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The Velocity Distribution of Nearby Stars from Hipparcos Data. I. The Significance of the Moving Groups
We present a three-dimensional reconstruction of the velocitydistribution of nearby stars (lsim100 pc) using a maximum likelihooddensity estimation technique applied to the two-dimensional tangentialvelocities of stars. The underlying distribution is modeled as a mixtureof Gaussian components. The algorithm reconstructs the error-deconvolveddistribution function, even when the individual stars have unique errorand missing-data properties. We apply this technique to the tangentialvelocity measurements from a kinematically unbiased sample of 11,865main-sequence stars observed by the Hipparcos satellite. We explorevarious methods for validating the complexity of the resulting velocitydistribution function, including criteria based on Bayesian modelselection and how accurately our reconstruction predicts the radialvelocities of a sample of stars from the Geneva-Copenhagen Survey (GCS).Using this very conservative external validation test based on the GCS,we find that there is little evidence for structure in the distributionfunction beyond the moving groups established prior to the Hipparcosmission. This is in sharp contrast with internal tests performed hereand in previous analyses, which point consistently to maximal structurein the velocity distribution. We quantify the information content of theradial velocity measurements and find that the mean amount of newinformation gained from a radial velocity measurement of a single staris significant. This argues for complementary radial velocity surveys toupcoming astrometric surveys.
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