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Unformatted text preview: 19. The interpretation of thermal spectra Reading: Shu, Vol.I, Ch.15 19.1 H II regions In the Milky Way and in other nearby galaxies on often observes small gas clouds in the vicinity of young, hot stars, that show a continuous emission spectrum throughtout the radio band and the infrared up to the Optical, superimposed on which are hydrogen lines like Ly α . Apparently these are clouds of ionized gas that is thermally emitting. We call them H II regions, where H II denotes singly ionized hydrogen. Let us have a closer look at an H II region which may be described by the following parameters: r = 5 pc n i = 100 cm- 3 T = 10 4 K (19 . 1) Then the absorption coefficient at hν ¿ kT is (in cgs units) α ν = 2 . 2 · 10- 17 n 2 i ν 2 mc 2 kT ! 3 / 2 ln β E e α E ph ! ’ 10- 3 ν- 2 (19 . 2) since the Coulomb logarithm ln β E e α E ph ! ’ 15 f¨ur ν = 10 8 Hz (19 . 3) The optical depth can be estimated using the radius as mean length of the line-of-sight, so τ ’ r α ν ’ 1 . 4 · 10 16 ν- 2 ’ ν 100 MHz- 2 (19 . 4) At frequencies below ν c ’ 100 MHz our H II region should be optically thick. In the optically thin part of the spectrum a frequency dependence of the intensity arises only from the Coulomb logarithm. Often one approximates that frequency dependence by a power- law with a view to characterize the spectrum by a power-law index, the so-called...
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- Fall '08
- Physics, HII Regions, Ly α, HII region