Spectroscopic analyses of neutron capture elements in open clusters

Abstract

The evolution of elements as a function or age throughout the Milky Way disk provides strong constraints for galaxy evolution models, and on star formation epochs. In an effort to provide such constraints, we conducted an investigation into r- and s-process elemental abundances for a large sample of open clusters as part of an optical follow-up to the SDSSIII/APOGEE-1 near infrared survey. To obtain data for neutron capture abundance analysis, we conducted a long-term observing campaign spanning three years (2013-2016) using the McDonald Observatory Otto Struve 2.1-meter telescope and Sandiford Cass Echelle Spectrograph (SES, R(¿/¿) 60,000). The SES provides a wavelength range of 1400 Å, making it uniquely suited to investigate a number of other important chemical abundances as well as the neutron capture elements.^

For this study, we derive abundances for 18 elements covering four nucleosynthetic families- light, iron-peak, neutron capture and a-elements- for 30 open clusters within 6 kpc of the Sun with ages ranging from 80 Myr to 10 Gyr. Both equivalent width (EW) measurements and spectral synthesis methods were employed to derive abundances for all elements. Initial estimates for model stellar atmospheres- effective temperature and surface gravity- were provided by the APOGEE data set, and then re-derived for our optical spectra by removing abundance trends as a function of excitation potential and reduced width (EW/¿). With the exception of Ba II and Zr I, abundance analyses for all neutron capture elements were performed by generating synthetic spectra from the new stellar parameters. In order to remove molecular contamination, or blending from nearby atomic features, the synthetic spectra were modeled by a best-fit Gaussian to the observed data.^

Nd II shows a slight enhancement in all cluster stars, while other neutron capture elements follow solar abundance trends. Ba II shows a large cluster-to-cluster abundance spread, consistent with other open cluster abundance studies. From log(Age) 8.5, this large spread as a function of age appears to replicate the findings from an earlier, much debated study by DOrazi et al. (2009) which found a linear trend of decreasing barium abundance with increasing age.