6 Summary and conclusions

We have observed the S(0) to S(5) pure-rotational lines with the SWS spectrometer on-board ISO toward a sample of 18 molecular clouds of the Galactic center region. The S(3) line is strongly affected by dust extinction due to the 9.7 $\mu$m band of the silicates. After correcting the data for extinction using a self-consistent method, and assuming that the ortho- and para- populations are in equilibrium one finds that the S(0) and S(1) lines indicate temperatures of $\sim \,$150 K. Extrapolating to the lowest levels at that temperature, a total column density of $\sim \,$1-2 10²² cm^-2 is derived. This is the first direct estimate of the column density of warm gas in the GC clouds. In addition, it shows a complex temperature structure of the warm gas.

The temperature derived from the S(5) and S(4) levels is $\sim \,$600 K for the sources in which it can be derived. However the column density of gas at this temperature is less than 1% of the column density at T=150 K. Assuming an OTP ratio of $\sim \,$2 the temperatures would be 10% larger than those derived assuming a LTE OTP ratio, while the total column densities at those temperatures would be a factor of $\sim \,$1.8 lower than the column densities derived assuming the ortho- and para- populations in equilibrium. Comparing the warm column densities with the column densities derived from our CO data by LVG calculations one finds that the average fraction of warm to the gas observed in CO is $\sim \,$30%. With our data and the NH₃ observations of Hüttemeister et al. (1993) we derive relatively high NH₃ abundances of a few 10^-7 in both the warm and the cold components.

Several indirect arguments point to shocks as the heating mechanism of the warm gas but PDRs may also play a role. Direct comparison of the data with PDRs and shocks models indicate that the S(4) and S(5) trace the densest gas in the GC clouds ( $\stackrel{<}{\scriptstyle \sim}$10⁶ cm^-3) heated in PDRs or shocks. Nevertheless, such dense PDRs or shocks fail to explain the S(0) and S(1) lines: several low density PDRs, low velocity shocks (< 10 kms^-1) or both, along the line of sight would be needed to explain the observed emission.

The cooling by H₂ in the warm component of GC clouds is comparable to the cooling by CO. Equating the   cooling rate with the heating rate by dissipation of supersonic turbulence, one finds that this mechanism could also contribute to the emission in the two lowest   lines. In one source (M -0.96+0.13), we have also found some evidence of large scale shocks that should be checked with higher spectral resolution observations.

Acknowledgements

We thank the referee, Rolf Güsten, for his useful comments. We acknowledge support from the ISO Spectrometer Data Center at MPE, funded by DARA under grant 50 QI 9402 3. NJR-F, JM-P, PdV, and AF have been partially supported by the CYCIT and the PNIE under grants PB96-104, 1FD97-1442 and ESP99-1291-E. NJR-F acknowledges Consejería de Educación y Cultura de la Comunidad de Madrid for a pre-doctoral fellowship.