Pinning Down Gravitational Settling

We analyse high-resolution archival UVES data of turnoff and subgiant stars in the nearby globular cluster NGC6397 ([Fe/H]≈−2). Balmer-profile analyses are performed to derive reddening-free effective temperatures. Due to the limited S/N and uncertainties related to blaze removal, we find the data quality insufficient to exclude the existence of gravitational settling. If the newly derived effective temperatures are taken as a basis for an abundance analysis, the photospheric iron (Fe ii) abundance in the turnoff stars is 0.11 dex lower than in the (well-mixed) subgiants. Practically all sophisticated stellar evolution models predict the existence of processes altering photospheric abundances on long timescales (see e.g. Pinsonneault, these proceedings). For example, Richard et al. [6] predict iron abundances in turnoff stars of NGC6397 to be lower by 0.2 dex than in red giants. TO201432 UVES pipeline 6400 6500 6600 6700 λ [Å] 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.05 no rm al iz ed f lu x Fig. 1. Co-addition of four UVES pipeline spectra of NGC6397/TO201432 (observing dates 2000-06-18 and 22, two spectra per night). The resulting spectrum was arbitrarily normalized at 6410 and 6690 Å. As blaze residuals are not properly accounted for in the pipeline order merging, the échelle order pattern is clearly visible in the merged spectrum. With an amplitude of 2%, these instrumental artifacts do not allow to derive Balmer-profile temperatures to better than 200–300 K.