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Astron. Astrophys. 357, 621-636 (2000)

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Internal variation of electron density in galactic HII regions

M.V.F. Copetti 1, J.A.H. Mallmann 1,2, A.A. Schmidt 1 and H.O. Castañeda 3

1 Universidade Federal de Santa Maria, Laboratório de Análise Numérica e Astrofísica, Departamento de Matemática, 97119-900 Santa Maria, RS, Brazil (mvfc@lana.ccne.ufsm.br)
2 Universidade de Ijuí, Departamento de Física, Estatística e Matemática, 98700-000 Ijuí, RS, Brazil
3 Instituto de Astronomía, Universidad Nacional Autónoma de México, Apartado Postal 877, BC 22860 Ensenada, México

Received 21 July 1999 / Accepted 20 March 2000

Abstract

A study on the internal variation of the electron density in galactic HII regions has been conducted on a sample of 15 objects of different sizes and evolutionary stages. The [SII ] [FORMULA]/[FORMULA] line ratio was adopted as electron density indicator. Long slit spectrophotometry of high signal-to-noise ratio with spectral dispersion of 0.75 Å pxl-1 and spatial scale of [FORMULA] pxl-1 were obtained at different slit positions and orientations. No systematic spatial variation of electron density was detected in nearly half of the objects studied (S 255, S 257, S 271, S 285, S 301, S 305, NGC 3372 and IC 1275). They are in general the most diffuse and probably evolved objects with low mean densities in the range [FORMULA] 20-140 cm-3. The remaining objects (S 288, S 307, NGC 2579, NGC 3503, Gum 62, Gum 64a and M 20) with mean densities [FORMULA] 80-360 cm-3, have shown a statistically significant electron density dependence on position. In most of these cases, the spatial variation of density may be interpreted as a radial gradient with the density decreasing from the centre to the edges. M 20 shows a systematic non-radial variation of electron density with maximum values occurring at its prominent dark lanes. A mean filling factor of the order of [FORMULA] = 0.1 was found compatible with the data. Based on their density profiles, NGC 2579, Gum 62, Gum 64a and possibly NGC 3503 were indicated as candidates of showing a `champagne flow'.

Key words: ISM: general – ISM: H ii regions

Send offprint requests to: M.V.F. Copetti

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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