The low [ \alpha /Fe ] ratio in the metal-poor ( [ Fe/H ] \sim - 1.50 ) common proper motion pair HD 134439 and HD 134440 has been variously attributed to chemical evolution in an extragalactic environment with an irregular star formation history , planetessimal accretion , and formation in an environment with an unusually high dust-to-gas ratio .
We explore these various putative origins using CNO , Be , Ag , and Eu abundances derived from high-resolution near-UV Keck/HIRES spectroscopy .
While we confirm a previously suggested correlation between elemental abundance ratios and condensation temperature at the 95 % confidence level , these ratios lie within the continuum of values manifested by extant dSph data .
We argue that the most plausible origin of our stars ’ distinctive abundance distribution relative to the Galactic halo field is formation in an environment chemically dominated by products of Type II SN of low progenitor mass ; such a progenitor mass bias has been previously suggested as an explanation of low \alpha -element ratios of dSph stars .
The proper motion pair ’ s heavy-to-light n -capture element ratio , which is { \geq } 0.3 - 0.5 dex lower than in the Galactic halo field and dSph stars , is discussed in the context of the truncated r -process , phenomenlogical n -capture production models , and \alpha -rich freezeout in a high neutron excess environment ; the latter simultaneously provides an attractive explanation of the difference in [ Ca , Ti/O , Mg , Si ] ratio in HD 134439/134440 compared to { \it in { } situ } dSph stars .