THE DIVERSE ORIGINS OF NEUTRON-CAPTURE ELEMENTS IN THE METAL-POOR STAR HD 94028: POSSIBLE DETECTION OF PRODUCTS OF i-PROCESS NUCLEOSYNTHESIS*

Abstract

We present a detailed analysis of the composition and nucleosynthetic origins of the heavy elements in the metalpoor ( We analyze an archival near-ultraviolet spectrum of HD94028, collected using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, and other archival optical spectra collected from ground-based telescopes. We report abundances or upper limits derived from 64species of 56elements. We compare these observations with s-process yields from low-metallicity AGB evolution and nucleosynthesis models. No combination of s-and r-process patterns can adequately reproduce the observed abundances, including the super-solar [As/Ge] ratio (+0.99 ± 0.23) and the enhanced [Mo/Fe] and [Ru/Fe] ratios. We can fit these features when including an additional contribution from the intermediate neutroncapture process (i process), which perhaps operated through the ingestion of H in He-burning convective regions in massive stars, super-AGB stars, or low-mass AGB stars. Currently, only the iprocess appears capable of consistently producing the super-solar [As/Ge] ratios and ratios among neighboring heavy elements found in HD94028. Other metal-poor stars also show enhanced [As/Ge] ratios, hinting that operation of the iprocess may have been common in the early Galaxy.

First determination ofs-process element abundances in pre-main sequence clusters

“…We have discussed several possible explanations for this curious behaviour, including chromospheric effects, microturbulence velocities, departures from the LTE approximation, or the activation of a different nucleosynthesis source such as the i-process, as has recently been suggested by Mishenina et al (2015). This last explanation seems to be very promising, but further theoretical investigations are needed, especially because of the uncertainties related to the models (see Mishenina et al 2015;Hampel et al 2016;Roederer et al 2016). The barium puzzle is still not solved and remains perhaps the most intriguing chemical feature of young clusters.…”

“…However, it is fundamental to stress that this Pb/Bi production strongly depends on the neutron exposure, which is substantially a free parameter in the models, so that the i-process can come in different "flavours" (see e.g. Roederer et al 2016 where instead a low neutron exposure has been adopted). Thus, a different source at different metallicity (CEMP stars are obviously much more metal-poor than our targets) might result in different yields.…”

From Nuclei to the Cosmos: Tracing Heavy-Element Production with the Oldest Stars

“…Overall, there is ample observational data to suggest that the i-process has contributed to the composition (see (Roederer et al 2016a) for a full list), for example, in the post-AGB star "Sakurai's object" (Herwig et al 2011), in pre-solar grains (Fujiya et al 2013), and in metal-poor stars with super-solar [As/Ge] and solar or sub-solar [Se/As] ratios (Roederer et al 2016a). More theoretical work is needed to firm up details on potential astrophysical sites, as well as yield predictions, to explain the full range of metal-poor stars with these unusual enhancements in neutron-capture elements.…”

The Intermediate Neutron-Capture Process and Carbon-Enhanced Metal-Poor Stars

“…The study of the influence of variations of the total neutron exposure on i-process abundance patterns goes beyond the scope of this work. In the future, the total neutron exposure should also be treated as a free parameter (as done for example by Roederer et al 2016). Including constraints from the observed lead abundances of CEMP-s/r stars can then be used to explore the effects of the total neutron exposure on the i process and reveal more information about its potential physical sites.…”