High resolution infrared spectra of NGC 6440 and NGC 6441: two massive Bulge Globular Clusters

TL;DR

This study presents high-resolution infrared spectra of giant stars in two massive bulge globular clusters, revealing their metallicity, alpha-enhancement, isotopic ratios, and chemical abundances, supporting rapid bulge formation.

Contribution

First high dispersion infrared abundance measurements for NGC6440 and detailed chemical analysis of both clusters, providing insights into bulge formation and stellar evolution.

Findings

NGC6440 has [Fe/H] = -0.56; NGC6441 has [Fe/H] = -0.50.

Both clusters show alpha-enhancement of ~+0.3 dex.

Low 12C/13C ratios (~5) indicate extra-mixing during red giant evolution.

Abstract

Using the NIRSPEC spectrograph at Keck II, we have obtained infrared echelle spectra covering the 1.5-1.8 micron range for giant stars in the massive bulge globular clusters NGC6440 and NGC6441. We report the first high dispersion abundance for NGC6440, [Fe/H]=-0.56+/-0.02 and we find [Fe/H]=-0.50+/-0.02 for the blue HB cluster NGC6441. We measure an average $\alpha$-enhancement of ~+0.3 dex in both clusters, consistent with previous measurements of other metal rich bulge clusters, and favoring the scenario of a rapid bulge formation and chemical enrichment. We also measure very low 12C/13C isotopic ratios (~5+/- 1), suggesting that extra-mixing mechanisms are at work during evolution along the Red Giant Branch also in the high metallicity regime. We also measure Al abundances, finding average [Al/Fe]=0.45+/-0.02 and [Al/Fe]=0.52+/-0.02 in NGC6440 and NGC6441, respectively, and some Mg-Al anti-correlation in NGC6441. We also measure radial velocities vr=-76+/-3 km/s and vr=+14+/-3 km/s and velocity dispersions sigma=9+/-2 km/s and sigma=10+/-2 km/s, in NGC6440 and NGC6441, respectively.