Astronomy and Astrophysics – Astrophysics
Scientific paper
Nov 1984
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984a%26a...140..141p&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 140, no. 1, Nov. 1984, p. 141-147.
Astronomy and Astrophysics
Astrophysics
6
Interstellar Extinction, Interstellar Matter, Planetary Nebulae, Astronomical Models, Cosmic Dust, Hydrogen Clouds, Ionized Gases, Opacity, Radii
Scientific paper
The author investigates the mean level of visual extinction in planetary nebulae as a function of observed radius R0, and finds the intrinsic variability in EB-V to be less than 0.2 mag over the range R0 < 0.53 pc. This is shown to be consistent with models of nebulae in which ionised mass is variable, and the H I halos are appreciably extended compared with observed H II shells. Substantial departures from this scenario (cf. compact halos and/or invariant H II mass) are found to require greatly depressed values of Av/(nH2+0.5nH) compared to the interstellar medium. Finally, the author briefly discusses four nebulae (NGC 7027, NGC 6302, M2-9, and NGC 6537) which appear to break this trend, and show clear evidence for high levels of intrinsic extinction. For these cases, the high column densities of absorbing material may reflect the large masses (3 → 5 M_sun;) shared by these and other type I planetaries.
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