CHEMICAL AND PHYSICAL CHARACTERISATION OF COMPACT HII REGIONS ACROSS THE GALAXY - PART 2 OF 2 -
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scientific abstract it is proposed to use the iso long wavelength spectrometer (lws), the short wavelength spectrometer (sws) and the camera (cam) to investigate the infrared spectra of compact hii regions. the selected sample of 60 sources covers in a nearly homogeneous way the galactic plane from the center out to 25 kpc. together with the study of the physical conditions pertaining in these regions of recent massive star formation, this will enable us to investigate in great detail the problem of the presentday abundance distribution in the galaxy. the study of chemical abundances and their variation as a function of the distance to the galactic center is of fundamental importance for our understanding of galactic evolution. the abundances of heavy elements in the interstellar medium provides a direct estimate of the enrichment due to nucleosynthesis in successive generations of stars. the variation of one element relative to another yields information on the formation paths of these elements. a detailed knowledge of the abundance variation within a galaxy is a prerequisite to a wide variety of studies such as the large scale distribution of the infrared radiation, gamma rays and cosmic rays. the infrared finestructure lines are better suited than the optical lines (as usually used) for abundance analysis in the galaxy for two reasons. first, the infrared lines are comparatively insensitive to the electron temperature so that the correction for electron temperature, crucial in the optical studies, is not required. second, infrared lines are less attenuated by interstellar extinction than their optical counterparts and provide a probe of the elemental abundances through the galactic disk (although substantial extinction corrections are still necessary), in particular in the inner regions, at galactocentric distances less than 6 kpc where no optical measurements are possible. observation summary the compact h t [truncated!, Please see actual data for full text]
