Chemical evolution of ytterbium in the Galactic disk*

Context. Measuring the abundances of neutron-capture elements in Galactic disk stars is an important part of understanding key stellar and galactic processes. In the optical wavelength regime a number of different neutron-capture elements have been measured; however, only the s-process-dominated element cerium has been accurately measured for a large sample of disk stars from the infrared H band. The more r-process dominated element ytterbium has only been measured in a small subset of stars so far. Aims. In this study we aim to measure the ytterbium (Yb) abundance of local disk giants using the Yb II line at λair = 16 498. We also compare the resulting abundance trend with cerium and europium abundances for the same stars to analyse the s- and r-process contributions. Methods. We analyse 30 K giants with high-resolution H band spectra using spectral synthesis. The very same stars have already been analysed using high-resolution optical spectra via the same method, but it was not possible to determine the abundance of Yb from those spectra due to blending issues for stars with [Fe/H] > 1. In the present analysis, we utilise the stellar parameters determined from the optical analysis. Results. We determined the Yb abundances with an estimated uncertainty for [Yb/Fe] of 0.1 dex. By comparison, we found that the [Yb/Fe] trend closely follows the [Eu/Fe] trend and has clear s-process enrichment in identified s-rich stars. This comparison confirms both that the validity of the Yb abundances is ensured and that the theoretical prediction that the s-/r-process contribution to the origin of Yb of roughly 40/60 is supported. Conclusions. These results show that, with a careful and detailed analysis of infrared spectra, reliable Yb abundances can be derived for a wider sample of cooler giants in the range1.1 < [Fe/H] < 0.3. This is promising for further studies of the production of Yb and for the r-process channel, key for galactochemical evolution, in the infrared.