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 based on high-resolution echelle spectra. The sample members have galactocentric distances covering 12 kpc <= R-GC <= 17.2 kpc, making them the most distant Galactic Cepheids upon which detailed abundance analyses have been performed. We find subsolar ratios of [Fe/H] and overabundances of [alpha/Fe], [La/Fe], and [Eu/Fe] in the program stars. 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 with Type II supernovae preferentially contributing ejecta to the interstellar medium and with Type Ia supernovae playing only a minor role. The enhancements for La suggest that asymptotic giant branch stars have contributed to the chemical evolution of the outer Galactic disk. Some of the young Cepheids are more metal-poor than the older open clusters and field stars at comparable galactocentric distances. This demonstrates that the outer disk is not the end result of the isolated evolution of an ensemble of gas and stars. We showed previously that the older open clusters and field stars reached a basement metallicity at about 10-11 kpc. The younger Cepheids reach the same metallicity but at larger galactocentric distances, roughly 14 kpc. This suggests that the Galactic disk has been growing with time, as predicted from numerical simulations. 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 S. M. 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 ongoing, with the Monoceros Ring and Canis Major galaxy being possible merger candidates.