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

Abstract

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&#39;s larger Cepheid sample and we refer to these stars as the &#34;Galactic Cepheids&#34;. A minority of the Cepheids show considerably lower [Fe/H] and higher [alpha/Fe] and we refer to these stars as the &#34;Merger Cepheids&#34;. One signature of a merger event would be composition differences between the &#34;Galactic&#34; and &#34;Merger&#34; Cepheids. The Cepheids satisfy this requirement and we speculate that the distinct compositions suggest that the &#34;Merger Cepheids&#34; 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]