Internal variation of electron density in galactic Hbt II regions
A study on the internal variation of the electron density in galactic HIi regions has been conducted on a sample of 15 objects of differentsizes and evolutionary stages. The [S Ii] lambda 6716/lambda 6731 lineratio was adopted as electron density indicator. Long slitspectrophotometry of high signal-to-noise ratio with spectral dispersionof 0.75 Ä,pxl-1 and spatial scale of 0\farcs90pxl-1 were obtained at different slit positions andorientations. No systematic spatial variation of electron density wasdetected in nearly half of the objects studied (S255, S 257, S 271,S 285, S 301, S305, NGC 3372 and IC1275). They are in general the most diffuse and probablyevolved objects with low mean densities in the range Ne ~20-140 cm-3. The remaining objects (S 288,S 307, NGC 2579, NGC3503, Gum 62, Gum 64aand M 20) with mean densities Ne ~ 80-360cm-3, have shown a statistically significant electron densitydependence on position. In most of these cases, the spatial variation ofdensity may be interpreted as a radial gradient with the densitydecreasing from the centre to the edges. M 20 shows asystematic non-radial variation of electron density with maximum valuesoccurring at its prominent dark lanes. A mean filling factor of theorder of phi = 0.1 was found compatible with the data. Based on theirdensity profiles, NGC 2579, Gum62, Gum 64a and possibly NGC3503 were indicated as candidates of showing a `champagneflow'.
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