A new Galactic Structure and Kinematic Model code: Comparisons to a few photometric and kinematic surveys

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Session 22 -- Galactic Structure; Galactic Center
Display presentation, Monday, 9, 1995, 9:20am - 6:30pm

[22.16D] A new Galactic Structure and Kinematic Model code: Comparisons to a few photometric and kinematic surveys

Rene A. Mendez (Yale University Observatory)

The results from a new Galactic Structure and Kinematic Model code (GSKM) are presented and discussed. The code predicts the expected number of stars as a function of apparent magnitude, observed (i.e., reddened) color, kinematics (proper motion and radial velocity), and position in the Galaxy by using the fundamental equation of stellar statistics. The needed luminosity and density functions, as well as Hess-diagrams for the three components implemented in the model (Disk, Thick-Disk, Halo), have been obtained from a critical examination of the current literature. The kinematic model uses Schwarzschild's velocity ellipsoids to describe the distribution of stars in velocity space for the different Galactic components. Velocity dispersions as well as tentative values for the Thick-Disk and Halo velocity lags have been extracted from the literature. The velocity lag for the Disk is computed self-consistently through the asymmetric drift equation and the assumed density function for the Disk component, including a proper treatment for the tilt of the velocity ellipsoid with distance from the Galactic plane. All these assumed parameters are used as initial guesses to be refined through a bootstrap method that involves comparisons of the model to different photometric and kinematic surveys. GSKM has been extensively tested against a few different photometric and proper motion surveys at low, intermediate and high Galactic latitudes. It is found that it is possible to reproduce approximately the magnitude and color counts, as well as the kinematics, in the whole magnitude range 14<= V< 20. For example, comparison with low-latitude proper motions toward the open clusters NGC 188 and NGC 3680 show that GSKM is able to fit the proper motion distribution of field stars in the range 14<= V < 17 with an accuracy that allows corrections to absolute proper motion for these two clusters with an uncertainty of 0.3 mas/yr in Galactic latitude and longitude within a 90% confidence interval. This implies that orbits for typical open clusters can be determined with a 5% uncertainty (10% for globular clusters) with this technique, even in those cases were there are no objects available in the field-of-view to provide a zero-point correction. A comparison of GSKM with a recently published pencil-beam photometric and proper motion survey at the SGP shows that the local relative normalization of Thick-Disk to Disk stars needs to be increased by about 50% above the current estimates of 2% in order to fit the color distribution in the range 17<= V< 20.

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