Physical and Chemical Properties of Molecular Gas in Star Forming Regions

In this thesis I study the physical and chemical properties of molecular gas in star forming regions. This work takes advantage of the newly developed CONDOR receiver, which allows spectrally resolved observations of emission lines at frequencies of about 1.4 THz. Thus the thesis contains observational data obtained with CONDOR as well as observations motivated by the science one can do with receivers at terahertz frequencies are largely included. In the massive star forming region NGC 2024, seven emission lines of 12CO and 13CO have been observed, including 12CO J=13-12 (f=1.497 THz) observations with CONDOR. These high-J CO data reveal a thin layer of hot (~300 K) and dense (~10e6 cm-3) molecular gas, which has not been detected with any other tracer before. This newly discovered component is located at the interface between an HII region and the molecular cloud. Furthermore, the narrow line width indicates that this layer is heated by radiation rather than by shocks. On the background of well established physical scenarios, such as the ''Blister Model'' and the PDR scenario, I developed a model of NGC 2024, using radiative transfer computations. This model explains both, the spatial distribution of density and temperature along the line of sight and the velocity structure of the source. The complex profiles of the observed 12CO and 13CO lines, which include emission by multiple velocity components as well as self-absorption by foreground material, are reproduced by this model remarkably well. Furthermore, the integrated intensities of five additional lines (J=14 to J=19) can also be explained. My study of low mass protostars was motivated by the H2D+ line observable with CONDOR, and shows the variation of chemical abundances along the star forming process. Next to its cosmological importance, deuterium plays also a significant role in the chemistry in the early phases of star formation. In such objects extremely high abundances of deuterated molecules have been measured. These abundances are mainly determined by the freeze out of CO onto dust grains, and the implication of the lowered CO abundance on gas phase chemistry. A consequence of the freeze out of CO at temperatures below 20 K, is that N2H+ is very abundant in the inner parts of prestellar cores, except for some possible depletion of N$_2$H$^+$ itself in the coldest spots. In objects, which harbor already a protostar, the CO depleted zone forms a shell around the warm core. The more evolved such an embedded protostar is, i.e., the more it heats up the surrounding material, the smaller gets the CO depleted shell. The lack of CO in the gas phase leads not only to an enhanced abundance of N2H+, but also deuterium bearing molecules, such as H2D+, N2D+ and NH2D, are formed in a significant amount. As I will show in this work, the abundance of these deuterated species, and especially the abundance ratio of the deuterated to the non-deuterated form of molecule can be used as a tracer for the evolutionary stage of young protostellar cores. A high N2D+/N2H+ ratio (>0.15) indicates, that in such a core the collapse, which forms a star, has just begun. This ratio decreases as the protostar evolves further. I conducted follow up observations of NH2D and NH3 in the same objects. These observations give a similar trend for the NH2D/NH3 ration. However, other ratios, e.g., DCO+/HCO+ do not show such an enhancement, and stay at a level of a few percent throughout the protostellar evolution. The differences in the deuterium fractionation during the stellar evolution help us to understand the physical and chemical processes taking place. The lines, now observable at high spectral resolution with CONDOR, provide new possibilities to study the physical processes in the interstellar medium. The observations of highly excited CO enable us for the first time to investigate the hot, molecular gas in the ISM specifically. Therefore the temperature and density of this material can be determined with much higher accuracy than previously. In addition, the shapes of the high-J CO lines draw conclusions on the heating mechanisms of the gas. H2D+ observations give us insights into the physics of the extremely cold part of the ISM, where standard tracers, like CO, are frozen out onto dust grains. As I will show in this work, observation at THz frequencies allow us to study the ISM under extreme physical conditions, which have been very difficult to investigate so far