Searching for merger debris in the Galactic halo: Chemodynamical evidence based on local blue HB stars

Abstract

We report on the discovery of a group of local A-type blue horizontal-branch (HBA) stars moving in a prograde, comet-like orbit with very similar kinematics and abundances. This serendipitously discovered group contains 5 or 6 local HBA stars venturing very close to the Galactic centre; their [Fe/H] is around -1.7, and they seem to present minimum scatter in at least Mg, Si, Ti, Fe, Al, and Cr abundances. This ``Cometary Orbit Group'' (COG) was found while we were testing a new method to detect the debris associated with the merger of smaller, specific protogalactic entities into our galaxy. The method is primarily intended to identify field HBA stars with similar kinematics and detailed, multi-species abundance patterns as seen among members of a surviving remnant (e.g., omega Centauri). Quite possibly, the COG is the remnant, on a highly decayed orbit, of a merging event that took place in the relatively remote past (i.e., at least one revolution ago).