Speaker
Description
While the r-process yield, especially the actinide-to-lanthanide abundance ratio, has been widely studied in metal-poor stars, their evolution in the metal-rich regime is poorly understood. One cause is the limited Th abundance measurement due to the difficulty of detecting the commonly used Th II 4019 Å absorption line in stellar spectra.
In this study, we present the [Th/Eu] ratio measurement for a sample of metal-rich disk stars. Our sample covers 89 giants with metallicities -0.7 < [Fe/H] < 0.4 and ages from a few hundred Myr to ~ 14 Gyr. Age information from stellar seismology is essential for studying the radioactive element Th, as it allows us to correct for decay and recover the initial production ratios. We derive Th and Eu abundances through spectral fitting of high-resolution spectra (R~80,000) obtained with the High Dispersion Spectrograph (HDS) at the Subaru Telescope. We also demonstrate the viability of using the less-studied Th II 5989 Å line to measure Th abundance, opening the possibility of extending such measurements to larger stellar samples.
We find no correlation between [Th/Eu] and [Fe/H], which indicates a stable [Th/Eu] production ratio across metallicities. On the other hand, our study shows, for the first time, a correlation between the [Th/Eu] production ratio and stellar age, with older stars showing a higher ratio than younger stars. This hints at the different contributions of short- and long-timescale sources in the early universe and the present day. Our results imply the presence of a short-timescale source(s) that can produce a high actinide-to-lanthanide ratio, which requires further investigations from the theoretical side. This work provides observational constraints to better understand the astrophysical origins of the r-process in the context of Galactic chemical evolution.
We will further explore our findings by analyzing stars with more accurate age measurements, i.e., those belonging to stellar clusters. We are currently working on a sample of giant stars in open and globular clusters, covering young and old populations, for which high-resolution spectra are available from the Gaia-ESO Survey. We expect to confirm the [Th/Eu]-age trend by analyzing this sample and further establishing our Th line analysis method for a larger sample of metal-rich stars.
| Category | Observation |
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