28-30 September 2016
Tagungstätte Schloss Ringberg, Kreuth
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Contribution Contributed Talk

MORNING SESSION 2 - SOLID PHASE

Simultaneous Hydrogenation and VUV-Irradiation Study of CO-CH$ _{3} $OH Ice Mixtures under Dense Interstellar Cloud Conditions

Speakers

  • Mr. Ko-Ju CHUANG

Primary authors

Co-authors

Content

Complex organic molecules (COMs) have been observed in hot cores of low- and high-mass protostars, and in cold dense regions of the interstellar medium. It is generally accepted that such COMs form in the solid state through radical diffusion reactions on icy dust grains. In the past decade, modeling and laboratory studies have been in favor of scenarios in which energetic triggers, i.e., impacting electrons, protons or UV-photons, are involved [1]. Recently, laboratory experiments showed that the detection of COMs in cold dense clouds, i.e., in regions where thermal processing can be excluded and photon driven chemistry is weaker, also can be explained by non-energetic H-atom additions [2, 3 and refs therein]. However, the direct comparison of COM formation efficiencies and composition ratios for conditions relevant to the CO-freeze-out stage still need to be achieved. Here we present new experimental results to comprehensively study COMs in both scenarios, with the aim to compare and to study the cumulative effect of H-atom bombardments and UV irradiation. Our work contains two sets of experiments. First, CO-CH3OH ice mixtures are codeposited with H-atoms at 14 K, resulting in the formation of methyl formate, glycolaldehyde and ethylene glycol, respectively (HC(O)OCH3), (HC(O)CH2OH) and (H2C(OH)CH2OH), under various H-atom flux and fluence conditions. Second, the vacuum UV-photon irradiation of identical ice mixtures is performed with and without H-atoms at 14K. The results in the figure show that the absolute abundance and composition ratio of COMs are H-atom flux and fluence dependent upon H-atom additions. The amount of COMs formed in the lower H-atom flux experiment is comparable to the results found in the separate photolysis experiment. In the combined co-deposition experiment with vacuum UV irradiation and H-atom additions, the absolute COM abundances drop by 50% of those found in the pure photolysis experiment, but the composition ratios remain the same.

!!The absolute abundance and composition ratios of COMs under various dense cloud conditions

References

[1] Öberg, K.I., Chemical Reviews(2016).

[2] Chuang, K.-J., Fedoseev, G., Ioppolo, S., van Dishoeck, E.F., Linnartz, H., MNRAS 455 (2016) 1702.

[3] Linnartz, H, Ioppolo, S, Fedoseev, S., Int. Rev. Phys. Chem 34 (2015) 205.

Summary

The aim of this laboratory work is to investigate the solid state formation of complex organic molecules (COMs) for different astrophysical conditions as typical for dense interstellar clouds. A direct comparison of COM formation efficiencies is made for energetic, i.e., upon VUV-irradiation and non-energetic processing, i.e., H-atom bombardment. Results are presented for experiments in which both triggers are applied simultaneously. This does not change the overall characteristics, but reduces the efficiency with roughly 50% compared to a pure UV irradiation experiment.