Deuterated interstellar and circumstellar molecules: D/H ratio and dominant formation processes

There are several constraints associated with the different models used in accounting for the D/H ratio observed of singly and multiply deuterated interstellar and circumstellar molecular species. Thermodynamically, the most distinctive difference between a molecule and its deuterated analogue is the zero point energy (ZPE). Applying high level quantum chemical calculations, the ZPE for all H-containing and their corresponding D-analogues for all interstellar/circumstellar molecular species considered in this study are determined. From the difference in the ZPE between the H-containing and the corresponding D-analogue, Boltzmann factor is computed for all the systems using the excitation temperature/molecular cloud temperature for the known D-molecules and a range of temperature for others. From the results, there is a direct correlation between the Boltzmann factors and the D/H ratios. Pronounced deuterium fractionation occurs at larger values of Boltzmann factor resulting in the observed high D/H ratios. Increased deuterium fractionation at low temperature suggests that grain surface reactions are the major formation processes for deuterated molecules. This implies that at lower temperature (higher Boltzmann factor), the exchange reaction involving deuterium or deuterium fractionation is much pronounced resulting in the distribution and redistribution of deuterium among various species. The implications of these results and the possibility of detecting more D-molecules are discussed. (author)