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

TL;DR

This study investigates the origins of the metal-poor star pair HD 134439/134440 by analyzing elemental abundances, suggesting they formed in an environment influenced by low-mass Type II supernovae and discussing their unique neutron-capture element ratios.

Contribution

It provides new high-resolution elemental abundance measurements and proposes a formation scenario involving low-mass Type II supernovae and truncated r-process nucleosynthesis.

Findings

Confirmed correlation between elemental ratios and condensation temperature.

Abundance ratios consistent with formation in a low-mass Type II supernova environment.

Heavy-to-light neutron-capture element ratio lower than in Galactic halo and dSph stars.

Abstract

The low [alpha/Fe] ratio in the metal-poor ([Fe/H]= -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 > 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 in situ dSph stars.