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Nearby Stars of the Galactic Disk and Halo

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Nearby Stars of the Galactic Disk and Halo Your thesaurus codes are: ASTROPHYSICS 08 (03.20.8; 08.04.1; 08.06.3; 08.12.1; 10.05.1) 3.7.1998 Nearby stars of the Galactic disk and halo ? Klaus Fuhrmann Institut fÈur Astronomie und Astrophysik der UniversitÈat MÈunchen, Scheinerstraûe 1, 81679 MÈunchen, Germany Received date; accepted date Abstract. Model atmosphere analyses ofechelle  spectra of stellar wind of giant stars, the occurrence of planetary nebulae, some ®fty nearby F- and G-type stars are presented. The sample or the very spectacular supernova events. Nowadays, it is there- is con®ned to the main-sequence, turnoff and subgiant region, fore no surprise that the ancient stars of the Galactic halo are regardless of the metal abundance. On the base of these data, found to be metal-poor objects ± as opposed to the understand- spectroscopic parallaxes are calculated and compared to the ing when they were ®rst analyzed in the 1950s. Interestingly, Hipparcos astrometry to explore the reliability of the derived these stars also reveal a very different kinematic behaviour in stellar parameters, notably the surface gravity and metal abun- that they do not take share in the rotation of the Galactic disk, dance scale. i.e. they constitutea differentstellar population,which provides The spectroscopic distance scale is found in good agreement important clues for scenarios of the early epochs. with the Hipparcos parallaxes and is characterized by a 5% With the rapidly increased number of observations, the more rms uncertainty. The results suggest a precision in log g of sophisticated theories, and the very much improved numeri- : : : 0 1dex,and005 0 10 dex for the metallicity. There is also cal modeling, the last decades saw many distinguished aspects reasonable evidence for the spectroscopic effective temperature and comprehensive insights into Galactic evolution. Inevitably, scale to be free of systematic errors; typical uncertainties are and as might have been anticipated, the picture has become assessed to 80 K. The basic spectroscopic parameters are progressively complicated, not only in its details, but also in supplemented by data for the microturbulence velocities, the the global characteristics of whether the Galaxy has evolved projected rotational velocities, stellar radii and -enhancement from halo-to-bulge or halo-to-disk, whether it evolved from abundances, the latter represented by the element magnesium. top-to-bottom, or, in contradistinction, from inside-out,oris Stellar masses are also given, though many are probably subject actually a combination of both. The role of accretion as a ma- to small adjustments (typically 5%) in forthcoming analyses. jor phenomenon has become a substantive issue, convincingly The well-de®ned distance correlation is also demonstrated to be demonstrated by the recent discovery of the Sagittarius dwarf an ef®cient means for an identi®cation of spectroscopic binaries. galaxy merger. This striking example, in particular, represents The results are discussed in terms of a spectroscopically a far-reaching alert to closed box models or other simpli®ed established distance scale, the sites of the stellar populations assumptions, in spite of their analytical beauties. Similarly, the in the [Mg/H]-[Fe/Mg] plane, a timescale for the Galactic thin system of Galactic globular clusters has been found to be mixed T g disk, and the potential of future ef f -log Kiel diagrams for a with some foreign intruders, which can alter our notions of, precise determination of Galactic globular clusters ages. for instance, the instrinsic age spread of the original ªfounder membersº if this polluting subgroup is not accounted for. Key words: Techniques: spectroscopic ± stars: distances ± stars: The concept of stellar populations has also become more fundamental parameters ± stars: late-type ± Galaxy: evolution complicated in recent years. Metallicity distributions of the halo,thethick disk and thin disk reveal considerable overlap and do not allow for an unequivocal classi®cation for many stars without reference to other criteria, such as age or kinematics. With respect to the latter, the situation has improved tremen- 1. Introduction dously on the base of the Hipparcos astrometry for nearby stars. But it is also immediately clear that the Hipparcos data take an One of the most intriguing issues of modern astrophysics refers active part in clarifying the accuracy of stellar age determina- to the formation and evolution of the Milky Way Galaxy. From stellar evolution we have learned of the general chemical en- tions, for they provide detailed measures of the stars' absolute richment of matter as a functionof time, as witnessed by e.g. the magnitudes. Along with the stellar effective temperature, evo- lutionary tracks from stellar interior calculations deliver this ? Based on observations at the German Spanish Astronomical important datum, and, at the same time, improve