Intermediate-metallicity, high-velocity stars and Galactic chemical evolution

High signal-to-noise ratio spectra were obtained of 10 high-proper-motion stars having  ?1 ?Fe/H] < 0  , and a comparable number of disc stars. All but two of the high-proper-motion stars were confirmed to have  Fe/H] > ?1.0  , some approaching solar metallicity, but, even so, earlier measurements overestimated the metallicities and velocities of some of these stars. Models of stellar populations were used to assign membership probabilities to the Galactic components to which the high-velocity stars might belong. Many were found to be more probably thick-disc than halo objects, despite their large space motions, and two might be associated with the inner Galaxy. It may be necessary to reassess contamination of previous halo samples, such as those used to define the metallicity distribution, to account for contamination by high-velocity thick-disc stars, and to consider possible subcomponents of the halo.
The change in α/Fe] ratios at  Fe/H]??1.0  is often used to constrain the degree and timing of Type Ia supernova nucleosynthesis in Galactic chemical-evolution models. Ti/Fe] values were measured for eight of the high-velocity stars. Both high- and low-Ti/Fe] halo stars exist; likewise high- and low-Ti/Fe] thick-disc stars exist. We conclude that the Ti/Fe]'break' is not well defined for a given population; nor is there a simple, continuous evolutionary sequence through the break. Implications for the interpretation of the α/Fe] break in terms of SN Ia time-scales and progenitors are discussed. The range of Ti/Fe] found for high -velocity (low rotation) thick-disc stars contrasts with that for the low -velocity (high rotation) thick-disc sample studied by Prochaska et al.