Abstract. High-resolution spectroscopic observations of about 150 nearby stars or star systems are presented and discussed. The study of these and another 100 objects of the previous papers of this series implies that the Galaxy became reality 13 or 14 Gyr ago with the implementation of a massive, rotationally-supported population of thick-disk stars. The very high star formation rate in that phase gave rise to a rapid metal enrichment and an expulsion of gas in supernovae-driven Galactic winds, but was followed by a star formation gap for no less than three billion years at the Sun's galactocentric distance. In a second phase, then, the thin disk -our "familiar Milky Way" -came on stage. Nowadays it traces the bright side of the Galaxy, but it is also embedded in a huge coffin of dead thick-disk stars that account for a large amount of baryonic dark matter. As opposed to this, cold-dark-matter-dominated cosmologies that suggest a more gradual hierarchical buildup through mergers of minor structures, though popular, are a poor description for the Milky Way Galaxy -and by inference many other spirals as well -if, as the sample implies, the fossil records of its long-lived stars do not stick to this paradigm. Apart from this general picture that emerges with reference to the entire sample stars, a good deal of the present work is however also concerned with detailed discussions of many individual objects. Among the most interesting we mention the blue straggler or merger candidates HD 165401 and HD 137763/HD 137778, the likely accretion of a giant planet or brown dwarf on 59 Vir in its recent history, and HD 63433 that proves to be a young solar analog at τ ∼ 200 Myr. Likewise, the secondary to HR 4867, formerly suspected non-single from the Hipparcos astrometry, is directly detectable in the highresolution spectroscopic tracings, whereas the visual binary χ Cet is instead at least triple, and presumably even quadruple. With respect to the nearby young stars a complete account of the Ursa Major Association is presented, and we provide as well plain evidence for another, the "Hercules-Lyra Association", the likely existence of which was only realized in recent years. On account of its rotation, chemistry, and age we do confirm that the Sun is very typical among its G-type neighbors; as to its kinematics, it appears however not unlikely that the Sun's known low peculiar space velocity could indeed be the cause for the weak paleontological record of mass extinctions and major impact events on our parent planet during the most recent Galactic plane passage of the solar system. Although the significance of this correlation certainly remains a matter of debate for years to come, we point in this context to the principal importance of the thick disk for a complete census with respect to the local surface and volume densities. Other important effects that can be ascribed to this dark stellar population comprise (i) the observed plateau in the shape of the luminosity function of the local FGK stars, (ii) a small though syst...

The Milky Way Galaxy has an age of about 13 billion years. Solar‐type stars evolve all the long way to the realm of degenerate objects on essentially this time‐scale. This, as well as the particular advantage that the Sun offers through reliable differential spectroscopic analyses, render these stars the ideal tracers for the fossil record of our parent spiral. Astrophysics is a science that is known to be notoriously plagued by selection effects. The present work – with a major focus in this fourth contribution on model atmosphere analyses of spectroscopic binaries and multiple star systems – aims at a volume‐complete sample of about 300 nearby F‐, G‐, and K‐type stars that particularly avoids any kinematical or chemical pre‐selection from the outset. It thereby provides an unbiased record of the local stellar populations – the ancient thick disc and the much younger thin disc. On this base, the detailed individual scrutiny of the long‐lived stars of both populations unveils the thick disc as a single‐burst component with a local normalization of no less than 20 per cent. This enormous fraction, combined with its much larger scaleheight, implies a mass for the thick disc that is comparable to that of the thin disc. On account of its completely different mass‐to‐light ratio the thick disc thereby becomes the dark side of the Milky Way, an ideal major source for baryonic dark matter. This massive, ancient population consequently challenges any gradual build‐up scenario for our parent spiral. Even more, on the supposition that the Galaxy is not unusual, the thick disc – as it emerges from this unbiased spectroscopic work – particularly challenges the hierarchical cold‐dark‐matter‐dominated formation picture for spiral galaxies in general.

This fifth in a series of papers finishes the model atmosphere analyses of an unbiased, volumecomplete 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 [Mg/H] = +0.006 dex Gyr −1 and [Fe/H] = +0.017 dex Gyr −1 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 ...

Abstract. We have designed and built theéchelle spectrograph FOCES fed by 100 µm optical fibres to be mounted at the Cassegrain focus of either the 2.2 m or the 3.5 m telescope at the Calar Alto Observatory. The spectrograph itself follows a white-pupil design collimated with two off-axis parabolic mirrors. The 15 cm beam leaving the 31.6 lines/mm R2échelle is refocussed in the vicinity of a small folding mirror which allows efficient removal of scattered light. The cross-dispersion is achieved with a tandem prism mounting, and the beam imaged with an f/3 transmission camera onto a field centered on a 1024 2 thinned Tektronix CCD with 24 µm pixel diameter. Thé echelle image covers the visible spectral region from 380 to 750 nm displayed in 70 spectral orders with full spectral coverage. Spectral orders are separated by 20 pixels in the blue and by 10 pixels in the red. The maximum spectral resolution is R = λ/∆λ = 40600 with a 2 pixel resolution element; unvignetted resolution as defined by the fibre alone would be obtained at R = 18000. Replacing the CCD by a 2048 2 chip with 15 µm pixel diameter and taking into account light losses from a reduced entrance slit width a full 2 pixel resolution of R = 65000 is obtained.The above concept has made FOCES an extremely well-defined instrument. A number of successful test installations at the Cassegrain foci of the Wendelstein 80 cm telescope, the Calar Alto 2.2 m and 3.5 m telescopes has produced spectra of high quality for up to 60 min exposures. The limiting magnitude for a 1 hr exposure with an S/N ratio of 100 scales to V = 12 for a 3.5 m telescope which is only slightly less than expected from laboratory tests. In an alternative mode FOCES offers a second fibre carrying the sky background signal to correct extremely faint object spectra. This mode obtains the required higher cross-dispersion from an additional grism resulting in a correspondingly reduced spectral coverage.Send offprint requests to: M. Pfeiffer (also download from