Elemental Abundance Ratios in Stars of the Outer Galactic Disk. III. Cepheids

We present metallicities, [Fe/H], and elemental abundance ratios, [X/Fe], for a sample of 24 Cepheids in the outer Galactic disk. The sample have Galactocentric distances covering 12 < R_GC (kpc) < 17.2 making them the most distant Galactic Cepheids upon which detailed abundance analyses have been performed. We find sub-solar ratios of [Fe/H] and overabundances of [alpha/Fe], [La/Fe], and [Eu/Fe]. All abundance ratios exhibit a dispersion that exceeds the measurement uncertainties. As seen in our previous studies of old open clusters and field giants, enhanced ratios of [alpha/Fe] and [Eu/Fe] reveal that recent star formation has taken place in the outer disk. The outer disk Cepheids appear to exhibit a bimodal distribution for [Fe/H] and [alpha/Fe]. Most of the Cepheids continue the trends with Galactocentric distance exhibited by Andrievsky's larger Cepheid sample and we refer to these stars as the "Galactic Cepheids". A minority of the Cepheids show considerably lower [Fe/H] and higher [alpha/Fe] and we refer to these stars as the "Merger Cepheids". One signature of a merger event would be composition differences between the "Galactic" and "Merger" Cepheids. The Cepheids satisfy this requirement and we speculate that the distinct compositions suggest that the "Merger Cepheids" may have formed under the influence of significant merger or accretion events. The short lifetimes of the Cepheids reveal that the merger event may be on-going with the Monoceros ring and Canis Major galaxy being possible merger candidates. [Abridged]