The diverse origins of neutron-capture elements in the metal-poor star HD 94028 : possible detection of products of i-process nucleosynthesis

Roederer, Ian U.; Karakas, Amanda I.; Pignatari, Marco; Herwig, Falk

Abstract

We present a detailed analysis of the composition and nucleosynthetic origins of the heavy elements in the metal-poor ([Fe/H] = −1.62 ± 0.09) star HD 94028. Previous studies revealed that this star is mildly enhanced in elements produced by the slow neutron-capture process (s process; e.g., [Pb/Fe] = +0.79 ± 0.32) and rapid neutron-capture process (r process; e.g., [Eu/Fe] = +0.22 ± 0.12), including unusually large molybdenum ([Mo/Fe] = +0.97 ± 0.16) and ruthenium ([Ru/Fe] = +0.69 ± 0.17) enhancements. However, this star is not enhanced in carbon ([C/Fe] = −0.06 ± 0.19). We analyze an archival near-ultraviolet spectrum of HD 94028, collected using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, and other archival optical spectra collected from ground-based telescopes. We report abundances or upper limits derived from 64 species of 56 elements. We compare these observations with s-process yields from low-metallicity AGB evolution and nucleosynthesis models. No combination of s- and r-process patterns can adequately reproduce the observed abundances, including the super-solar [As/Ge] ratio (+0.99 ± 0.23) and the enhanced [Mo/Fe] and [Ru/Fe] ratios. We can fit these features when including an additional contribution from the intermediate neutron-capture process (i process), which perhaps operated through the ingestion of H in He-burning convective regions in massive stars, super-AGB stars, or low-mass AGB stars. Currently, only the i process appears capable of consistently producing the super-solar [As/Ge] ratios and ratios among neighboring heavy elements found in HD 94028. Other metal-poor stars also show enhanced [As/Ge] ratios, hinting that operation of the i process may have been common in the early Galaxy.

Citation

Roederer, I. U., Karakas, A. I., Pignatari, M., & Herwig, F. (2016). The diverse origins of neutron-capture elements in the metal-poor star HD 94028 : possible detection of products of i-process nucleosynthesis. The Astrophysical journal, 821(1), Article ARTN 37. https://doi.org/10.3847/0004-637X/821/1/37

• Acceptance Date: Feb 27, 2016
• Online Publication Date: Apr 6, 2016
• Publication Date: Apr 10, 2016
• Deposit Date: Apr 19, 2016
• Publicly Available Date: Apr 19, 2016
• Journal: Astrophysical journal
• Print ISSN: 0004-637X
• Electronic ISSN: 1538-4357
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 821
• Issue: 1
• Article Number: ARTN 37
• DOI: https://doi.org/10.3847/0004-637X/821/1/37
• Keywords: I-process nucleosynthesis, Metal-poor stars
• Public URL: https://hull-repository.worktribe.com/output/436413
• Publisher URL: http://iopscience.iop.org/article/10.3847/0004-637X/821/1/37/meta;jsessionid=6019D150AAB7EB6CCC0B0EAA2988A39D.c2
• Additional Information: Copy of article first published in: Astrophysical journal, 2016, v.821, issue 1