[Paper Review] Old open clusters as key tracers of Galactic chemical evolution. I. Fe abundances in NGC 2660, NGC 3960, and Berkeley 32

P. Sestito, A. Bragaglia+4 more|arXiv (Cornell University)|Jul 19, 2006

Stellar, planetary, and galactic studiesPhysics and Astronomy49 references54 citations

TL;DR

This study presents the first high-resolution spectroscopic metallicity measurements for the old open clusters NGC 2660 and Berkeley 32, along with a refined determination for NGC 3960, using UVES/FLAMES data on red clump stars. It finds [Fe/H] = +0.04 ± 0.04 for NGC 2660, +0.02 ± 0.04 for NGC 3960, and [Fe/H] = −0.29 ± 0.04 for the older Be 32, supporting a negative radial metallicity gradient in the Galactic disk and establishing a homogeneous abundance scale for future chemical evolution modeling.

ABSTRACT

We obtained high-resolution UVES/FLAMES observations of a sample of nine old open clusters spanning a wide range of ages and Galactocentric radii. The goal of the project is to investigate the radial metallicity gradient in the disk, as well as the abundance of key elements (alpha and Fe-peak elements). In this paper we present the results for the metallicity of three clusters: NGC 2660 (age ~1 Gyr, Galactocentric distance of 8.68 kpc), NGC 3960 (~ 1 Gyr, 7.80 kpc), and Be 32 (~6-7 Gyr, 11.30 kpc). For Be 32 and NGC 2660, our study provides the first metallicity determination based on high-resolution spectra. We performed equivalent width analysis with the spectral code MOOG, which allows us to define a metallicity scale and build a homogeneous sample. We find that NGC 3960 and NGC 2660 have a metallicity that is very close to solar ([Fe/H]=+0.02 and +0.04, respectively), while the older Be 32 turns out to have [Fe/H]=$-$0.29.

Motivation & Objective

• To determine the iron abundance ([Fe/H]) in three old open clusters—NGC 2660, NGC 3960, and Berkeley 32—using high-resolution spectroscopy.
• To establish a homogeneous metallicity scale across a sample of old open clusters for reliable Galactic chemical evolution modeling.
• To investigate the radial metallicity gradient in the Galactic disk using clusters spanning diverse Galactocentric distances and ages.
• To resolve discrepancies in literature [Fe/H] values by applying a consistent spectroscopic analysis method across all clusters.

Proposed method

• High-resolution UVES/FLAMES spectroscopy was obtained for red clump giant stars in the target clusters.
• Spectral synthesis and equivalent width analysis were performed using the MOOG code with a consistent set of atomic parameters and model atmospheres.
• Effective temperatures, surface gravities, and microturbulent velocities were derived from ionized and neutral iron lines, ensuring internal consistency.
• A homogeneous abundance scale was established by applying identical analysis procedures across all clusters, minimizing systematic offsets.
• Distances and ages were adopted from the literature or WEBDA database to enable radial metallicity gradient analysis.
• The analysis accounted for differential reddening and used multiple stars per cluster to improve metallicity precision.

Experimental results

Research questions

• RQ1 What is the iron abundance ([Fe/H]) in the old open clusters NGC 2660, NGC 3960, and Berkeley 32, as determined from high-resolution spectroscopy?
• RQ2 How do the [Fe/H] values of these clusters compare with previous literature estimates, and what explains discrepancies?
• RQ3 Does the observed [Fe/H] distribution across these clusters support a negative radial metallicity gradient in the Galactic disk?
• RQ4 To what extent does high-resolution spectroscopy reduce scatter in the [Fe/H] vs. Galactocentric distance relation compared to low-resolution studies?

Key findings

• The iron abundance in NGC 2660 is [Fe/H] = +0.04 ± 0.04, consistent with solar metallicity.
• The iron abundance in NGC 3960 is [Fe/H] = +0.02 ± 0.04, also consistent with solar metallicity.
• The older cluster Berkeley 32 has a significantly lower iron abundance of [Fe/H] = −0.29 ± 0.04, indicating sub-solar metallicity.
• This study provides the first high-resolution spectroscopic metallicity determination for both NGC 2660 and Berkeley 32.
• The results support a negative radial metallicity gradient in the Galactic disk, with higher [Fe/H] at smaller Galactocentric distances.
• High-resolution data show reduced scatter in the [Fe/H] vs. Rgc relation compared to low-resolution studies, especially in the inner disk (Rgc < 10 kpc).