Molybdenum, Ruthenium, and the Heavy r-process Elements in Moderately Metal-Poor Main-Sequence Turnoff Stars

Abstract

The ratios of elemental abundances observed in metal-poor stars of the Galactic halo provide a unique present-day record of the nucleosynthesis products of its earliest stars. While the heaviest elements were synthesized by the r- and s-processes, dominant production mechanisms of light trans-ironic elements were obscure until recently. This work investigates further our 2011 conclusion that the low-entropy regime of a high-entropy wind (HEW) produced molybdenum and ruthenium in two moderately metal-poor turnoff stars that showed extreme overabundances of those elements with respect to iron. Only a few, rare nucleosynthesis events may have been involved. Here we determine abundances for Mo, Ru, and other trans-Fe elements for 28 similar stars by matching spectral calculations to well-exposed near-UV Keck HIRES spectra obtained for beryllium abundances. In each of the 26 turnoff stars with Mo or Ru line detections and no evidence for s-process production (therefore old), we find Mo and Ru to be three to six times overabundant. In contrast, the maximum overabundance is reduced to factors of three and two for the neighboring elements zirconium and palladium. Since the overproduction peaks sharply at Mo and Ru, a low-entropy HEW is confirmed as its origin. The overabundance level of the heavy r-process elements varies significantly, from none to a factor of four, but is uncorrelated with Mo and Ru overabundances. Despite their moderate metallicity, stars in this group trace the products of different nucleosynthetic events: possibly very few events, possibly events whose output depended on environment, metallicity, or time.

Accepted April 2, 2013, for publication in the Astrophysical Journal Letters (7 pages, 3 figures)

References in corpus (4)

• First Stars VIII -- Enrichment of the neutron-capture elements in the early Galaxy
• Silver and palladium help unveil the nature of a second r-process
• New Hubble Space Telescope Observations of Heavy Elements in Four Metal-Poor Stars
• First stars XVI. STIS/HST abundances of heavy-elements in the uranium-rich star CS 31082-001

Cited by in corpus (20)

• The production of proton-rich isotopes beyond iron: The $\gamma$ process in stars
• A Comparison of Stellar Elemental Abundance Techniques and Measurements
• Galactic Chemical Evolution: the Impact of the 13C-pocket Structure on the s-process Distribution
• Testing the role of SNe Ia for Galactic chemical evolution of p-nuclei with 2D models and with s-process seeds at different metallicities
• Abundance patterns of the light neutron-capture elements in very and extremely metal-poor stars
• Stellar abundances and presolar grains trace the nucleosynthetic origin of molybdenum and ruthenium
• Constraining nucleosynthesis in neutrino-driven winds: observations, simulations and nuclear physics
• New Fe I level energies and line identifications from stellar spectra
• Nuclear processes in Astrophysics: Recent progress
• 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
• Enrichment of the Galactic disc with neutron-capture elements: Mo and Ru
• Nucleosynthesis signatures of neutrino-driven winds from proto-neutron stars: a perspective from chemical evolution models
• High resolution study of the abundance pattern of the heavy elements in very metal-poor field stars
• Palladium and silver abundances in stars with [Fe/H] > -2.6
• On the Origin of Metal-poor Stars in the Solar Neighborhood
• Something borrowed, something blue: The nature of blue metal-poor stars inferred from their colours and chemical abundances
• Abundances of disk and bulge giants from high-resolution optical spectra V. Molybdenum -- the p-process element
• The first r-process enhanced star confirmed to be a member of the Galactic bulge
• Neutron-capture elements in a sample of field metal-poor N-rich dwarfs
• Heavy Elements -- They came out of the blue