The dangers of deprojection of proper motions

Abstract

We re-examine the method of deprojection of proper motions, which has been used for finding the velocity ellipsoid of stars in the nearby Galaxy. This method is only legitimate if the lines of sight to the individual stars are uncorrelated with the stars' velocities. Very simple models are used to show that spurious results similar to ones recently reported are obtained when velocity dispersion decreases with galactocentric radius in the expected way. A scheme that compensates for this bias is proposed.

5 pages, 5 figures (3 colour), MNRAS submitted

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• The stellar kinematics in the solar neighborhood from LAMOST data
• The peculiar in-plane velocities in the outer disc of the Milky Way
• The White Dwarf Luminosity Functions from the Pan-STARRS 1 3π Steradian Survey
• The kinematics of Galactic disc white dwarfs in Gaia DR2
• Kinematic origin of white dwarfs in the solar neighborhood