Evaporation kinetics of minerals and isotopic fractination ; application to the solar nebula

实验研究了橄榄石在真空中的蒸发速率和不同压力下的氢气流量。蒸发率与氢气压力有很大关系。当P<H2>低于10^~(-6)bar时，蒸发速率与真空度相等，与P_<H_2>有关，P_<H_2>与P_<H_2>的乘积在10^~(-6)~10^~(-4)bar之间，与P_<H_2>在10^~(-4)~(-4)bar之间的关系不大。当P<H2>低于10^&lt；-4&gt；bar时，蒸发速率与氢压的关系由方程j=0.33p<H2>+8×10~(-7)&gt；表示，其中j是单位时间内单位表面积的蒸发通量。单位幂表明，表面的一摩尔镁橄榄石与一摩尔吸附的氢分子之间发生了基元反应，并据此估算了太阳星云中镁橄榄石颗粒的寿命。假设星云为均匀大小的球形颗粒，星云由H_2、O_2和C组成，系统假定为…更多的是瞬间升到高温，并保持在那个温度很长一段时间。在加热初期，颗粒的蒸发速率与真空中的蒸发速率相同，但随着时间的延长，由于含镁或含硅气体分压的增加，颗粒的蒸发速率变小。得到了颗粒大小和蒸发通量随无因次参数ETA的变化。Ethe代表尘埃相对于氢气的浓缩程度，这些气体归一化到太阳系元素丰度。当温度小于3时，镁橄榄石颗粒完全蒸发，蒸发速率与真空相当。反之，当ETA大于3时，体系达到固气平衡，导致蒸发停止。随着ETA的降低，达到平衡所需的时间变长。在太阳星云中，估计在0.1到100之间，因此，星际尘埃在丰度较高时可以通过星云加热而存活，当丰度较小时星际尘埃完全蒸发，导致系统的均一化。较少

Evaporation rate of olivine in vacuum and hydrogen gas flow of various pressures were experimentally investigated. The evaporation rate has large dependence on the hydrogen gas pressures. The evaporation rate is equal to that in vacuum at P_<H2> below 10^<-6> bar, is dependent on P_<H2> with power of unity at P_<H2> between 10^<-6> and 10^<-4> bar, and has small dependence on P_<H2> above 10^<-4> bar. The relationship between evaporation rate and hydrogen pressure at P_<H2> below 10^<-4> bar is shown by the equation j=0.33P_<H2>+8x10^<-7>, where j is the evaporation flux from a unit surface area in a unit time. The power of unity suggests that the elementary reaction is between one mole of forsterite at the surface and one mole of adsorbed hydrogen molecule.By using above results, the life-time of forsterite grains in the solar nebula was estimated. The grains are assumed to be spherical with a uniform size, the nebula is assumed to consist of H_2, O_2 and C,and the system was assumed … More to go up to a high temperature instantaneously and kept at that temperature for a long time. The evaporation rate from the grains are the same as that in vacuum at the beginning of heating and gets smaller with time due to increase of partial pressure of Mg-or Si-bearing gas species. The change of grain size and evaporation flux are obtained as a function of a dimensionless parameter eta. Ethe represents the degree of enrichment of dust against hydrogen gas normalized to the solar system elemental abundance. At etasmaller than 3, forsterite grains evaporate totally and the evaporation rate is equal to that in vacuum. On the contrary at eta larger than 3, the system gets equilibrium between solid and gas resulted in cease of evaporation. The time required for getting equilibrium becomes longer with decreasing eta. It requires infinite time at eta around 3. In the solar nebula, is estimated to be between 0.1 to 100, and therefore, interstellar dust could survive from nebular heating when the degree of enrichment was high, and that they totally evaporated when the degree was small which resulted in homogenization of the system. Less