Other
Scientific paper
Dec 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996pasp..108.1139b&link_type=abstract
Publications of the Astronomical Society of the Pacific, v.108, p.1139
Other
5
Scientific paper
Photometric and spectroscopic observations have been obtained for 1028 G- and K-type stars near the cardinal directions corresponding to Galactic rotation (V) and anti-rotation (-V). The spectra have been used to obtain radial velocities by cross-correlation, and estimates of stellar metallicity via a spectrophotometric index. The DDO and BV photometric observations yield estimates of distance, color excess, spectral type and metallicity. Reddening trends as a function of distance are found to be consistent with previous investigations in each of the two directions. An extracted subsample of 570 late-type giants has been used with maximum-likelihood modeling to study the kinematic and abundance characteristics of the local Galactic disk. Two types of models were used to fit the observed distribution of "V" - velocity vs. metallicity: 1) "discrete-component" models involving a superposition of components with Gaussian distributions in both observed quantities, and 2) "continuous" models in which the kinematic properties are represented as Gaussians with parameters that are functions of abundance. The data are well represented by a two-component disk model in which the weaker-metal component has been identified with the Galactic "thick disk". The "thick disk" properties derived in this study are: mean abundance -0.35 +/- 0.06 dex; asymmetric drift 43 +/- 9 km s^-1; and local normalization 13 +/- 3%. These values were derived without any restrictions or assumptions, other than that of two underlying Gaussian components. A three-component disk model fit was also investigated, but the necessity of a third component was not found (for this sample) to be justifiable through a chi^2 test. Two families of parameterized "continuous" models were also fitted to the data: 1) "spline" models, in which continuous functions in the model properties were derived from cubic spline interpolation through four fiducial points; and 2) "parameterized NRED" models in which zero-point and scale adjustments were left as free parameters in fitting to the data the "extended-disk" model of Norris and Ryan (1991). In both cases, the computed best-fit models of these extended-disk families were shown through a likelihood ratio test to be significantly inferior to the best-fit two-component model. This study thus supports the notion of the "thick disk" as a separate Galactic component, as opposed to being merely the extreme tail of a continuous disk structure. The thick disk's discrete nature is supportive of disk-formation models in which this component is created by a separate process or distinguishable phase relative to the thin disk, such as through a merger event early in the disk's history. It is less consistent with models in which the thick disk forms through continuous processes, such as during a dissipational disk collapse or through secular kinematic diffusion of stars from a thin-disk configuration into a thick one. (SECTION: Dissertation Summaries)
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