the size-luminosity diagram, and showing several chemical abundance anomalies. Here, we present Stromgren uvby photometry of the cluster. Using the gravity-and metallicity-sensitive c(1) and m(1) indices, we identify a sample of likely cluster members extending well beyond the formal tidal radius. The estimated contamination by cluster non-members is only one per cent, making our catalogue ideally suited for spectroscopic follow-up. We derive photometric [Fe/H] of red giants, and depending on which metallicity calibration from the literature we use, we find reasonable to excellent agreement with spectroscopic [Fe/H], both for the cluster mean metallicity and for... (More)
NGC 2419 is a peculiar Galactic globular cluster off set from the others in the size-luminosity diagram, and showing several chemical abundance anomalies. Here, we present Stromgren uvby photometry of the cluster. Using the gravity-and metallicity-sensitive c(1) and m(1) indices, we identify a sample of likely cluster members extending well beyond the formal tidal radius. The estimated contamination by cluster non-members is only one per cent, making our catalogue ideally suited for spectroscopic follow-up. We derive photometric [Fe/H] of red giants, and depending on which metallicity calibration from the literature we use, we find reasonable to excellent agreement with spectroscopic [Fe/H], both for the cluster mean metallicity and for individual stars. We demonstrate explicitly that the photometric uncertainties are not Gaussian and this must be accounted for in any analysis of the metallicity distribution function. Using a realistic, non-Gaussian model for the photometric uncertainties, we find a formal internal [Fe/H] spread of sigma = 0.11(-0.01)(+0.02) dex. This is an upper limit to the cluster's true [Fe/H] spread and may partially, and possibly entirely, reflect the limited precision of the photometric metallicity estimation and systematic effects. The lack of correlation between spectroscopic and photometric [Fe/H] of individual stars is further evidence against a [Fe/H] spread on the 0.1 dex level. Finally, the CN-sensitive delta(4), among other colour indices, anti-correlates strongly with magnesium abundance, indicating that the second-generation stars are nitrogen enriched. The absence of similar correlations in some other CN-sensitive indices supports the second generation being enriched in He, which in these indices approximately compensates the shift due to CN. Compared to a single continuous distribution with finite dispersion, the observed delta(4) distribution of red giants is slightly better fit by two distinct populations with no internal spread, with the nitrogen-enhanced second generation accounting for 53 +/- 5 per cent of stars. Despite its known peculiarities, NGC 2419 appears to be very similar to other metal-poor Galactic globular clusters with a similarly nitrogen-enhanced second generation and little or no variation in [Fe/H], which sets it apart from other suspected accreted nuclei such as omega Cen. (Less)
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author
organization
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type
Contribution to journal
publication status
published
subject
keywords
globular clusters: individual: NGC 2419, stars: abundances, techniques:, photometric
in
Astronomy & Astrophysics
volume
581
publisher
EDP Sciences
external identifiers
  • wos:000361803900072
  • scopus:84941196184
ISSN
0004-6361
DOI
10.1051/0004-6361/201526555
language
English
LU publication?
yes
id
f5bc6f4f-49e6-41a6-a784-f326e5365c52 (old id 8214548)
date added to LUP
2015-11-27 14:54:00
date last changed
2017-01-01 05:27:53
@article{f5bc6f4f-49e6-41a6-a784-f326e5365c52,
  abstract     = {NGC 2419 is a peculiar Galactic globular cluster off set from the others in the size-luminosity diagram, and showing several chemical abundance anomalies. Here, we present Stromgren uvby photometry of the cluster. Using the gravity-and metallicity-sensitive c(1) and m(1) indices, we identify a sample of likely cluster members extending well beyond the formal tidal radius. The estimated contamination by cluster non-members is only one per cent, making our catalogue ideally suited for spectroscopic follow-up. We derive photometric [Fe/H] of red giants, and depending on which metallicity calibration from the literature we use, we find reasonable to excellent agreement with spectroscopic [Fe/H], both for the cluster mean metallicity and for individual stars. We demonstrate explicitly that the photometric uncertainties are not Gaussian and this must be accounted for in any analysis of the metallicity distribution function. Using a realistic, non-Gaussian model for the photometric uncertainties, we find a formal internal [Fe/H] spread of sigma = 0.11(-0.01)(+0.02) dex. This is an upper limit to the cluster's true [Fe/H] spread and may partially, and possibly entirely, reflect the limited precision of the photometric metallicity estimation and systematic effects. The lack of correlation between spectroscopic and photometric [Fe/H] of individual stars is further evidence against a [Fe/H] spread on the 0.1 dex level. Finally, the CN-sensitive delta(4), among other colour indices, anti-correlates strongly with magnesium abundance, indicating that the second-generation stars are nitrogen enriched. The absence of similar correlations in some other CN-sensitive indices supports the second generation being enriched in He, which in these indices approximately compensates the shift due to CN. Compared to a single continuous distribution with finite dispersion, the observed delta(4) distribution of red giants is slightly better fit by two distinct populations with no internal spread, with the nitrogen-enhanced second generation accounting for 53 +/- 5 per cent of stars. Despite its known peculiarities, NGC 2419 appears to be very similar to other metal-poor Galactic globular clusters with a similarly nitrogen-enhanced second generation and little or no variation in [Fe/H], which sets it apart from other suspected accreted nuclei such as omega Cen.},
  articleno    = {A72},
  author       = {Frank, Matthias J. and Koch, Andreas and Feltzing, Sofia and Kacharov, Nikolay and Wilkinson, Mark I. and Irwin, Mike},
  issn         = {0004-6361},
  keyword      = {globular clusters: individual: NGC 2419,stars: abundances,techniques:,photometric},
  language     = {eng},
  publisher    = {EDP Sciences},
  series       = {Astronomy & Astrophysics},
  title        = {Stromgren uvby photometry of the peculiar globular cluster NGC 2419},
  url          = {http://dx.doi.org/10.1051/0004-6361/201526555},
  volume       = {581},
  year         = {2015},
}