Deuterium

Chapter 6 described observations of the ground state rotational line of HD in emission. These observations were used to deduce a value for the deuterium abundance in the Sgr B2 envelope. This is particularly important because very few measurements of D/H have been made beyond the local ISM. The main results and conclusions were as follows:

• Measurement of HD. A detection of the 112 $\mu$m $J=1\rightarrow0$ rotational transition of HD was made at more than 99.9% confidence. Outside of the Solar System, this line has only ever been detected towards two other sources (Orion and W49N; Wright et al., 1999; Caux et al., 2002). Non-prime data were used to determine a detection limit for the $J=2\rightarrow1$ transition at 56 $\mu$m and this showed that the HD emission must originate in the warm envelope of Sgr B2 with a temperature $<80$ K. Previous studies in the literature gave a lower value for the temperature in this region of 40 K (e.g. de Vicente et al., 1997) and in this case the column density of HD was calculated to be $N(\mathrm{HD})=(1$- $13)\times10^{18}$ cm$^{-2}$ (corresponding to 40-80 K).
• Deuterium abundance. In order to calculate the D/H ratio in the Sgr B2 envelope, the corresponding column density of molecular hydrogen was estimated. Analysis of SWS observations of the rotational transitions of H$_{2}$ showed that these lines traced warmer gas than that associated with the HD emission. The column density of cool H$_{2}$ was determined from dust measurements in the literature to be $N(\mathrm{H_2})=$(0.5-2)$\times10^{24}$ cm$^{-2}$, giving a final deuterium abundance of $\mathrm{D/H}=(0.26$- $13)\times10^{-6}$. This is consistent with two previous measurements (Lubowich et al., 2000; Jacq et al., 1999), showing that there is a gradient in deuterium abundance, with decreasing values towards the Galactic Centre. This is due to the higher rates of stellar processing that exist in the Galactic Centre region. However, some models of galactic chemical evolution predict that there should be a much larger decrease than the observations allow. Infall of primordial material can solve this problem and when this is included in the models (e.g. Prantzos, 1996) the results are much more consistent with the observations. Many more measurements of deuterium abundances are needed throughout the Galactic Disk to constrain these models further.