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Paper: VLBA Observations of AU-scale HI Structures
Volume: 365, SINS — Small Ionized and Neutral Structures in the Diffuse Interstellar Medium
Page: 12
Authors: Brogan, C.L.; Goss, W.M.; Lazio, T.J.W.; Faison, M.D.
Abstract: Several recent studies of Galactic H I toward background quasars and pulsars have provided evidence that there are opacity changes in the neutral interstellar medium on size scales as small as a few AU. The nature of these opacity variations has remained a matter of debate but could reflect a variety of physical processes, including changes in the H I spin temperature or gas density. A recent three-epoch series of observations of the quasar 3C 138 with the Very Long Baseline Array by Brogan et al. have shown clear evidence for opacity variations at the level of 0.50±0.05 over spatial scales of about 25 AU or 50 mas. In addition, clear evidence for temporal variations in the H I opacity over the seven-year time scale were observed, consistent with transverse motions of a few tens of kilometers per second. The plane-of-sky covering fraction for the absorbing medium was determined to be ~ 10% with an inferred filling factor for the small scale H I of ~ 1%. The line widths of the VLBA small scale features are comparable to single dish data, suggesting that the opacity variations cannot be due to change in the H I spin temperature. Based on these results, density enhancements are the favored interpretation. If this is the case, the enhancements are expected to be far from equilibrium and necessarily of short duration. Recently (2005 August), new VLBA observations of the quasar 3C 147 were carried out, which form a second epoch observation following the previous observations by Faison & Goss in 1998. The on-going analyses of these observations suggests that the properties of the small-scale H I opacity variations along these two lines of sight are broadly consistent.
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