Star Counts Redivivus. IV. Density Laws through Photometric Parallaxes

In an effort to more precisely define the spatial distribution of Galactic field stars, we present an analysis of the photometric parallaxes of stars in seven Kapteyn selected areas. Our photometry database covers ~14.9 deg2 and contains over 130,000 stars, of which approximately 70,000 are in a color range (0.4 ≤ R-I ≤ 1.5) for which relatively unambiguous photometric parallaxes can be derived. We discuss our photometry pipeline, our method of determining photometric parallaxes, and our analysis efforts. We also address the affects of Malmquist bias, subgiant/giant contamination, metallicity, and binary stars upon the derived density laws. The affect of binary stars is the most significant of these biases—a binary star fraction of 50% could result in derived scale heights that are 80% of the actual values. We find that while the disklike populations of the Milky Way are easily constrained in a simultaneous analysis of all seven fields, no good simultaneous solution for the halo is found. We have applied halo density laws taken from other studies and find that the Besançon flattened power-law halo model (c/a = 0.6,ρ ∝ r-2.75) produces the best fit to our data. With this halo, the thick disk has a scale height of 750 pc with an 8.5% normalization to the old disk. The old-disk scale height is ~280-300 pc for our early-type (5.8 ≤ MR < 6.8) dwarfs and rises to ~ 350 pc for our late-type (8.8 ≤ MR ≤ 10.2) dwarf stars. Corrected for a binary fraction of 50%, these scale heights are 940 and 350-375 pc, respectively. Even with this model, there are systematic discrepancies between the observed and predicted density distributions—discrepancies apparent only at the faint magnitudes reached by our survey. Specifically, our model produces density overpredictions in the inner Galaxy and density underpredictions in the outer Galaxy. A possible escape from this dilemma is offered by modeling the stellar halo as a two-component system, as favored by studies of blue horizontal branch/RR Lyrae stars and recent analyses of the kinematics of metal-poor stars. In this paradigm, the halo has a flattened inner distribution and a roughly spherical but substructured outer distribution. Further reconciliation could be provided by a flared thick disk, a structure consistent with a merger origin for that population.