Molecular fraction in the Galactic Centre: The Central Molecular and H <scp>i</scp> Zones

By mapping the molecular fraction of Galactic Center (GC), we quantitatively address question how much and central CMZ (Central Molecular Zone) is. For this purpose analyze CO HI-line archival data, determine column- (surface-) volume-molecular fractions, $f_{\rm mol}^\Sigma$ mol}^\rho$, which are ratio column-mass density H$_2$ projected on sky to that total gas (H$_2$ + HI) from line intensities, volume-mass densities \Htwo\ brightness temperature, respectively. It is shown as high $ \sim 0.9-0.95$ in CMZ, mol}^\rho$ $0.93-0.98$ GC Arms I II attaining highest value $\sim 0.98$ toward Sgr B2. The expanding ring (EMR, or parallelogram) has a slightly smaller 0.9-0.93$. We define region with mol}^\Sigma \ge 0.8-0.9$ between shoulders plateau-like distribution column $l=-1^\circ.1$ $+1^\circ.8$ having Gaussian vertical half thickness $\pm 0^\circ.2$. embedded Central HI Zone (CHZ), defined an disc $l\sim -2^\circ$ $+2^\circ.5$, $b=-0^\circ.5$ $+0^\circ.5$. Based analysis, discuss origin interstellar physics such volume filling factors gases inferred difference f_{\rm mol}^\rho$.