2011MNRAS.414.2893F

This fifth in a series of papers finishes the model atmosphere analyses of an unbiased, volume-complete sample of more than 300 nearby solar-type stars. The sample first disintegrates into thick-disc and thin-disc stars on account of their different distributions in the [Mg/H]–[Fe/Mg] chemical abundance plane. Detailed stellar age-datings of the locally few, but very relevant, subgiants show this map to be the sequel of a major star formation gap and hence the legacy for the two stellar populations. Among the stars of the thin disc the subgiant 70 Vir with τ= 8.1±0.6 Gyr remains the best case to constrain the age of this comparatively young population. Uncertainties for the existing thick-disc subgiants are about twice as much, but they consistently promote ages of 12–13 Gyr. The unbiased thin-disc metallicity distribution functions of magnesium and iron average at <[Mg/H]>=-0.009 ±0.012 dex and <[Fe/H]>=-0.034±0.015 dex, thereby demonstrating that the Sun is a typical thin-disc star in terms of these two elements. The small but likewise unbiased sample of thick-disc stars leads to <[Mg/H]>=-0.207 ±0.049 dex and <[Fe/H]>=-0.584±0.057 dex, which implies that about two-thirds of the α-element magnesium was already synthesized in the early Milky Way. Similarly, the age–metallicity relations for the thin disc result in Δ[Mg/H]=+0.006 dex/Gyr and Δ[Fe/H]=+0.017 dex/Gyr and lead to the conclusion that a similar percentage of iron was synthesized before the birth of the thin disc. In comparison with the considerable metallicity dispersions σ_[Mg/H]= 0.151 dex and σ_[Fe/H]= 0.191 dex for the stars of the thin disc, this immediately explains the coexistence of old, metal rich as well as young, fairly metal poor stars within this stellar population. The stellar multiplicities of the solar-type thin-disc stars show a minority of less than 47 per cent to be single and at least 15 per cent to belong to triple and higher level systems. More concisely, in terms of the primary masses there is a clear correlation with mass with a cross-over of preferentially single to binary stars at M≃ 1.25 M_☉and a fraction of 21 per cent of triple and higher level systems at this particular mass. The steady record of new discoveries of nearby companion stars renders these numbers, however, as yet provisional, with an ever-decreasing fraction of single objects. For the thick disc the multiplicity statistics are necessarily weak, yet tentatively imply even less single stars and eventually more triple or higher level systems. Most importantly, and as already addressed in the previous papers of this series, the rigorous census of the long-lived stars of the sample uncovers the thick disc as a massive, dark population with a 20 per cent local normalization, and, on account of its larger scaleheight, a mass comparable to that of the thin disc. With the ages of its stars at or above 12 Gyr the thick-disc epoch then leads to a very different star formation rate and qualifies this stellar population as a single-burst component, much at variance with the thin disc whose stars cover the complete stellar age range of 1 Myr up to 8 Gyr. Such information conveyed by the stars of the solar neighbourhood then clearly do not support a gradual build-up scenario, a hierarchical cold-dark-matter-dominated formation picture for our parent spiral. Provided the Milky Way is not an unusual spiral galaxy, it implies that this paradigm is also challenged in a much broader context.