J/A+A/599/A97 Chemical composition of globular cluster NGC 6426 (Hanke+, 2017)
Insights into the chemical composition of the metal-poor Milky Way halo globular
cluster NGC 6426.
Hanke M., Koch A., Hansen C.J., McWilliam A.
<Astron. Astrophys. 599, A97 (2017)>
=2017A&A...599A..97H 2017A&A...599A..97H (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, nearby ; Stars, giant ; Stars, metal-deficient ;
Abundances ; Equivalent widths
Keywords: stars: abundances - Galaxy: abundances - Galaxy: evolution -
Galaxy: halo - globular clusters: individual: NGC 6426
Abstract:
We present our detailed spectroscopic analysis of the chemical
composition of four red giant stars in the halo globular cluster NGC
6426. We obtained high-resolution spectra using the Magellan2/MIKE
spectrograph, from which we derived equivalent widths and subsequently
computed abundances of 24 species of 22 chemical elements. For the
purpose of measuring equivalent widths, we developed a new
semi-automated tool, called EWCODE. We report a mean Fe content of
[Fe/H]=-2.34±0.05dex (stat.) in accordance with previous studies.
At a mean α-abundance of [(Mg,Si,Ca)/3Fe]=0.39±0.03dex,
NGC 6426 falls on the trend drawn by the Milky Way halo and other
globular clusters at comparably low metallicities. The distribution of
the lighter α-elements as well as the enhanced ratio
[Zn/Fe]=0.39dex could originate from hypernova enrichment of the
pre-cluster medium. We find tentative evidence for a spread in the
elements Mg, Si, and Zn, indicating an enrichment scenario, where
ejecta of evolved massive stars of a slightly older population
polluted a newly born younger one. The heavy element abundances in
this cluster fit well into the picture of metal-poor globular
clusters, which in that respect appear to be remarkably homogeneous.
The pattern of the neutron-capture elements heavier than Zn point
towards an enrichment history governed by the r-process with only
little - if any - sign of s-process contributions. This finding is
supported by the striking similarity of our program stars to the
metal-poor field star HD 108317.
Description:
Our spectra were gathered with the Magellan Inamori Kyocera Echelle
(MIKE) spectrograph, which is mounted at the 6.5-m Magellan2 Telescope
located at Las Campanas Observatory, Chile.
Throughout this work, in order to compute chemical abundances we have
computed equivalent widths using the newly developed semi-automated
tool EWCODE.
Objects:
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RA (2000) DE Designation(s)
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17 44 54.71 +03 10 12.5 NGC 6426 = NGC 6426
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
stars.dat 52 4 List of studied red giants
table2.dat 86 272 Line list including equivalent widths
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See also:
J/AJ/112/1054 : VI photometry of NGC 6426 (Zinn+ 1996)
J/A+A/573/A13 : Metal abundances in 6 globular cluster (Dias+, 2015)
Byte-by-byte Description of file: stars.dat
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Bytes Format Units Label Explanations
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1- 5 I5 --- Star Star number
7- 8 I2 h RAh Right ascension (J2000)
10- 11 I2 min RAm Right ascension (J2000)
13- 16 F4.1 s RAs Right ascension (J2000)
18 A1 --- DE- Declination sign (J2000)
19- 20 I2 deg DEd Declination (J2000)
22- 23 I2 arcmin DEm Declination (J2000)
25- 28 F4.1 arcsec DEs Declination (J2000)
30- 52 A23 --- Name 2MASS name
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 2 A2 --- El Element
4- 5 A2 --- Ion Ionization stage
10- 16 F7.2 0.1nm lambda Wavelength
21- 24 F4.2 eV chiex Excitation potential
29- 34 F6.3 [-] loggf Oscillator strength
39- 41 I3 0.1pm EW14853 ? Equivalent width for star 14853
45- 47 I3 0.1pm e_EW14853 ? Equivalent width error for star 14853 (1)
52- 54 I3 0.1pm EW5659 ? Equivalent width for star 5659
58- 60 I3 0.1pm e_EW5659 ? Equivalent width error for star 5659 (1)
65- 67 I3 0.1pm EW736 ? Equivalent width for star 736
71- 73 I3 0.1pm e_EW736 ? Equivalent width error for star 736 (1)
78- 80 I3 0.1pm EW2314 ? Equivalent width for star 2314
84- 86 I3 0.1pm e_EW2314 ? Equivalent width error for star 2314 (1)
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Note (1): Equivalent width uncertainties provided by EWCODE.
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Acknowledgements:
Michael Hanke, mhanke(at)ari.uni-heidelberg.de
(End) Michael Hanke [ARI, Germany], Patricia Vannier [CDS] 08-Dec-2016