High-resolution CRIRES spectra of Terzan1: a metal-poor globular cluster toward the inner bulge

TL;DR

This study presents high-resolution infrared spectra of 16 giant stars in the metal-poor globular cluster Terzan 1, located in the innermost Galactic bulge, revealing its chemical composition and supporting rapid chemical enrichment in the region.

Contribution

First high-resolution spectroscopic analysis of a metal-poor bulge globular cluster in the innermost Galaxy region, providing detailed chemical abundances and radial velocities.

Findings

Average [Fe/H] = -1.26 dex for cluster stars

Alpha-element enhancement around +0.4 dex

Cluster stars show rapid chemical enrichment evidence

Abstract

Containing the oldest stars in the Galaxy, globular clusters toward the bulge can be used to trace its dynamical and chemical evolution. In the bulge direction, there are ~50 clusters, but only about 20% have been subject of high-resolution spectroscopic investigations. So far, the sample observed at high resolution spans a moderate-to-high metallicity regime. In this sample, however, very few are located in the innermost region ($R_{GC}\leq$1.5\,Kpc and $|l, b|\leq5^{\circ}$). To constrain the chemical evolution enrichment of the innermost region of Galaxy, accurate abundances and abundance patterns of key elements based on high-resolution spectroscopy are necessary. Here we present the results we obtained for Terzan 1, a metal-poor cluster located in the innermost bulge region. Using the near-infrared spectrograph CRIRES at ESO/VLT, we obtained high-resolution (R$\approx$50,000) H-band spectra of 16 bright giant stars in the innermost region ($r\leq 60"$) of Terzan1. Full spectral synthesis techniques and equivalent width measurements of selected lines, isolated and free of significant blending and/or contamination by telluric lines, allowed accurate chemical abundances and radial velocities to be derived. Fifteen out of 16 observed stars are likely cluster members, with an average heliocentric radial velocity of +57$\pm$1.8\,km/s and mean iron abundance of [Fe/H]=--1.26$\pm$0.03\,dex. For these stars we measured some [$\alpha$/Fe] abundance ratios, finding average values of [O/Fe]=+0.39$\pm$0.02\,dex, [Mg/Fe]=+0.42$\pm$0.02\,dex, [Si/Fe]=+0.31$\pm$0.04\,dex, and [Ti/Fe]=+0.15$\pm$0.04\,dex The $\alpha$ enhancement ($\approx +0.4$\,dex) found in the observed giant stars of Terzan1 is consistent with previous measurements on other, more metal-rich bulge clusters, which suggests a rapid chemical enrichment.