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The low [alpha/Fe] ratio in the metal-poor ([Fe/H]= -1.50) common propermotion pair HD 134439 and HD 134440 has been variously attributed to chemicalevolution in an extragalactic environment with an irregular star formationhistory, planetessimal accretion, and formation in an environment with anunusually high dust-to-gas ratio. We explore these various putative originsusing CNO, Be, Ag, and Eu abundances derived from high-resolution near-UVKeck/HIRES spectroscopy. While we confirm a previously suggested correlationbetween elemental abundance ratios and condensation temperature at the 95%confidence level, these ratios lie within the continuum of values manifested byextant dSph data. We argue that the most plausible origin of our stars'distinctive abundance distribution relative to the Galactic halo field isformation in an environment chemically dominated by products of Type II SN oflow progenitor mass; such a progenitor mass bias has been previously suggestedas an explanation of low alpha-element ratios of dSph stars. The proper motionpair's heavy-to-light $n$-capture element ratio, which is > 0.3-0.5 dex lowerthan in the Galactic halo field and dSph stars, is discussed in the context ofthe truncated r-process, phenomenlogical n-capture production models, andalpha-rich freezeout in a high neutron excess environment; the lattersimultaneously provides an attractive explanation of the difference in[Ca,Ti/O,Mg,Si] ratio in HD 134439/134440 compared to in situ dSph stars.

The Origin of the Metal-Poor Common Proper Motion Pair HD 134439/134440: Insights from New Elemental Abundances