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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 (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: --------------------------------------------------- RA (2000) DE Designation(s) --------------------------------------------------- 17 44 54.71 +03 10 12.5 NGC 6426 = NGC 6426 --------------------------------------------------- File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file stars.dat 52 4 List of studied red giants table2.dat 86 272 Line list including equivalent widths
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
Bytes Format Units Label Explanations
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
Byte-by-byte Description of file: table2.dat
Bytes Format Units Label Explanations
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)
Note (1): Equivalent width uncertainties provided by EWCODE.
Acknowledgements: Michael Hanke, mhanke(at)
(End) Michael Hanke [ARI, Germany], Patricia Vannier [CDS] 08-Dec-2016
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