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 metal-poor ([Fe/H]=-1.62+/-0.09) star HD94028. Previous studies revealed that this star is mildly enhanced in elements produced by the slow neutron-capture process (s-process; e.g., [Pb/Fe]=+0.79+/-0.32) and rapid neutron-capture process (r-process; e.g., [Eu/Fe]=+0.22+/-0.12), including unusually large molybdenum ([Mo/Fe]=+0.97+/-0.16) and ruthenium ([Ru/Fe]=+0.69+/-0.17) enhancements. However, this star is not enhanced in carbon ([C/Fe]=-0.06+/-0.19). We analyze an archival near-ultraviolet spectrum of HD94028, 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 neutron-capture process (i process), which perhaps operated by 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 HD94028. 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.

arXiv commentAccepted for publication in the Astrophysical Journal. (13 pages, 7 figures)

References in corpus (74)

• The chemical composition of the Sun
• Validation of the new Hipparcos reduction
• Compact Remnant Mass Function: Dependence on the Explosion Mechanism and Metallicity
• New constraints on the chemical evolution of the solar neighbourhood and Galactic disc(s). Improved astrophysical parameters for the Geneva-Copenhagen Survey
• The Dawes Review 2: Nucleosynthesis and stellar yields of low and intermediate-mass single stars
• Galactic Evolution of Sr, Y, Zr: A Multiplicity of Nucleosynthetic Processes
• The Extremely Metal-Poor, Neutron-Capture-Rich Star CS 22892-052: A Comprehensive Abundance Analysis
• A Search for Stars of Very Low Metal Abundance. VI. Detailed Abundances of 313 Metal-Poor Stars
• Nucleosynthesis and mixing on the Asymptotic Giant Branch. III. Predicted and observed s-process abundances
• Non-LTE calculations for neutral Na in late-type stars using improved atomic data
• The Rise of the s-Process in the Galaxy
• The Chemical Composition and Age of the Metal-Poor Halo Star BD +17^\circ 3248
• Departures from LTE for neutral Li in late-type stars
• The Binary Frequency Among Carbon-Enhanced, s-Process Rich, Metal-Poor Stars
• The Abundances of Neutron Capture Species in the Very Metal-Poor Globular Cluster M15: An Uniform Analysis of RGB and RHB Stars
• s Process in low-mass Asymptotic Giant Branch Stars
• Electron-capture supernovae as the origin of elements beyond iron
• The s process in asymptotic giant branch stars of low metallicity and the composition of carbon-enhanced metal-poor stars
• Convective-reactive proton-C12 combustion in Sakurai's object (V4334 Sagittarii) and implications for the evolution and yields from the first generations of stars
• Production of Light Element Primary Process nuclei in neutrino-driven winds
• Non-standard s process in low metallicity massive rotating stars
• The s process in massive stars at low metallicity. Effect of primary N14 from fast rotating stars
• Nucleosynthesis in Electron Capture Supernovae of AGB Stars
• Detection of Elements at All Three r-process Peaks in the Metal-Poor Star HD 160617
• Silver and palladium help unveil the nature of a second r-process
• Integrated Nucleosynthesis in Neutrino Driven Winds
• New Hubble Space Telescope Observations of Heavy Elements in Four Metal-Poor Stars
• NLTE determination of the aluminium abundance in a homogeneous sample of extremely metal-poor stars
• Improved Ti II log(gf) Values and Abundance Determinations in the Photospheres of the Metal-Poor Star HD 84937
• Uncertainties in the nu p-process: supernova dynamics versus nuclear physics
• HST Observations of Heavy Elements in Metal-Poor Galactic Halo Stars
• Non-LTE Line-Formation and Abundances of Sulfur and Zinc in F, G, and K Stars
• s-Process in Low Metallicity Stars. III. Individual analysis of CEMP-s and CEMP-s/r with AGB models
• The End of Nucleosynthesis: Production of Lead and Thorium in the Early Galaxy
• Helium enrichment and Carbon-star Production in Metal-rich Populations
• Iron-Group Abundances in the Metal-Poor Main Sequence Turnoff Star HD~84937
• Improved Ni I log(gf) Values and Abundance Determinations in the Photospheres of the Sun and Metal-poor Star HD 84937
• NLTE analysis of Sr lines in spectra of late-type stars with new R-matrix atomic data
• Global non-spherical oscillations in 3-D $4\pi$ simulations of the H-ingestion flash
• H ingestion into He-burning convection zones in super-AGB stellar models as a potential site for intermediate neutron-density nucleosynthesis
• Three carbon-enhanced metal-poor dwarf stars from the SDSS - Chemical abundances from CO^5BOLD 3D hydrodynamical model atmospheres
• First stars XVI. STIS/HST abundances of heavy-elements in the uranium-rich star CS 31082-001
• Nucleosynthesis Modes in the High-Entropy-Wind of Type II Supernovae: Comparison of Calculations with Halo-Star Observations
• 3D hydrodynamical simulations of a proton ingestion episode in a low-metallicity asymptotic giant branch star
• Barium Isotopic Composition of Mainstream Silicon Carbides from Murchison: Constraints for s-Process Nucleosynthesis in AGB Stars
• New insights on Ba over-abundance in open clusters. Evidence for the intermediate neutron-capture process at play?
• Neutral oxygen spectral line formation revisited with new collisional data: large departures from LTE at low metallicity
• Asymptotic Giant Branch models at very low metallicity
• Are There Any Stars Lacking Neutron-Capture Elements? Evidence from Strontium and Barium
• Homogeneous photometry and metal abundances for a large sample of Hipparcos metal-poor stars
• Metal-Poor Stars Observed with the Magellan Telescope I. Constraints on Progenitor Mass and Metallicity of AGB Stars Undergoing s-Process Nucleosynthesis
• On the Abundance of Potassium in Metal-Poor Stars
• New Abundance Determinations of Cadmium, Lutetium, and Osmium in the r-process Enriched Star BD+173248
• Carbon and Strontium Abundances of Metal-Poor Stars
• A Long, Cold, Early r-process? Neutrino-induced Nucleosynthesis in He Shells Revisited
• New Detections of Arsenic, Selenium, and Other Heavy Elements in Two Metal-Poor Stars
• Detection of the Second r-process Peak Element Tellurium in Metal-Poor Stars
• Germanium, Arsenic, and Selenium Abundances in Metal-Poor Stars
• Hubble Space Telescope Near-Ultraviolet Spectroscopy of the Bright CEMP-no Star BD+44 493
• Detection of Neutral Phosphorus in the Near Ultraviolet Spectra of Late-Type Stars
• Hubble Space Telescope Near-Ultraviolet Spectroscopy of Bright CEMP-s Stars
• Nucleosynthesis in O-Ne-Mg Supernovae
• Impact of supernova dynamics on the νp-process
• Relics of ancient post-AGB stars in a primitive meteorite
• Post-AGB stars in the Magellanic Clouds and neutron-capture processes in AGB stars
• Stellar abundances and presolar grains trace the nucleosynthetic origin of molybdenum and ruthenium
• The Extreme Overabundance of Molybdenum in Two Metal-Poor Stars
• Nucleosynthesis in helium-enriched asymptotic giant branch models: Implications for Heavy Element Enrichment in Omega Centauri
• Non-LTE analysis of neutral copper in the late-type metal-poor stars
• The Chemical Evolution of Phosphorus
• New Fe I level energies and line identifications from stellar spectra
• Evolution and nucleosynthesis of helium-rich asymptotic giant branch models
• Nucleosynthesis in Neutrino-Driven Supernovae
• Metal-rich carbon stars in the Sagittarius Dwarf Spheroidal galaxy

