The Distance to the Vela Pulsar Gauged withHubble Space TelescopeParallax Observations

The distance to the Vela pulsar (PSR B0833-45) has been traditionally assumedto be 500 pc. Although affected by a significant uncertainty, this value stuckto both the pulsar and the SNR. In an effort to obtain a model free distancemeasurement, we have applied high resolution astrometry to the pulsar V~23.6optical counterpart. Using a set of five HST/WFPC2 observations, we haveobtained the first optical measurement of the annual parallax of the Velapulsar. The parallax turns out to be 3.4 +/- 0.7 mas, implying a distance of294(-50;+76) pc, i.e. a value significantly lower than previously believed.This affects the estimate of the pulsar absolute luminosity and of its emissionefficiency at various wavelengths and confirms the exceptionally high value ofthe N_e towards the Vela pulsar. Finally, the complete parallax data baseallows for a better measurement of the Vela pulsar proper motion(mu_alpha(cos(delta))=-37.2 +/- 1.2 mas/yr; mu_delta=28.2 +/- 1.3 mas/yr aftercorrecting for the peculiar motion of the Sun) which, at the parallax distance,implies a transverse velocity of ~65 km/s. Moreover, the proper motion positionangle appears specially well aligned with the axis of symmetry of the X-raynebula as seen by Chandra. Such an alignment allows to assess the spacevelocity of the Vela pulsar to be ~81 km/s.Comment: LaTeX, 21 pages, 5 figures. Accepted for publication in Ap