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abstract
We present and discuss a comparison between a newly developed Galactic Structure and Kinematics Model code and a recent deep survey which provides B,V photographic photometry and absolute proper motions for a set of 1,800 field stars, complete down to V=21.0, in the direction of the Globular Cluster NGC 288. Membership probabilities, based on proper motions, are used to perform a statistical separation of the cluster and field stars.
Comparisons are made using both a two-component Galaxy model, containing a disk and an oblate spheroid, and a three-component model that contains in addition to the disk and the spheroid a thick-disk component represented by a 47 Tuc-like luminosity function and Hess-diagram.
The following representations are explored for the spheroid: an M3-like Hess-diagram (with a well populated blue and red HB), an M13-like Hess-diagram (with a red HB), and an NGC 7006-like Hess-diagram (with a blue HB). These three clusters provide extreme prototype cases of the second-parameter effect, enabling us to explore the dependence of the predicted magnitude and color counts of the field on the Hess-diagram morphology of the spheroid component. Implications of the best-fit model parameters on the nature and characteristics of the thick-disk and spheroid are discussed.
The observed distributions in absolute proper motion are also compared to the model for different assumptions on the tilt of the velocity ellipsoid of the various components. Two extreme cases are computed from the model and compared to the data: a velocity ellipsoid that is always parallel to the Galactic plane and a velocity ellipsoid that points to the Galactic center. The dynamical significance of the best-fit model is presented.
Finally, the adequacy of the model to reproduce the observed run of secular proper motion as a function of apparent magnitude is discussed in the context of the statistical correction to absolute proper motion.
