Physics
Scientific paper
Sep 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989msngr..57...56g&link_type=abstract
The Messenger, vol. 57, p. 56-57
Physics
1
H Ii Regions: Ccd Observations
Scientific paper
Understanding the physical and dynamical evolution of galactic and extragalactic H11 regions requires a knowledge of the dust component and its distribution. To date the extinction Av has been derived by various methods: optical and infrared line ratios, comparison of radio and emission line fluxes, stellar photometry, etc. In particular the intensity ratios of the strong and spectroscopically easily accessible Balmer lines Halpha, Hbeta are frequently used to derive Av via their decrement. Since these lines originate from different upper levels, the interpretation of the observed line ratios requires recombination line model calculations (cf. Osterbrock 1974) which in many extragalactic cases have failed to give consistent results (Ward et al. 1987, Malkan 1983, Rieke and Lebofsky 1981). This difficulty can be avoided by using multiplet line ratios originating from the same upper level so that the theoretical line ratios depend primarily on their relative transition probabilities.
Castles James J.
Greve Albert
McKeith Conal D.
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