Neutral Hydrogen in the Direction of the Vela Supernova Remnant

Abstract

We have carried out a study of thedistribution and kinematics of the neutral hydrogen in the direction ofthe Vela supernova remnant (SNR). A field of 6.8d x 5.4d centeredat l = 264.1d, b = -1.6d was surveyed using the Parkes 64 m radiotelescope (half-power beamwidth 14.7' at 21 cm). Nearly 2300 H iprofiles were obtained with a grid spacing of 7.5'. The presence ofa thin, almost circular H i shell, centered at v = 1.6 +/- 0.8 kms^-1, is revealed. This shell delineates the outer border of theX-ray emission as shown in the ROSAT observations of Aschenbach,Egger, & Tr�mper and wraps around the receding part of the remnant.In addition, two higher velocity features possibly associated withVela are observed at about -30 and 30 km s^-1. These features areinterpreted as gas accelerated by the expansion of the supernovashock. The low systemic velocity observed suggests a distance shorterthan 500 pc for the Vela SNR. The H i shell is ~7 deg in diameterand expands at v ~ 30 km s^-1. By assuming a distance of 350 pc,we calculate for this shell a linear radius of 22 pc, a swept-upmass of ~1200-2300 M_⊙, and an atomic preshock density of ~1-2cm^-3. The kinetic energy transferred by the supernova shock into theinterstellar medium is ~(1-2) x 10^49 ergs, while the initial energy of theexplosion is estimated to be ~(1-2.5) x 10^51 ergs. We present thedistribution of the column density of the neutral material absorbing theX-radiation, an essential parameter in the analysis of X-ray data. Acomparison between the H i and Halpha emission suggests that the H ishell contains embedded dust that might be responsible for increasedoptical absorption in this region. On the other hand, the brightestarc-shaped optical filaments associated with the western side of Velashow good correspondence with the H i features. From a comparisonbetween the H i and Molonglo Observatory Synthesis Telescope 843 MHzradio continuum emission, we find that the outermost arched radiofilaments correlate well with the main ridge of the H i shell. No stronginhomogeneities were found in the ambient H i medium in the direction ofVela X (the central nebula, powered by the pulsar PSR B0833-45).