ar X iv : a st ro - p h / 96 07 16 9 v 1 3 1 Ju l 1 99 6 Galactic structure towards the open clusters NGC 188 and NGC 3680

We present the first comparisons of a newly developed Galactic Structure and Kinematic Model to magnitude and color counts, as well as relative proper motions, in the fields of the open clusters NGC 188 ((l, b) = (122.8 o , +22.4 o)) and NGC 3680 ((l, b) = (286.8 o , +16.9 o)). In addition to determining the reddening toward these two clusters, it is shown that starcounts at intermediate Galactic latitudes in the range 11 ≤ V ≤ 17 allow us to constrain the model scale-height for disk subgiants. We obtain a mean value of 250 ± 32 pc, in agreement with previous determinations of the scale-height for red-giants. We are also able to constrain the scale-height of main-sequence stars, and the distance of the sun from the Galactic plane, ruling out the possibility of a value of +40 pc, in favor of a smaller value. Comparisons with the observed proper-motion histograms indicate that the velocity dispersion of disk main-sequence stars must increase with distance from the Galactic plane in order to match the observed proper-motion dispersion. The required increase is consistent with the values predicted by dynamical models, and provides a clear observational evidence in favor of such gradients. The shape of the observed proper-motion distribution is well fitted within the Poisson uncertainties. This implies that corrections to absolute proper motion (and, therefore, space velocities) for open clusters may be obtained using our model when no inertial reference frame is available. Using this approach, the derived tangential motions for NGC 188 and NGC 3680 are presented.