4.4. The Galactic Center

The central parsec of the galaxy, identified with the Sagittarius A nebula, contains ionized gas powered by about 10⁴⁰ ionizing photons sec^-1 (Lacy el al. 1980). A cluster of He I emission line stars has been observed and spectroscopically analyzed (Tamblyn et al. 1996, Najarro et al. 1997). The complete spectrum of infrared fine structure lines that has been observed, combined with the H Br and Br lines (see Shields & Ferland 1994 for a compilation) should in principle allow to perform an abundance analysis. From a two-component photoionization model Shields & Ferland (1994) estimate that the abundance of Ar should be about twice solar, but Ne seems rather to have the solar value. The evidence for over solar metallicity is thus mixed. The N/O ratio is estimated to about 3 - 4 times solar. However, the derived abundances may be clouded by errors in the reddening corrections (the extinction is as high as A_V = 31, so, even at far infrared wavelengths, reddening become important) and uncertainties in the atomic parameters (mainly those determining the ionization structure). As a consistency check, Shields & Ferland (1994) compared the electron temperature measured from recombination lines with their model predictions. For that, they included heating by dust, and assumed the same grain content as in the model of Baldwin et al. (1991) for Orion. They found the measured temperatures to be consistent with a metallicity 1 - 2 times solar, while 3 times solar would be only marginally consistent. However, with a population of small grains, photoelectric heating would be more important, and larger metal abundances could be acceptable.

The Galactic center has since then been reobserved by ISO (Lutz et al. 1996), but a detailed discussion of the new results remains to be done.