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• Neutron-capture elements in dwarf galaxies II: Challenges for the s- and i-processes at low metallicity
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• Enrichment of Strontium in Dwarf Galaxies
• Trans-iron Ge, As, Se, and heavier elements in the dwarf metal-poor stars, HD~19445, HD~84937, HD~94028, HD~140283, and HD~160617
• First determination of s-process element abundances in pre-main sequence clusters: Y, Zr, La, and Ce in IC 2391, the Argus association and IC 2602
• Enrichment of the Galactic disc with neutron capture elements: Sr
• The impact of (n,$\gamma$) reaction rate uncertainties on the predicted abundances of i-process elements with $32\le Z\le 48$ in the metal-poor star HD94028
• A high-precision abundance analysis of the nuclear benchmark star HD 20
• Synthesis of thorium and uranium in asymptotic giant branch stars
• A strong neutron burst in jet-like supernovae of spinstars
• Detection of Pb II in the Ultraviolet Spectra of Three Metal-Poor Stars
• Be and O in the ultra metal-poor dwarf 2MASS J18082002-5104378: The Be-O correlation
• Reanalysis of neutron-capture elements in the benchmark r-rich star CS 31082-001
• The Pristine Inner Galaxy Survey (PIGS) X. Probing the early chemical evolution of the Sagittarius dwarf galaxy with carbon abundances
• Spectroscopic study of Ba and CEMP-s stars: Mass distribution of AGB progenitors
• Nuclear Level Density and $\gamma$-ray Strength Function of ${}^{67}\mathrm{Ni}$ and the impact on the i-process
• Isotopic ratios for C, N, Si, Al, and Ti in C-rich presolar grains from massive stars
• High-resolution spectroscopic analysis of four new chemically peculiar stars
• A wide angle view of the Sagittarius dwarf spheroidal galaxy -- II. A CEMP-r/s star in the Sagittarius dwarf Spheroidal Galaxy
• The s process in massive stars, a benchmark for neutron capture reaction rates
• The Abundance of Lead in Four Metal-Poor Stars
• Study of the abundance features of the metal-poor star HD 94028
• The Impact of Inelastic Collisions with Hydrogen on NLTE Copper Abundances in Metal-Poor Stars
• New Fe I Level Energies and Line Identifications from Stellar Spectra. III. Initial Results from UV, Optical, and Infrared Spectra
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• Heavy Elements -- They came out of the blue
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• The chemical DNA of the Magellanic Clouds VI. Origin and evolution of neutron-capture elements in the SMC
• UVES analysis of red giants in the bulge globular cluster NGC 6522
• The R-Process Alliance: A Nearly Complete R-Process Abundance Template Derived from Ultraviolet Spectroscopy of the R-Process-Enhanced Metal-Poor Star HD 222925
• An ancient system hidden in the Galactic plane?