2. Quality of the spectroscopic data
We have analysed a sample of about 20 dwarf and subgiant stars with [Fe/H] , i.e. one tenth of the solar metallicity. This metallicity is generally assumed to correspond to the most metal-rich part of the halo of our Galaxy. The spectra were obtained with the Coudé Echelle Spectrometer (CES) fed by the 1.4m Coudé Auxiliary Telescope (CAT) at the European Southern Observatory (La Silla, Chile). Four spectral regions, chosen to contain lines of neutron-capture elements, were observed. The spectral resolution is of the order of and the signal-to-noise ratio in the continuum is for each spectrum. In order to reduce the analysis uncertainties, the lines were chosen to have similar dependences on the stellar atmospheric parameters (effective temperature, surface gravity, microturbulence velocity, overall metallicity) whenever possible. Moreover, the analysis was carried out differentially inside the sample, i.e., each star was compared to all other stars in the sample.
The sample of stars, together with some key abundance ratios and the total velocity with respect to the Local Standard at Rest, are presented in Table 1.
Table 1. Basic observational data, abundance ratios and total space velocity.
Following the traditional abundance analyses (e.g. Magain 1989, Edvardsson et al. 1993) we would show for example [Ti/Fe] as a function of [Fe/H]. The scatter in that plot for our sample amounts to 0.08 dex (20 ). Is this scatter real or is it due to observational and/or analysis uncertainties? To answer this question we compare in Fig. 1 the values of [Ti/Fe] deduced from neutral lines with the ones deduced from lines of the singly ionized species. We can see a very nice correlation between those two values, with a scatter of only 0.026 dex (6 ). Since the neutral and ionized lines have different dependences on the stellar atmospheric parameters, this shows that the scatter in element abundances due to analysis uncertainties does not exceed 6 . Therefore, the scatter in the abundance of Ti relative to Fe is real cosmic scatter. We will now investigate the cosmic scatter in the relative abundances of the other chemical elements.
© European Southern Observatory (ESO) 1998
Online publication: January 16, 1998