Other
Scientific paper
Jan 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt........66c&link_type=abstract
PhD Dissertation, Florida Univ. Gainesville, FL United States
Other
Distance Measuring Equipment, Galaxies, Open Clusters, Parallax, Radial Velocity, Least Squares Method, Maxwell-Boltzmann Density Function, Calibrating, Spatial Distribution, Velocity Distribution
Scientific paper
Due to its proximity, the Hyades open cluster plays a vital role in the establishment of the galactic and extragalactic distance scales, as many of the distance indicators used to estimate the distances to other galaxies are calibrated using stars which are members of the Hyades. This importance of the Hyades stars as calibrators demands that their distances be determined as accurately as possible. Past investigations have made use of convergent point approaches, which impose the physically ridiculous requirement that all stars which are members of the cluster must share exactly the same velocity. In addition, these investigations treated the proper motion and radial velocity data in a piecemeal manner, and have totally ignored the information provided by the trigonometric parallaxes. For the first time, the method of least squares with probabilistic constraints has been applied to the Hyades cluster. This technique constrains the stars' velocities to obey a univariate Gaussian (i.e., Maxwellian) distribution with a dispersion of approximately 0.25 km/s, which seems to closely approximate the actual velocity distribution of the Hyades, and also restricts the stellar parallaxes to prevent a distortion of the cluster along the line of sight. All kinematic observations -- proper motions, radial velocities and trigonometric parallaxes -- can be simultaneously incorporated into a single adjustment which directly yields accurate estimates of the parallaxes and velocities of the Hyades stars. Application of the technique to model clusters shows that the least squares with probabilistic constraints does indeed yield more accurate distances than those obtained by the classical convergent point. The models also indicate that the parameter estimates are not terribly sensitive to the form of the cluster velocity distribution, the results being similar for both Gaussian and uniform spatial and velocity distributions. Finally, an adjustment incorporating observations for 140 known members of the Hyades yields a cluster distance of 49.7 +/- 1.0 parsecs, in good agreement with recent determinations by other investigators.
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