Isotopic anomalies and condensation sequence

The observed overabundance of deuterated species in molecular clouds and outer disks compared to the measured interstellar D/H ratio of 10 5 is well established. A classical isotopic deuterium fractionation is possible at low temperatures of 10-20 K owing to disbalance between forward and reversed reaction efficiencies H+ + HD 5 H2D+ + H2 + 232K (e.g. Millar et al. 1989 Gerlich et al. 2002). The temperature dependency in an isotope exchange reaction is a consequence of the zero-point vibrational energy difference for the isotopically substituted molecules (Bigeleisen Mayer 1947 Urey 1947). This leads to an elevated ratio ol H2t)+/H compared to HD/H2, which is quickly transferred into other molecules by ion-molecule reactions (see e.g. Roberts Millar 2000 Roberts et al. 2003). For example, the dominant reaction pathway to produce DCO+ is via ion-molecule reactions of CO with H2D+. In disks it results in a DCO+ to HCO+ ratio that increases with radius owing to the outward decrease of temperature (Aikawa Herbst 2001 Willacy 2007 Qi et al. 2008). 

Other important fractionation reactions effective at higher temperatures (up to 70 K) are CH+ + HD CH2D+ + H2 + 390 K (Asvany et al. 2004) and C2H + HD C2HD+ + H2 + 550 K (Herbst et al. 1987). Both reactions lead to DCN by the ion-molecule reaction N + CH2D+ — DCN+, followed by reaction with H2 and dissociative recombination (Roueff et al. 2007). An alternative route is surface addition of D to CN (Hiraoka et al. 2006). The measured abundance ratio of DCN to HCN is about 1% in the disk around TW Hya (Qi et al. 2008). Effective 

There is evidence from chondrites that the solar nebula was well mixed between 0.1 and 10 AU during its first several million years of the evolution, as shown by the homogeneity in concentrations of many isotopes of refractory elements (Boss 2004 Chapter 9). This is likely caused by the evaporation and recondensation of solids in the very hot inner nebula, followed by outward transport due to turbulent diffusion and angular momentum removal. Materials out of which terrestrial planets and asteroids are built have been heated to temperatures above 1300 K and are thus depleted in volatile elements. The inner solar nebula, with some exceptions, does not retain memories of the pristine interstellar medium (ISM) chemical composition (Palme 2001 Trieloff Palme 2006). 

According to Palme (2001), four major condensation components can be isolated, see Table 4.3. While highly refractory elements have been locked in CAIs already at T 1800 K, the bulk of meteoritic samples are made of less refractory materials, like silicates and metals (T 1400 K). Iron condenses almost entirely in metallic form, and silicates are mostly as Mg-rich forsterite and enstatite. At about 700 K 

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