Published in

Astronomy & Astrophysics, (622), p. A159, 2019

DOI: 10.1051/0004-6361/201834241

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Unusual neutron-capture nucleosynthesis in a carbon-rich Galactic bulge star

This paper was not found in any repository, but could be made available legally by the author.
This paper was not found in any repository, but could be made available legally by the author.

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Data provided by SHERPA/RoMEO

Abstract

Metal-poor stars in the Galactic halo often show strong enhancements in carbon and/or neutron-capture elements. However, the Galactic bulge is notable for its paucity of these carbon-enhanced metal-poor (CEMP) and/or CH-stars, with only two such objects known to date. This begs the question whether the processes that produced their abundance distribution were governed by a comparable nucleosynthesis in similar stellar sites as for their more numerous counterparts in the halo. Recently, two contenders of these classes of stars were discovered in the bulge, at [Fe/H] = −1.5 and −2.5 dex, both of which show enhancements in [C/Fe] of 0.4 and 1.4 dex (respectively), [Ba/Fe] in excess of 1.3 dex, and also elevated nitrogen. The more metal-poor of the stars can be well matched by standard s-process nucleosynthesis in low-mass asymptotic giant branch (AGB) polluters. The other star shows an abnormally high [Rb/Fe] ratio. Here, we further investigate the origin of the abundance peculiarities in the Rb-rich star by new, detailed measurements of heavy element abundances and by comparing the chemical element ratios of 36 species to several models of neutron-capture nucleosynthesis. The i-process with intermediate neutron densities between those of the slow (s-) and rapid (r)-neutron-capture processes has been previously found to provide good matches of CEMP stars with enhancements in both r- and s-process elements (class CEMP-r/s), rather than invoking a superposition of yields from the respective individual processes. However, the peculiar bulge star is incompatible with a pure i-process from a single ingestion event. Instead, it can, statistically, be better reproduced by more convoluted models accounting for two proton ingestion events, or by an i-process component in combination with s-process nucleosynthesis in low-to-intermediate mass (2–3 M) AGB stars, indicating multiple polluters. Finally, we discuss the impact of mixing during stellar evolution on the observed abundance peculiarities.

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