Th/Eu abundance ratio of red giants in the Kepler Field

Abstract

The r-process production in the early universe has been well constrained by the extensive studies of metal-poor stars. However, the r-process enrichment in the metal-rich regime is still not well understood. In this study, we examine the abundance ratios of Th and Eu, which represent the actinides and lanthanides, respectively, for a sample of metal-rich disk stars. Our sample covers 89 giant stars in the Kepler field with metallicities $-0.7 \leq \rm{[Fe/H]} \leq 0.4$ and ages from a few hundred Myr to $\sim 14$ Gyr. Age information for this sample is available from stellar seismology, which is essential for studying the radioactive element Th. We derive Th and Eu abundances through $\chi^2$ fitting of high-resolution archival spectra ($R \sim 80,000$) observed with the High Dispersion Spectrograph (HDS) at the Subaru Telescope. We create synthetic spectra for individual stars using a 1D LTE spectral synthesis code, Turbospectrum, adopting MARCS model atmospheres. Our study establishes the use of a less extensively studied Th II line at 5989 angstrom, carefully taking into account the blends of other spectral lines to derive the Th abundance. We successfully determine Eu abundance for 89 stars in our sample and Th for 81 stars. For the remaining 8 stars, we estimate the upper limits of Th abundance. After correcting the Th abundance for the decay, we find no correlation between $\rm{[Th/Eu]}$ and $\rm{[Fe/H]}$, which indicates that actinides production with respect to lanthanides does not depend on metallicity. On the other hand, we find a positive correlation of $\rm{[Th/Eu]}$ with age, with a slope of $0.10 \pm 0.04$. This may hint at the possibility that the dominant r-process sources are different between the early and late universe.