Galactic Evolution of Sr, Y, and Zr: A Multiplicity of Nucleosynthetic Processes

Abstract

In this paper we follow the Galactic enrichment of three easily observed light n-capture elements -Sr, Y, and Zr. Input stellar yields have been first separated into their respective main and weak s-process components, and r-process component. The s-process yields from Asymptotic Giant Branch (AGB) stars of low to intermediate mass are computed, exploring a wide range of efficiencies of the major neutron source, 13 C, and covering both disk and halo metallicities. AGB stars have been shown to reproduce the main s-component in the solar system, i.e., the s-process isotopic distribution of all heavy isotopes with atomic mass number A > 90, with a minor contribution to the light s-process isotopes up to A ∼ 90. The concurrent weak s-process, which accounts for the major fraction of the light s-process isotopes in the solar system and occurs in massive stars by the operation of the 22 Ne neutron source, is discussed in detail. Neither the main s-, nor the weak s-components are shown to contribute significantly to the neutron capture element abundances observed in unevolved halo stars. Knowing the sprocess distribution at the epoch of the solar system formation, we first employed the r-process residuals method to infer the isotopic distribution of the r-process. We assumed a primary r-process production in the Galaxy from moderately massive Type II supernovae that best reproduces the observational Galactic trend of metallicity versus Eu, an almost pure r-process element. We present a detailed analysis of a large published database of spectroscopic observations of Sr, Y, Zr, Ba, and Eu for Galactic stars at various metallicities, showing that the observed trends versus metallicity can be understood in light of a multiplicity of stellar neutron-capture components. Spectroscopic observations of the Sr, Y, and Zr to Ba and Eu abundance ratios versus metallicity provide useful diagnostics of the types of neutron-capture processes forming Sr, Y and Zr. In particular, the observed [Sr,Y,Zr/Ba,Eu] ratio is clearly not flat at low metallicities, as we would expect if Ba, Eu and Sr, Y, Zr all had the same r-process nucleosynthetic origin. We discuss our chemical evolution predictions, taking into account the interplay between different processes to produce Sr-Y-Zr. Making use of the very r-process-rich and very metal-poor stars like CS 22892-052 and CS 31082-001, we find hints, and discuss the possibility of a primary process in low-metallicity massive stars, different from the 'classical s-process' and from the 'classical rprocess', that we tentatively define LEPP (Lighter Element Primary Process). This allows us to revise the estimates of the r-process contributions to the solar Sr, Y and Zr abundances, as well as of the contribution to the s-only isotopes 86,87 Sr and 96 Mo.

Nucleosynthetic signatures of the first stars

“…The weak s-process occurring in formerly more massive stars (M > 10 M ⊙ might yield Sr with little or no Ba, although theoretical calculations suggest that this process is inefficient at low metallicity. 16 Alternatively, the main "rapid" neutron-capture (r-) process might have produced the Sr, given that [Sr/Ba] non-LTE > −0.4 is consistent with the ratio observed in metal-poor, r-process enhanced stars, in which 17 [Sr/Ba] non-LTE ∼ −0.3 (non-LTE corrections were applied as for HE 1327−2326). It is therefore possible that the Sr was formed in a type II supernova (SNII), currently believed to be a site of the main r-process, that expelled material from which HE 1327−2326 formed.…”

“…Another possibility is the recently proposed process that might have provided light neutron-capture elements in the early Galaxy. 16 Although the site and details of this newly suggested process are not yet known, it might provide an explanation for the excesses of light neutron-capture elements, including Sr, seen in very metal-poor stars. To discriminate the contribution of such processes, further constraints on the Ba abundance, as well as further information on other neutron-capture elements, from higher quality data, are needed.…”

SYSTEMATIC NON-LTE STUDY OF THE −2.6 ≤ [Fe/H] ≤ 0.2 F AND G DWARFS IN THE SOLAR NEIGHBORHOOD. II. ABUNDANCE PATTERNS FROM Li TO Eu*

“…According to Travaglio et al (2004), 67% of the solar Zr was contributed by the main and weak s-process, and 15% and 18% was attributed to the r-and LEPP-process, respectively. Considering contributions from the r-and LEPP-process together, we deduce the solar r-residual [Zr/Ba] r ;=0.22.…”

s-process enrichment in the planetary nebula NGC 3918. Results from deep echelle spectrophotometry

“…In order to establish which level of [X/O], where X is a n-capture element, counts as significant, it is necessary to take into account the primordial scatter in the abundances of the progenitor gas. Based on stellar data by Travaglio et al (2004) Table 15. If we adopted the more recent solar composition of Asplund et al (2009), the derived abundances would change by less than 0.06 dex, with the exception of [Rb/O] which would be 0.11 dex larger.…”