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==> In this proposal, more time is being requested for DGARNETT.SWS_NES
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This is a joint proposal for programs DGARNETT.SWS_NES and MEDMUNDS.PROPISO.
We request supplemental ISO observing time to recover science lost because
of the reduced sensitivity of the SWS. We will make deep full grating scans
with SWS and LWS of one very metal-rich H II region in the spiral galaxy M51
(twice solar Z) and the very metal-poor H II galaxy SBS 0335-052 (2% solar Z).
The data will be used to measure a variety of nebular IR emission lines which
are important coolants and abundance diagnostics in the ionized gas. Because
of their weak sensitivity to electron temperature, the IR fine-structure lines
can be used to measure heavy element abundances with greater reliability than
high-excitation optical forbidden lines, especially in metal-rich nebulae where
electron temperature measurements are not available. We will confront two very
pressing problems of H II region physics: (1) the relation between optical
emission line strengths and abundances for low-excitation H II regions in
the inner parts of spiral galaxies; this is poorly known because electron
temperatures can not be measured in such objects, and unique photoionization
models for such nebulae are sensitive to competing influences of dust, density
and abundance variations; and (2) are there large systematic uncertainties in
the abundance scale due to electron temperature fluctuations within ionized
nebulae. Measurement of the IR emission lines in these nebulae will allow us
to (i) determine, for the first time, accurate chemical abundances in the gas
at metallicities above solar; (ii) greatly improve the calibration of strong-
line abundance indicators for giant H~II regions at high metallicities, with
considerable benefit to the field of chemical evolution of galaxies; and (iii)
test the importance of temperature fluctuations by comparing abundances from
optical and IR forbidden lines. Deep spectra may reveal the presence of organic
molecular features and rotational lines of H2, whose presence in very metal
poor dwarf galaxies is completely uncertain.