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 similar to 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 <= 1.5 Kpc and vertical bar l,b vertical bar <= 5 degrees). 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 approximate to 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 [alpha/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 alpha enhancement (approximate to+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.