our knowledge Center, Calar Alto, Spain of the stellar masses. In addition, the combination of effective Fig. 1. A FOCES spectrum of the iron-de®cient turnoff star CrB with resolution R 60000 and wavelength coverage 4200-9000. The 5 mergedechelle  spectrum has an equidistant spacing of = 20mAÊ and a total length of 2 10 data points. The top panel shows the complete spectrum with successive enlargements below te peatuesa dpaa aesesuts t estea ad,w c eso ut o ( = 60000). ou t obse v g u ay in turn are combined with the masses to provide the surface 1997 repeated this setting and supplied about half of the spectra gravities. Some independent information about this often badly of the present analysis. Fig. 1 gives an illustrative representation determined parameter is of utmost interest, since it incorporates for one of the observed stars. the important advantage to ®x the metallicity scale as well. Almost all stars were observed twice, with nominal signal- For an understanding of the Galactic evolution scenario, to-noisevaluesup to 700. Unfortunately,thereal quality of the rather bright and long-lived stars in not too advanced stages of data does not exceed S/N 200, due to some lasting problems evolution constitute well-suited candidates. Consequently, we with the ®ber interface, that, however, are currently ®xed. concentrate here on F- and G-type stars of the main-sequence, the turnoff and subgiant region, irrespective of the metal con- 3. Stellar atmospheric parameters tent. Our analysis provides data for approximately ®fty nearby stars and is by no means a complete or unbiased sample. Its We adopt a purely spectroscopic approach. The analysis is main purpose is to explore the reliability of certain analysis strictly differential with respect to the Sun. techniques and stellar parameters in different regions of the HR The modeling of the spectral lines is based on T. Gehren's diagram. (1975) atmosphere program that makes use of the standard as- In particular, wewill confront thespectroscopically deduced sumptions such as hydrostatic equilibrium, plane-parallel lay- parallaxes or distances with the Hipparcos astrometry. From ers and local thermodynamic equilibrium, and is fed by the this comparison it will turn out that for certain regions in the opacity distribution functions described in Kurucz (1992) for HR diagram the surface gravities implied from the LTE iron an inclusion of the effects of line blanketing. The underlying ionization equilibrium produce con¯icting results and we have abundance pattern assumes either a solar mixture close to the to recourse to a differential method that employs the wings of one given in Grevesse, Noels & Sauval (1996) or the so-called strong absorption lines. -enhancement. The latter is a well-known feature of many Although our sample is not very large, we will present HR metal-poor stars that tend to be overabundant by approximately diagrams for the distribution of the microturbulence parameter 0.4 dex in -chain nuclei O, Ne, Mg, Si, S, Ar, Ca and probably v i t and the projected rotational velocity, sin . In addition, we Ti. Hence, basically two grids of model atmospheres encompass T g present data for the stellar radii and masses, although part of the our program stars for values in ef f and log as displayed in latter may require small corrections on the base of revised evo- the Kiel diagram of Fig. 2 and for about ®ve orders of magni- lutionary tracks. Some results are also given for spectroscopic tude in metallicity, up to [Fe/H]= +1:0. binaries or suspected objects. They were either immediately Effective temperatures are deduced from the wings of the detected as such from the spectrograms, or, in the course of the Balmer lines, since they are very sensitive to this parameter in analyses, from discrepancies with the distance scale. our temperature range (e.g. Schmidt 1972). Surface gravities Within the last few years several studies were concerned are usually derived from the ionization equilibria of abundant with the metallicity distribution function of the solar neigh- species, we focus here on the most often used iron. In addition, borhood. For those analyses, where a reasonable number of the wings of strong lines, such as the Mg Ib triplet, are also common objects is present, we confront these data with our well-known to represent pressure-dependent features and may ®ndings. A recent sample of stars with effective temperature serve as a diagnostics for the surface gravity parameter (e.g. T determinations from the infrared ¯ux method is also compared. Deeming 1960). It is important to realize that values of ef f Finally, the important behaviour of the magnesium-over-iron and log g are interdependent if the latter is derived from the ion- abundance as a function of metallicity is discussed and com- ization equilibrium, whereas both stellar parameters are rather pared to other data from the literature. decoupled if we make use of the Mg Ib lines. This may have advantages and disadvantages, as we will see in what follows.
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