The Distance to the Vela Pulsar Gauged withHubble Space TelescopeParallax Observations

The distance to the Vela pulsar (PSR B0833-45) has been traditionally assumed to be 500 pc. Although affected by a significant uncertainty, this value stuck to both the pulsar and the supernova remnant. In an effort to obtain a model-free distance measurement, we have applied high-resolution astrometry to the pulsar V ~ 23.6 optical counterpart. Using a set of five Hubble Space Telescope Wide Field Planetary Camera 2 observations, we have obtained the first optical measurement of the annual parallax of the Vela pulsar. The parallax turns out to be 3.4 ± 0.7 mas, implying a distance of 294 pc, i.e., a value significantly lower than previously believed. This affects the estimate of the pulsar absolute luminosity and of its emission efficiency at various wavelengths and confirms the exceptionally high value of N e toward the Vela pulsar. Finally, the complete parallax database allows for a better measurement of the Vela pulsar proper motion [μ α cos(δ) = -37.2 ± 1.2 mas yr -1 ; μ δ = 28.2 ± 1.3 mas yr -1 after correcting for the peculiar motion of the Sun], which, at the parallax distance, implies a transverse velocity of ≈65 km s -1 . Moreover, the proper-motion position angle appears especially well aligned with the axis of symmetry of the X-ray nebula as seen by Chandra . Such an alignment allows us to assess the space velocity of the Vela pulsar to be 81 km s -1 .