Abundances in bulge stars from high-resolution, near-IR spectra I. The CNO elements observed during the science verification of CRIRES at VLT

The formation and evolution of the Milky Way bulge is not yet well understood and its classification is ambiguous. Constraints can, however, be obtained by studying the abundances of key elements in bulge stars. The aim of this study is to determine the chemical evolution of CNO, and a few other elements in stars in the Galactic bulge, and to discuss the sensitivities of the derived abundances from molecular lines. High-resolution, near-IR spectra in the H band were recorded using VLT/CRIRES. Due to the high and variable visual extinction in the line-of-sight towards the bulge, an analysis in the near-IR is preferred. The CNO abundances can all be determined simultaneously from the numerous molecular lines in the wavelength range observed. The three giant stars in Baade’s window presented here are the first bulge stars observed with CRIRES. We have especially determined the CNO abundances, with uncertainties of less than 0.20 dex, from CO, CN, and OH lines. Since the systematic uncertainties in the derived CNO abundances due to uncertainties in the stellar fundamental parameters, notably Teff, are significant, a detailed discussion of the sensitivities of the derived abundances is included. We find good agreement between near-IR and optically determined O, Ti, Fe, and Si abundances. Two of our stars show a solar [C+N/Fe], suggesting that these giants have experienced the first dredge-up and that the oxygen abundance should reflect the original abundance of the giants. The two giants fit into the picture, in which there is no significant difference between the O abundance in bulge and thick-disk stars. Our determination of the S abundances is the first for bulge stars. The high [S/Fe] values for all the stars indicate a high star-formation rate in an early phase of the bulge evolution.

💡 Research Summary

The paper presents a pioneering chemical‑abundance analysis of three Galactic bulge giant stars observed with the high‑resolution near‑infrared spectrograph CRIRES on the VLT. Because visual extinction toward the bulge is severe and highly variable, the authors chose to work in the H‑band (1.5–1.8 µm), where interstellar dust attenuation is much lower and a wealth of molecular lines (CO, CN, OH) provides a powerful diagnostic for the CNO elements. The three targets lie in Baade’s Window, a low‑extinction window that still suffers enough reddening to make optical work challenging.

Observations were carried out at a resolving power of R≈100 000, delivering spectra with high signal‑to‑noise ratios. Stellar parameters (effective temperature, surface gravity, metallicity, microturbulence) were initially estimated from existing optical analyses, photometric colours, and evolutionary tracks, then refined iteratively during the abundance analysis. Synthetic spectra were generated with LTE model atmospheres and the MOOG code, and fitted to the observed lines. The abundance determination proceeded in a logical sequence: OH lines first fixed the oxygen abundance, then CO lines constrained carbon, and finally CN lines yielded nitrogen. This approach exploits the interdependence of the molecular equilibria and ensures a self‑consistent solution. Over twenty unblended lines were used for each element, dramatically reducing random errors.

A thorough sensitivity study quantified how uncertainties in the stellar parameters propagate into the derived abundances. Raising Teff by 100 K changes