High-resolution CRIRES spectra of Terzan 1: a metal-poor globular cluster toward the inner bulge

Aims: 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 (RGC ≤ 1.5 Kpc and | l,b | ≤ 5°). 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. Methods: Using the near-infrared spectrograph CRIRES at ESO/VLT, we obtained high-resolution (R ≈ 50 000) H-band spectra of 16 bright giant stars in the innermost region (r ≤ 60'') of Terzan 1. 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. Results: Fifteen out of 16 observed stars are likely cluster members, with an average heliocentric radial velocity of +57 ± 1.8 km s-1 and mean iron abundance of [ Fe/H ] = -1.26 ± 0.03 dex. For these stars we measured some [α/Fe] abundance ratios, finding average values of [ O/Fe ] = +0.39 ± 0.02 dex, [ Mg/Fe ] = +0.42 ± 0.02 dex, [ Si/Fe ] = +0.31 ± 0.04 dex, and [ Ti/Fe ] = +0.15 ± 0.04 dex Conclusions: The α enhancement (≈+0.4 dex) found in the observed giant stars of Terzan 1 is consistent with previous measurements on other, more metal-rich bulge clusters, which suggests a rapid chemical enrichment.