Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.naic.edu/~astro/talks/abstracts/deshpande.html Дата изменения: Thu Feb 14 20:49:44 2002 Дата индексирования: Sat Dec 22 13:43:00 2007 Кодировка: Поисковые слова: ultraviolet

During the past decade or so, many measurements of Galactic HI absorption using VLBI against extra-galactic sources as well as those using multi-epoch observations in pulsar directions have reported detectable differences in the absorption when two or more sight-lines with small angular separations are compared. The angular separations translate to transverse spatial scales down to tens of AU at the distance of the absorbing matter. These measurements have invariably been interpreted as evidence for significant wide-spread small-scale structure in the HI distribution with volume densities orders of magnitude higher ($n_{HI}\sim 10^4-10^5$ cm$^{-3}$) than those implied by the parsec-scale component. Naturally, there are serious difficulties about such structures being in equilibrium with the other components of the medium, and hence about how to generate and maintain such apparently commonly encountered structures. Some recent suggestions for a geometric solution (invoking cold, dense curved filaments or sheets that line-up along sight-lines), while easing some, appear to have their own set of difficulties.

In this talk, we will see how the observations have been {\it mis-interpreted}, particularly in estimating and attributing such high densities to the structure on the sub-parsec scale, making it appear as a physically distinct population of structures. We will also show that the observed opacity differences, {\it when interpreted correctly}, are not surprising, but are consistent with a single power-law description of the distribution of HI-opacity in the interstellar medium. There appears to be no compelling evidence to treat the HI structure at small-scales (tens to hundreds of AU) as a component distinct from the rest of the neutral medium. In fact, the small-scales contribute only a tiny fraction of the observed column densities along any sight-line, consistent with their equilibrium coexistence with the wide range of hierarchical scales. This, in our opinion, should {\it put an end to the decades long puzzle of the so-called small-scale structure} in HI and other species in the Galaxy.