K. Farouqi

Charged-particle and neutron-capture processes in the high-entropy wind of core-collapse supernovae

The astrophysical site of the r-process is still uncertain, and a full exploration of the systematics of this process in terms of its dependence on nuclear properties from stability to the neutron drip-line within realistic stellar environments has still to be undertaken. Sufficiently high neutron-to-seed ratios can only be obtained either in very neutron-rich low-entropy environments or moderately neutron-rich high-entropy environments, related to neutron star mergers (or jets of neutron star matter) and the high-entropy wind of core-collapse supernova explosions. As chemical evolution models seem to disfavor neutron star mergers, we focus here on high-entropy environments characterized by entropy S, electron abundance Y-e, and expansion velocity V-exp. We investigate the termination point of charged-particle reactions, and we define a maximum entropy S-final for a given V-exp and Y-e, beyond which the seed production of heavy elements fails due to the very small matter density. We then investigate whether an r-process subsequent to the charged-particle freeze-out can in principle be understood on the basis of the classical approach, which assumes a chemical equilibrium between neutron captures and photodisintegrations, possibly followed by a beta-flow equilibrium. In particular, we illustrate how long such a chemical equilibrium approximation holds, how the freeze-out from such conditions affects the abundance pattern, and which role the late capture of neutrons originating from beta-delayed neutron emission can play. Furthermore, we analyze the impact of nuclear properties from different theoretical mass models on the final abundances after these late freeze-out phases and beta-decays back to stability. As only a superposition of astrophysical conditions can provide a good fit to the solar r-abundances, the question remains how such superpositions are attained, resulting in the apparently robust r-process pattern observed in low metallicity stars.

Explorations of the r-processes: Comparisons between calculations and observations of low-metallicity stars

Abundances of heavier elements (barium and beyond) in many neutron-capture-element-rich halo stars accurately replicate the solar system r-process pattern. However, abundances of lighter neutron-capture elements in these stars are not consistent with the solar system pattern. These comparisons suggest contributions from two distinct types of r-process synthesis events, a so-called main r-process for the elements above the second r-process peak and a weak r-process for the lighter neutron-capture elements. We have performed r-process theoretical predictions to further explore the implications of the solar and stellar observations. We find that the isotopic composition of barium and the elemental Ba/Eu abundance ratios in r-process-rich low-metallicity stars can only be matched by computations in which the neutron densities are in the range 23 log nn 28, values typical of the main r-process. For r-process conditions that successfully generate the heavy element pattern extending down to A = 135, the relative abundance of 129I produced in this mass region appears to be at least ~90% of the observed solar value. Finally, in the neutron number density ranges required for production of the observed solar/stellar third r-process-peak (A ≈ 200), the predicted abundances of interpeak element hafnium (Z = 72, A ≈ 177-180) follow closely those of third-peak elements and lead. Hf, observable from the ground and close in mass number to the third r-process-peak elements, might also be used as part of a new nuclear chronometer pair, Th/Hf, for stellar age determinations.

Nucleosynthesis in the early galaxy

Recent observations of r-process-enriched metal-poor star abundances reveal a nonuniform abundance pattern for elements -->Z ≤ 47. Based on noncorrelation trends between elemental abundances as a function of Eu richness in a large sample of metal-poor stars, it is shown that the mixing of a consistent and robust light element primary process (LEPP) and the r-process pattern found in r-II metal-poor stars explains such apparent nonuniformity. Furthermore, we derive the abundance pattern of the LEPP from observation and show that it is consistent with a missing component in the solar abundances when using a recent s-process model. As the astrophysical site of the LEPP is not known, we explore the possibility of a neutron-capture process within a site-independent approach. It is suggested that scenarios with neutron densities -->nn ≤ 1013 cm−3 or in the range -->nn ≥ 1024 cm−3 best explain the observations.

