THE METALLICITY OF GALACTIC CENTER GAS

scientific abstract galactic evolution models lead us to believe that the interstellar gas occupying the central few hundred parsecs of the galaxy has probably undergone a relatively large amount of nuclear processing in stars. this is supported by observations of highly unusual isotopic abundance ratios of molecular lines there, indicative of an enhanced abundance of nuclear burning products via both primary and secondary processes. however, the metallicity of the galactic center region is only crudely determined at present, and there is essentially no unambiguous information on how the metallicity varies with radius in the inner few hundred parsecs. a few measures have been made of midir lines of ar and ne, and of the farir fine structure lines of n iii and o iii, and these suggest a metallicity enhancement of about a factor of two, but the treatment of the abundances of unobserved ionization states is quite uncertain. stars near the galactic center provide some r^ant clues the peak metallicity of k giants in baade.s window is about twice the solar value, though there is a broad range. in sum, the metallicity of the galactic center is poorly known, and yet we cannot understand this environment well without knowing it. the metallicity is needed to 1) constrain models of galactic evolution, 2) model the heating, cooling, and chemistry of the interstellar medium near the galactic center, 3) use molecular lines to derive cloud masses, and 4) understand how metallicity may affect the imf in the star formation process. in order to determine accurate abundance ratios and to establish the galactic center metallicity and its dependence on galactocentric radius with some reliability, we propose to use the sws and lws on iso for a total of 2.9 hours to observe the accessible ionization states of h (alpha lines at 4.05 & 7.45 microns), n (nii at 121.9 and niii at 57.4 microns), o (oiii at 51.8 & tr [truncated!, Please see actual data for full text]