Deuterium in comets—and AGB stars?

Abstract

We discuss the deuterium chemistry of cometary comae, and show how the fractionation observed in coma molecules relates to the D/H ratios in the nuclear ices. For most molecules, we find that the fractionation is unchanged in the coma. In addition, we calculate the potential effects of degradation of D-rich polymers/organic refractory material on selected gas-phase fractionation ratios. We demonstrate that for molecules known to have an extended coma source, the observed D/H ratios may not necessarily reflect those in the nuclear ice. The recent detection of water vapour in the stellar wind of the carbon-rich red giant IRC+10°216 has been attributed to the vaporization of a remnant population of comets surrounding this evolved star. This theory would be confirmed by the detection of deuterated molecules in this object: as deuterium is destroyed in stars, any D-bearing molecules must originate from cometary or planetary objects. We quantitatively assess the possibility of detecting HDO in this source if such comets contain similar D/H ratios to those in our solar system, and show that the 1 11–0 00 transition at 894 GHz is the most promising candidate for detection.