Abundances in Turn-off Stars in the Old, Metal-Rich Open Cluster, NGC 6791

TL;DR

This study analyzes the chemical abundances of two turn-off stars in the old, metal-rich open cluster NGC 6791, confirming high metallicity and comparing element abundances with field stars to understand Galactic chemical evolution.

Contribution

It provides detailed elemental abundances for stars in NGC 6791, confirming high metallicity and suggesting possible origins near the Galactic center based on orbital evidence.

Findings

Confirmed high [Fe/H] of +0.30 in NGC 6791

Found similar element abundances between cluster and field stars

Suggested Ni enhancement may result from SN Ia enrichment

Abstract

Open clusters have long been used to illuminate both stellar evolution and Galactic evolution. The oldest clusters, though rather rare, can reveal the chemical and nucleosynthetic processes early in the history of the Galaxy. We have studied two turn-off stars in the old, metal-rich open cluster, NGC 6791. The Keck + HIRES spectra have a resolution of 45,000 and signal-to-noise ratios of 40 per pixel. We confirm the high value for [Fe/H] finding +0.30 $\pm$0.08, in agreement with earlier results from evolved stars in other parts of the HR diagram. We have also determined abundances for Na, Si, Ca, Ti, Cr, Ni, Y and Ba. These are compared to a sample of old, metal-rich field stars. With the probable exception of enhanced Ni in the cluster stars, the field and cluster stars show similar abundances of the elements. Model predictions show that the Ni enhancement could result from enrichment of the pre-cluster gas by SN Ia. Orbital evidence indicates that NGC 6791 could have originated near the inner regions of the Galaxy where the metallicity is generally higher than it is in the disk or halo. Subsequent perturbations and migrations may have resulted in its current heliocentric distance of 4 kpc and 1 kpc above the Galactic plane.