Program: GS-2009A-C-6

We propose to use the Phoenix Spectrometer to observe H$_3^+$ and CO toward newly identified Galactic center sources in order to further characterize the diffuse cloud environment there. Our research has already identified a previously unrecognized but very extensive region of warm ($T$=200--300~K) and diffuse ($n\leq$50~--~200~cm$^{-3}$) gas in the Central Molecular Zone (CMZ) of the Galactic center, mainly by using metastable H$_3^+$ $R$(3,3)$^l$ absorption line. Although our probing of the CMZ has been based only on spectroscopy of IR sources within 30 pc of Sgr A*, circumstantial evidence suggests that the warm and diffuse gas pervades the entire CMZ. If correct, it would completely overturn previous understanding of the CMZ environment. Using newly identified IR sources suitable for spectroscopy of H$_3^+$ and CO we can now extend the study of the newly found gas from the vicinity of SgrA* ($\sim$ 30 pc) to a much wider region of the CMZ ($\sim$ 170 pc), an increase of $\sim$50 in the angular area. Our extended investigation will reveal how the warm and diffuse clouds are distributed in the CMZ. We will obtain information on temperature, density, volume filling factor, and ionization rate of the diffuse CMZ gas. The extent and the nature of the cloud will in turn provide crucial information on the astrophysics in the core.