D/H enrichment at astrophysically-relevant temperatures

Details D/H enrichment at astrophysically-relevant temperatures D/H enrichment at astrophysically-relevant temperatures

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p11c1359e&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #P11C-1359

Mathematics

Probability

[6020] Planetary Sciences: Comets And Small Bodies / Ices, [6210] Planetary Sciences: Solar System Objects / Comets

Scientific paper

Although the relative abundance of elemental deuterium to hydrogen is around 1.5 x 10^(-5) [1], a variety of molecules in different regions of space have shown [2,3] to contain a higher deuteration ratio, up to 10^(-2) or even higher values, which cannot be explained by pure standard gas phase chemistry. One of the proposed explanations for this involves low temperature gas-grain reactions, as the lower zero-point energy of deuterated molecules favours longer residence times on ice mantles surfaces, where they could react with other not fully reduced species trapped on the ices. After evaporation these species could present the high D/H ratio observed in gas phase. This investigation explores the possibility of D/H interchange between gas phase D2 and water ice within a range of temperatures that extends down to 14 K. To this aim, we have prepared samples by vapour deposition on a cold substrate of mixtures of deuterium and water, keeping a larger D2 partial pressure to favour D/H interchange. We will present evidence of formation of deuterated species based on their infrared signature, namely the O-D stretching band in the 4 - 4.16 μm region. The spectroscopic behaviour of this band differs depending on the temperature and relative concentration of formation of the ices. Our results show that the amount of deuterium interchange is fixed for a given H2O/D2 pressure ratio within the 40 - 140 K range, but the amorphous to crystalline phase change in ice at 150 K induces a reordering of the deuterated species revealed by a drastic change in the O-D stretching band shape. Based on our results, we estimate the probability of D/H interchange for collisions between D2 and water ice above 30 K of the order of 1/1000 to 1/5000. [1] Linsky, J. L. 2003, Space Sci. Rev. 106, 49. [2] Ratajczak, A., Quirico, E., Faure, A., Schmitt, B., & Ceccarelli, C. 2009, A&A, 496, L21. [3] Weber, A. S., Hodyss, R., Johnson, P. V., Willacy, K., & Kanik, I. 2009, ApJ, 703. 1030.

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