Simulating the D/H ratio of water formed in the early solar nebula

Abstract

Our solar system originated from a protoplanetary disk about 4.6 billion years ago. We simulate the formation of this disk by a three-stage model of the solar nebula (SN) which describes the hydrodynamic and chemical evolution of a cold cloud core consisting of gas and one mass percent dust. Considering the first two stages of this SN-model we have studied the formation and deuteration of water, which is an important precondition of life. During the quasi-stationary stage of the cloud core, corresponding to the first SN stage, water has been formed on the surface of dust grains by the hydrogenation of oxygen. The gas and dust temperatures, which differ at the outer boundary of the core, are nearly 14 K and reach 9 K in its center. Therefore an icy mantle forms on the dust grains in less than 105 years and changes slowly afterwards. Because of the large abundance of hydrogen and a carbon to oxygen (C/O) ratio of 0.44 the major component of this mantle is water ice. We found that the water produced in the gas phase amounts to less than 20 ppm of the water formed on dust grains. In both phases, the deuterium enrichment δD (‰) relative to the Standard Mean Ocean Water varies at 1 AU from 15,050 to 63,100‰ (or a D/H ratio from 2 to 0.5%) and indicates the low formation temperature of water molecules. In the second stage of our SN-model, the collapse of the cold cloud core is simulated using a semi-analytical solution of the magneto-hydrodynamic equations. Due to relatively high temperatures around the center (10^(2-3) K), this range is identified with the hot corino observed in regions of low mass star formation in our galaxy. There, the icy mantles of the grains vanish due to desorption of water molecules from their surfaces. As a result the water to hydrogen ratio in the gas phase increases to 10^(-5)-10^(-4). Since this water was formed in a cold region and a collision related destruction of water molecules (occurring at ~10^5 K) can be neglected everywhere except for the protostellar source in the core center (< 10^(-2) AU), the deuterium enrichment in the outer hot corino (1 AU) reaches δD of 2,210‰ (or D/H of 0.1%) at the end of the main collapse phase. Different reasons for this relatively high value are discussed.