The widths and peak metallicities of thin-disc metallicity distributions for solar neighbourhood dwarfs and giants

For dwarfs near the Sun, published metallicity distributions commonly have peak metallicities that are a few tenths of a dex below the solar value. However, Haywood has recently used data from photometric calibrations to obtain a peak metallicity of about -0.05 dex. Haywood argues that uncorrected sampling biases explain the difference between his result and previous results. To check Haywood's peak metallicity, a statistical analysis is applied to Haywood's sample and also to a set of averaged high-dispersion metallicities for nearby dwarfs. In addition, a magnitude-limited sample of evolved stars is considered. Care is taken to make sure that all the data in these samples have a common zero-point which there is good reason to regard as reliable. In addition, sampling biases are duly considered, and full allowance is made for the contribution of the thick-disc population. Only statistical analysis is used; no conclusions are drawn by inspecting data or histograms without the aid of statistical analysis. The analysis yields a best-fitting Gaussian for dwarfs whose peak metallicity is fully consistent with Haywood's results. The 2 width of that Gaussian is 0.37 dex, and its peak falls at a metallicity of -0.041 +/- 0.013 dex. Although that peak differs detectably from zero, the difference is not appreciable when compared with a number of previous results. For evolved stars, a problem appears: their mean metallicity turns out to be significantly lower than that for dwarfs (by 0.054 +/- 0.016 dex). In addition, the metallicity distribution for evolved stars is found to be narrower than its counterpart for dwarfs. It is suggested that these discrepancies can be traced to the present lack of knowledge about giants with [Fe/H] > +0.2 dex. This suggestion is supported (although not proven) by analysing an augmented test sample which includes data for a number of high-metallicity dwarfs. It is suggested that this problem should be examined further when more has been learned about the numbers of high-metallicity giants in the solar neighbourhood.