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Silver and palladium help unveil the nature of a second r-process | | Camilla Juul Hansen
; Francesca Primas
; Henrik Hartman
; Karl-Ludwig Kratz
; Shinya Wanajo
; Bruno Leibundgut
; Khalil Farouqi
; Oliver Hallmann
; Norbert Christlieb
; Hampus Nilsson
; | | Date: |
21 May 2012 | | Abstract: | The rapid neutron-capture process, creating about half of the heaviest
elements in the Solar System was believed to be unique. Many recent studies
have shown that this does not include the formation of lighter elements (in
particular 38 < Z < 48). Among those, palladium (Pd) and especially silver (Ag)
are expected to be key indicators of a possible second r-process, but until
recently they have been studied only in a few stars. Therefore we target Pd and
Ag in a large sample of stars and compare these abundances to those of Sr, Y,
Zr, Ba and Eu produced by the slow (s-) and rapid (r-) neutron-capture
processes. Hereby we investigate the nature of the formation process of Ag and
Pd. Through a homogeneous 1D LTE analysis of 71 stars we derive stellar
abundances using the spectrum synthesis code MOOG, and MARCS model atmospheres.
We calculate abundance ratio trends and compare the derived abundances to
site-dependent yield predictions (low mass O-Ne-Mg cc SN, and parametrised high
entropy winds), to extract characteristics of the second r-process. The
abundance ratios of the heavy elements yield correlations and
anti-correlations. These trends lead to clear indications of the existence of a
second/weak r-process, responsible for the formation of Pd and Ag. By comparing
to the model predictions, we find that the conditions under which this process
takes place differ from the main r-process in needing lower neutron number
densities, neutron-to-seed ratios, entropies and/or favour higher electron
abundances. Our analysis confirms that Pd and Ag form via a r-process that
differs from the main r-process, the main and weak s-processes, and charged
particle freeze-outs. This process is efficiently working down to [Fe/H] = -3.3
(where our sample ends). Our results may indicate that a combination of these
explosive sites is needed to explain the observationally-derived abundance
patterns. | | Source: | arXiv, 1205.4744 | | Services: | Forum | Review | PDF | Favorites | |
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