Optical high-resolution spectroscopy of 14 young $\alpha$-rich stars

Abstract

We report chemical abundances of 14 young $\alpha$-rich stars including neutron-capture elements based on high-quality optical spectra from HIRES/Keck I and differential line-by-line analysis. From the comparison of the abundance patterns of young $\alpha$-rich stars to those of nearby bright red giants with a similar metallicity range ($-0.7<[Fe/H]<-0.2$), we confirm their high $\alpha$-element abundances reported by previous studies based on near-infrared spectroscopy. We reveal for the first time low abundances of $s$-process elements and high abundances of $r$-process elements. All the abundances are consistent with those seen in typical $\alpha$-rich population of the Galactic disk, and no abundance anomalies are found except for Li-enhancement in one object previously reported and mild enhancement of Na in two stars. In particular, the lack of $s$-process enhancement excludes the hypothesis that mass transfer from asymptotic giant branch stars plays an important role in the formation of young $\alpha$-rich stars. The high frequency of radial velocity variation (more than $50\%$) is also confirmed. We argue that mass transfer from low-mass red giants is the likely dominant formation mechanism for young $\alpha$-rich stars.

Accepted for publication in ApJ