Galactic evolution of sulphur as traced by globular clusters

• Verfasst von: Kacharov, Nikolay [VerfasserIn]
• Verfasst von: Koch-Hansen, Andreas [VerfasserIn]
• Verfasst von: Caffau, Elisabetta [VerfasserIn]
• Verfasst von: Sbordone, Luca [VerfasserIn]
• Titel: Galactic evolution of sulphur as traced by globular clusters
• Verf.angabe: N. Kacharov, A. Koch, E. Caffau, and L. Sbordone
• E-Jahr: 2015
• Jahr: 24 April 2015
• Umfang: 9 S.
• Fussnoten: Gesehen am 17.06.2020
• Titel Quelle: Enthalten in: Astronomy and astrophysics
• Ort Quelle: Les Ulis : EDP Sciences, 1969
• Jahr Quelle: 2015
• Band/Heft Quelle: 577(2015) Artikel-Nummer A18, 9 Seiten
• ISSN Quelle: 1432-0746
• DOI: doi:10.1051/0004-6361/201425058
• Datenträger: Online-Ressource
• Sprache: eng
• K10plus-PPN: 1700329448

Abstract

Sulphur is an important volatile <i>α<i/> element, but its role in the Galactic chemical evolution is still uncertain, and more observations constraining the sulphur abundance in stellar photospheres are required.<i>Aims. <i/>We derive the sulphur abundances in red giant branch (RGB) stars in three Galactic halo globular clusters (GC) that cover a wide metallicity range (−2.3 < [Fe/H] < −1.2): M 4 (NGC 6121), M 22 (NGC 6656), and M 30 (NGC 7099). The halo field stars show a large scatter in the [S/Fe] ratio in this metallicity span, which is inconsistent with canonical chemical evolution models. To date, very few measurements of [S/Fe] exist for stars in GCs, which are good tracers of the chemical enrichment of their environment. However, some light and <i>α<i/> elements show star-to-star variations within individual GCs, and it is as yet unclear whether the <i>α<i/> element sulphur also varies between GC stars.<i>Methods. <i/>We used the infrared spectrograph CRIRES to obtain high-resolution (<i>R<i/> ~ 50 000), high signal-to-noise (<i>S<i/>/<i>N<i/> ~ 200 per px) spectra in the region of the S I multiplet 3 at 1045 nm for 15 GC stars selected from the literature (six stars in M 4,six stars in M 22, and three stars in M 30). Multiplet 3 is better suited for S abundance derivation than the more commonly used lines of multiplet 1 at 920 nm, since its lines are not blended by telluric absorption or other stellar features at low metallicity.<i>Results. <i/>We used spectral synthesis to derive the [S/Fe] ratio of the stars assuming local thermodynamic equilibrium (LTE). We find mean [S/Fe]<sub>LTE<sub/> = 0.58 ± 0.01 ± 0.20 dex (statistical and systematic error) for M 4, [S/Fe]<sub>LTE<sub/> = 0.57 ± 0.01 ± 0.19 dex for M 22, and [S/Fe]<sub>LTE<sub/> = 0.55 ± 0.02 ± 0.16 dex for M 30. The negative NLTE corrections are estimated to be in the order of the systematic uncertainties. We do not detect star-to-star variations of the S abundance in any of the observed GCs, with the possible exception of two individual stars, one in M 22 and one in M 30, which appear to be highly enriched in S.<i>Conclusions. <i/>With the tentative exception of two stars with measured high S abundances, we conclude that sulphur behaves like a typical <i>α<i/> element in the studied Galactic GCs, showing enhanced abundances with respect to the solar value at metallicities below [Fe/H]−1.0 dex without a considerable spread.