Non-equilibrium H_2 ortho-to-para ratio in two molecular clouds of the Galactic Center

Abstract

We present ISO observations of the S(0), S(1), S(2), and S(3) rotational lines of molecular hydrogen from two molecular clouds near the Galactic Center (GC). We have also measured continuum dust emission at infrared wavelengths with ISO and the rotational radio lines J=1-0 of ¹³CO and C¹⁸O and J=2-1 of C¹⁸O with the IRAM-30m telescope. Using the dust continuum spectra and the CO lines we derive a total visual extinction of s^{-1}im 15-20 magnitudes toward these GC clouds. After correcting the H_2 data for extinction, the gas temperatures are s^{-1}im 250 K and the column densities of warm gas are s^{-1}im 2x 10²¹ cm^-2. This is the first direct measure of the H_2 column densities of the warm component; with this, we estimate an NH_3 abundance in the warm gas of s^{-1}im 2 10^-7. The column density of warm gas is, at least, a factor of 100 larger than the corresponding column densities derived from the warm dust. The observed ortho-to-para ratio (OTPR) is s^{-1}im 1, clearly below the local thermodynamical equilibrium (LTE) OTPR for gas at 250 K of s^{-1}im 3. Low velocity shocks (s^{-1}im 10 km s^-1) are the most likely explanation for the column densities of warm gas and dust and the non-LTE H_2 OTPR. ISO is an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.