Silver and palladium help unveil the nature of a second r-process

Context. The rapid neutron-capture process, which created about half of the heaviest elements in the solar system, is believed to have been unique. Many recent studies have shown that this uniqueness is not true for the formation of lighter elements, in particular those in the atomic number range 38 < Z < 48. Among these, 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. We therefore 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. Aims. We study the abundances of seven elements (Sr, Y, Zr, Pd, Ag, Ba, and Eu) to gain insight into the formation process of the elements and explore in depth the nature of the second r-process. Methods. By adopting a homogeneous one-dimensional local thermodynamic equilibrium (1D LTE) analysis of 71 stars, we derive stellar abundances using the spectral synthesis code MOOG, and the MARCS model atmospheres. We calculate abundance ratio trends and compare the derived abundances to site-dependent yield predictions (low-mass O-Ne-Mg core-collapse supernovae and parametrised high-entropy winds), to extract characteristics of the second r-process. Results. The seven elements are tracers of different (neutron-capture) processes, which in turn allows us to constrain the formation process(es) of Pd and Ag. The abundance ratios of the heavy elements are found to be correlated and anti-correlated. These trends lead to clear indications that a second/weak r-process, is responsible for the formation of Pd and Ag. On the basis of the comparison to the model predictions, we find that the conditions under which this process takes place differ from those for the main r-process in needing lower neutron number densities, lower neutron-to-seed ratios, and lower entropies, and/or higher electron abundances. Conclusions. Our analysis confirms that Pd and Ag form via a rapid neutron-capture process that differs from the main r-process, the main and weak s-processes, and charged particle freeze-outs. We find that this process is efficiently working down to the lowest metallicity sampled by our analysis ([Fe/H] = −3.3). Our results may indicate that a combination of these explosive sites is needed to explain the variety in the observationally derived abundance patterns.

Nucleosynthesis Modes in The High-Entropy Wind of Type II Supernovae: Comparison of Calculations With Halo-Star Observations

While the high-entropy wind (HEW) of Type II supernovae remains one of the more promising sites for the rapid neutron-capture (r-) process, hydrodynamic simulations have yet to reproduce the astrophysical conditions under which the latter occurs. We have performed large-scale network calculations within an extended parameter range of the HEW, seeking to identify or to constrain the necessary conditions for a full reproduction of all r-process residuals N r,☉ = N ☉–N s,☉ by comparing the results with recent astronomical observations. A superposition of weighted entropy trajectories results in an excellent reproduction of the overall N r,☉ pattern beyond Sn. For the lighter elements, from the Fe group via Sr-Y-Zr to Ag, our HEW calculations indicate a transition from the need for clearly different sources (conditions/sites) to a possible co-production with r-process elements, provided a range of entropies are contributing. This explains recent halo-star observations of a clear noncorrelation of Zn and Ge and a weak correlation of Sr-Zr with heavier r-process elements. Moreover, new observational data on Ru and Pd also seem to confirm a partial correlation with Sr as well as the main r-process elements (e.g., Eu).

The Hamburg/ESO R-process enhanced star survey (HERES) IV. Detailed abundance analysis and age dating of the strongly r-process enhanced stars CS 29491-069 and HE 1219-0312 ,

We report on a detailed abundance analysis of two strongly r-process enhanced, very metal-poor stars newly discovered in the HERES project, CS 29491−069 ([Fe/H] = −2.51, [r/Fe] =+ 1.1) and HE 1219−0312 ([Fe/H] = −2.96, [r/Fe] =+ 1.5). The analysis is based on high-quality VLT/UVES spectra and MARCS model atmospheres. We detect lines of 15 heavy elements in the spectrum of CS 29491−069, and 18 in HE 1219−0312; in both cases including the Th II 4019 A line. The heavy-element abundance patterns of these two stars are mostly well-matched to scaled solar residual abundances not formed by the s-process. We also compare the observed pattern with recent high-entropy wind (HEW) calculations, which assume core-collapse supernovae of massive stars as the astrophysical environment for the r-process, and find good agreement for most lanthanides. The abundance ratios of the lighter elements strontium, yttrium, and zirconium, which are presumably not formed by the main r-process, are reproduced well by the model. Radioactive dating for CS 29491−069 with the observed thorium and rare-earth element abundance pairs results in an average age of 9.5 Gyr, when based on solar r-process residuals, and 17.6 Gyr, when using HEW model predictions. Chronometry seems to fail in the case of HE 1219−0312, resulting in a negative age due to its high thorium abundance. HE 1219−0312 could therefore exhibit an overabundance of the heaviest elements, which is sometimes called an “actinide boost”.

Co-production of light p-, s- and r-process isotopes in the high-entropy wind of type II supernovae

We have performed large-scale nucleosynthesis calculations within the high-entropy-wind (HEW) scenario of Type II supernovae. The primary aim was to constrain the conditions for the production of the classical ‘p-only’ isotopes of the light trans-Fe elements. We find, however, that for electron fractions in the range 0.458 ≤ Ye ≤ 0.478, sizeable abundances of p-, s- and r-process nuclei between 64Zn and 98Ru are coproduced in the HEW at low entropies (S ≤ 100) by a primary charged-particle process after an α-rich freezeout. With the above Ye–S correlation, most of the predicted isotopic abundance ratios within a given element, e.g. 64Zn(p)/70Zn(r) or 92Mo(p)/94Mo(p), as well as of neighboring elements, e.g. 70Ge(s + p)/74Se(p) or 74Se(p)/78Kr(p) agree with the observed Solar-System ratios. Taking the Mo isotopic chain as a particularly challenging example, we show that our HEW model can account for the production of all 7 stable isotopes, from ‘p-only’ 92Mo, via ‘s-only’ 96Mo up to ‘r-only’ 100Mo. Furthermore, our model is able to reproduce the isotopic composition of Mo in presolar SiC X-grains.

