R-process enrichment from a single event in an ancient dwarf galaxy
It has long been debated whether elements heavier than zinc are formed continually, for example in core-collapse supernovae, or in rare events, such as neutron star mergers; here, studies of element abundances in a local ultrafaint dwarf galaxy provide evidence that these elements are formed during...
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Veröffentlicht in: | Nature (London) 2016-03, Vol.531 (7596), p.610-613 |
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Zusammenfassung: | It has long been debated whether elements heavier than zinc are formed continually, for example in core-collapse supernovae, or in rare events, such as neutron star mergers; here, studies of element abundances in a local ultrafaint dwarf galaxy provide evidence that these elements are formed during rare yet prolific stellar events.
Nucleosynthesis in dwarf galaxy Reticulum II
The recently discovered Milky Way satellite Reticulum II, an ultra-faint dwarf galaxy, provides an ideal model for the study of stellar nucleosynthesis, the creation of heavier elements from hydrogen, helium and other lighter elements and particles. It has long been debated whether elements heavier than zinc are formed continually, for example in core-collapse supernovae, or in rare events, such as neutron star mergers. Alexander Ji
et al
. used high-resolution spectroscopy to determine element abundances in nine young stars in Reticulum II, and find that seven of the nine show strong enhancements in heavy neutron-capture elements with abundances that follow the universal r-process pattern above barium. The enhancement is several orders of magnitude greater than that seen in other ultra-faint dwarf galaxies, implying that a single rare event produced the r-process material.
Elements heavier than zinc are synthesized through the rapid (r) and slow (s) neutron-capture processes
1
,
2
. The main site of production of the r-process elements (such as europium) has been debated for nearly 60 years
2
. Initial studies of trends in chemical abundances in old Milky Way halo stars suggested that these elements are produced continually, in sites such as core-collapse supernovae
3
,
4
. But evidence from the local Universe favours the idea that r-process production occurs mainly during rare events, such as neutron star mergers
5
,
6
. The appearance of a plateau of europium abundance in some dwarf spheroidal galaxies has been suggested as evidence for rare r-process enrichment in the early Universe
7
, but only under the assumption that no gas accretes into those dwarf galaxies; gas accretion
8
favours continual r-process enrichment in these systems. Furthermore, the universal r-process pattern
1
,
9
has not been cleanly identified in dwarf spheroidals. The smaller, chemically simpler, and more ancient ultrafaint dwarf galaxies assembled shortly after the first stars formed, and are ideal systems with which to study nucleosynthesis events such as the r-process
10
,
11
. Reticulum II is one such g |
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ISSN: | 0028-0836 1476-4687 |
DOI: | 10.1038/nature17425 |