J/A+A/619/A178 Abundance analysis of 4 red giants in NGC 6558 (Barbuy+, 2018)
High-resolution abundance analysis of four red giants in the globular
cluster NGC 6558.
Barbuy B., Muniz L., Ortolani S., Ernandes H., Dias B., Saviane I.,
Kerber L., Bica E., Perez-Villegas A., Rossi L., Held E.V.
<Astron. Astrophys. 619, A178 (2018)>
=2018A&A...619A.178B 2018A&A...619A.178B (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way ; Clusters, globular ; Stars, giant ; Abundances
Keywords: stars: abundances - Galaxy: bulge - globular clusters: NGC 6558
Abstract:
NGC 6558 is a bulge globular cluster with a blue horizontal branch
(BHB), combined with a metallicity of [Fe/H]~-1.0. It is similar to HP
1 and NGC 6522, which could be among the oldest objects in the Galaxy.
Element abundances in these clusters could reveal the nature of the
first supernovae.
We aim to carry out detailed spectroscopic analysis for four red
giants of NGC 6558, in order to derive the abundances of the light
elements C, N, O, Na, Al, the α-elements Mg, Si, Ca, Ti, and the
heavy elements Y, Ba, and Eu.
High-resolution spectra of four stars with FLAMES-UVES@VLT UT2-Kueyen
were analysed. Spectroscopic parameter-derivation was based on
excitation and ionization equilibrium of FeI and FeII.
This analysis results in a metallicity of [Fe/H]=-1.17±0.10 for
NGC 6558. We find the expected α-element enhancements in O and
Mg with [O/Fe]=+0.40, [Mg/Fe]=+0.33, and low enhancements in Si and
Ca. Ti has a moderate enhancement of [Ti/Fe]=+0.22. The r-element Eu
appears very enhanced with a mean value of [Eu/Fe]=+0.63. The first
peak s-elements Y and Sr are also enhanced, these results have however
to be treated with caution, given the uncertainties in the continuum
definition; the use of neutral species (Sr I, YI), instead of the
dominant ionized species is another source of uncertainty. Ba appears
to have a solar abundance ratio relative to Fe.
NGC 6558 shows an abundance pattern that could be typical of the
oldest inner bulge globular clusters, together with the pattern in the
similar clusters NGC 6522 and HP 1. They show low abundances of the
odd-Z elements Na and Al, and of the explosive nucleosynthesis
α-elements Si, Ca, and Ti. The hydrostatic burning
α-elements O and Mg are normally enhanced as expected in old
stars enriched with yields from core-collapse supernovae, and the
iron-peak elements Mn, Cu, Zn show low abundances, which is expected
forMn and Cu, but not for Zn. Finally, the cluster trio NGC 6558,
NGC 6522 and HP 1 have relatively high abundances of first-peak heavy
elements, variable second-peak element Ba, and the r-element Eu is
enhanced. The latter is particularly high in NGC 6558.
Description:
The spectra of individual stars of NGC 6558 were obtained at the VLT
using the UVES spectrograph, in FLAMES-UVES mode. The red chip
(5800-6800Å) uses ESO CCD #20, an MIT backside illuminated, of
4096x2048pixels, and pixel size 15x15um. The blue chip (4800-5800Å)
uses ESO Marlene EEV CCD#44, backside illuminated, of 4102x2048
pixels, and pixel size 15x15um. The UVES standard setup 580 yields a
resolution R∼45000 for a 1 arcsec slit width. The pixel scale is
0.0147Å/pix, with ∼7.5 pixels per resolution element at 6000Å.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
stars.dat 48 4 List of studied stars
tablea1.dat 55 109 FeI and FeII lines employed, their wavelengths,
excitation potential, oscillator strengths and
van der Waals broadening constant C6 adopted
tablea2.dat 54 78 *Abundance ratios [X/Fe] of C, N, O, odd-Z elements
Na, Al, alpha-elements O, Mg, Si, Ca, Ti, and
heavy elements Eu, Ba, La, Y, Sr, and atomic
parameters adopted.
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Note on tablea2.dat: Comments on unreliable lines of La, Y, Sr are given in
the text.
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Byte-by-byte Description of file: stars.dat
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Bytes Format Units Label Explanations
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1- 4 I4 --- No Star number
6- 21 A16 --- 2MASS 2MASS name (HHMMSSss+DDMMSSs)
23- 24 I2 h RAh right ascension (J2000)
26- 27 I2 min RAm right ascension (J2000)
29- 34 F6.3 s RAs right ascension (J2000)
36 A1 --- DE- Declination sign (J2000)
37- 38 I2 deg DEd Declination (J2000)
40- 41 I2 arcmin DEm Declination (J2000)
43- 48 F6.3 arcsec DEs Declination (J2000)
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Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 4 A4 --- Ion Atomic species (FeI or FeII)
6- 12 F7.2 0.1nm lambda Wavelength
14- 17 F4.2 eV chiex Excitation potential χex
19- 23 F5.2 --- loggf Oscillator strength
25- 33 E9.4 --- C6 Van der Waals broadening constant C6
35- 38 F4.1 0.1nm EW283 ?=- Equivalent Width for star 283
40- 43 F4.1 0.1nm EW364 ?=- Equivalent Width for star 364
45- 49 F5.1 0.1nm EW1072 ?=- Equivalent Width for star 1072
51- 55 F5.1 0.1nm EW1160 ?=- Equivalent Width for star 1160
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Byte-by-byte Description of file: tablea2.dat
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Bytes Format Units Label Explanations
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1- 5 A5 --- Ion Atomic species
7- 14 F8.3 0.1nm lambda Wavelength
17- 20 F4.2 eV chiex ? Excitation potential χex
23- 27 F5.2 --- loggf ?=- Oscillator strength
29- 33 F5.2 --- [X/Fe]283 ?=- Logarithmic abundance relative to
Iron [X/Fe] for star 283
34 A1 --- u_[X/Fe]283 Uncertainty flag on [X/Fe]283
36- 40 F5.2 --- [X/Fe]364 ?=- Logarithmic abundance relative to
Iron [X/Fe] for star 364
41 A1 --- u_[X/Fe]364 Uncertainty flag on [X/Fe]364
43- 47 F5.2 --- [X/Fe]1072 ?=- Logarithmic abundance relative to
Iron [X/Fe] for star 1072
48 A1 --- u_[X/Fe]1072 Uncertainty flag on [X/Fe]1072
50- 54 F5.2 --- [X/Fe]1160 ?=- Logarithmic abundance relative to
Iron [X/Fe] for star 1160
55 A1 --- u_[X/Fe]1160 Uncertainty flag on [X/Fe]1160
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Acknowledgements:
Beatriz Barbuy, b.barbuy(at)iag.usp.br
(End) Patricia Vannier [CDS] 20-Nov-2018