New attempts to understand nanodiamond stardust

We report on a concerted effort aimed at understanding the origin and history of the pre-solar nanodiamonds in meteorites including the astrophysical sources of the observed isotopic abundance signatures. This includes measurement of light elements by secondary ion mass spectrometry (SIMS), analysis of additional heavy trace elements by accelerator mass spectrometry (AMS) and dynamic calculations of r-process nucleosynthesis with updated nuclear properties. Results obtained indicate that: (i) there is no evidence for the former presence of now-extinct 26Al and 44Ti in our diamond samples other than what can be attributed to silicon carbide and other ‘impurities’, and this does not offer support for a supernova (SN) origin but neither does it negate it; (ii) analysis by AMS of platinum in ‘bulk diamond’ yields an overabundance of r-only 198Pt that at face value seems more consistent with the neutron burst than with the separation model for the origin of heavy trace elements in the diamonds, although this conclusion is not firm given analytical uncertainties; (iii) if the Xe–H pattern was established by an unadulterated r-process, it must have been a strong variant of the main r-process, which possibly could also account for the new observations in platinum.

News from r‐Process Nucleosynthesis: Consequences from the ESS to Early Stars

The exact conditions for the supemova high‐entropy wind (HEW) as one of the favored sites for the rapid neutron‐capture (r‐) process still cannot be reproduced self‐consistently in present hydrodynamic astrophysical models. Therefore, we have performed large‐scale dynamical network calculations within a parameterized HEW model to constrain the necessary conditions for a full r‐process and to compare our results with recent observations. A model‐inherently weighted superposition of entropy trajectories results in an excellent reproduction of the overall Solar‐System isotopic abundances (Nr,⊙) of the “main” r‐process elements beyond Sn. For the lighter r‐elements in the range 26⩽Z⩽42, our HEW model supports earlier qualitative ideas about a multiplicity of nucleosynthetic processes leading to the total Nr,⊙ distribution. In the HEW scenario, these suggestions are quantified, and the origin of the missing primary process is confirmed to be a rapid charged‐particle process, thus excluding a “weak” neutron‐capture component. This explains the recent halo‐star observations of a Z‐dependent non‐correlation, respectively partial correlation of the lighter r‐elements with metallicity and/or enrichment of “main” r‐process elements.The exact conditions for the supemova high‐entropy wind (HEW) as one of the favored sites for the rapid neutron‐capture (r‐) process still cannot be reproduced self‐consistently in present hydrodynamic astrophysical models. Therefore, we have performed large‐scale dynamical network calculations within a parameterized HEW model to constrain the necessary conditions for a full r‐process and to compare our results with recent observations. A model‐inherently weighted superposition of entropy trajectories results in an excellent reproduction of the overall Solar‐System isotopic abundances (Nr,⊙) of the “main” r‐process elements beyond Sn. For the lighter r‐elements in the range 26⩽Z⩽42, our HEW model supports earlier qualitative ideas about a multiplicity of nucleosynthetic processes leading to the total Nr,⊙ distribution. In the HEW scenario, these suggestions are quantified, and the origin of the missing primary process is confirmed to be a rapid charged‐particle process, thus excluding a “weak” neutron‐cap...

Nucleosynthesis Modes in the High‐Entropy‐Wind Scenario of Type II Supernovae

In an attempt to constrain the astrophysical conditions for the nucleosynthesis of the classical r‐process elements beyond Fe, we have performed large‐scale dynamical network calculations within the model of an adiabatically expanding high‐ entropy wind (HEW) of type II supernovae (SN II). A superposition of several entropy‐components (S) with model‐inherent weightings results in an excellent reproduction of the overall Solar System (SS) isotopic r‐process residuals (Nr,⊙), as well as the more recent observations of elemental abundances of metal‐poor, r‐process rich halo stars in the early Galaxy. For the heavy r‐process elements beyond Sn, our HEW model predicts a robust abundance pattern up to the Th, U r‐chronometer region. For the lighter neutron‐capture region, an S‐dependent superposition of (i) a normal α‐component directly producing stable nuclei, including s‐only isotopes, and (ii) a component from a neutron‐rich α‐freezeout followed by the rapid recapture of β‐delayed neutrons (βdnrpar; emitted ...