Desorption Kinetics and Binding Energies of Small Hydrocarbons



Small hydrocarbons are an important organic reservoir in protostellar and protoplanetary environments. Constraints on desorption temperatures and binding energies of such hydrocarbons are needed for accurate predictions of where these molecules exist in the ice vs. gas-phase during the different stages of star and planet formation.

Through a series of temperature programmed desorption (TPD) experiments, we constrain the binding energies of 2 and 3-carbon hydrocarbons (C2H2 - acetylene, C2H4 - ethylene, C2H6 - ethane, C3H4 - propyne, C3H6 - propene, and C3H8 - propane) to 2200-4200 K in the case of pure amorphous ices, to 2400-4400 K on compact amorphous H2O, and to 2800-4700 K on porous amorphous H2O. The 3-carbon hydrocarbon binding energies are always larger than the 2-carbon hydrocarbon binding energies. Within the 2- and 3-carbon hydrocarbon families, the alkynes (i.e., least-saturated) hydrocarbons exhibit the largest binding energies, while the alkane and alkene binding energies are comparable. Binding energies are ∼5-20% higher on water ice substrates compared to pure ices, which is a small increase compared to what has been measured for other volatile molecules such as CO and N2. Thus in the case of hydrocarbons, H2O has a less pronounced effect on sublimation front locations (i.e., snowlines) in protoplanetary disks.

Aida Behmard, Edith C. Fayolle, Dawn M. Graninger, Jennifer B. Bergner, Rafael Martín-Doménech, Pavlo Maksyutenko, Mahesh Rajappan, Karin I. Öberg
(Submitted on 22 Mar 2019)

Comments: 12 pages, 15 figures. Accepted for publication in The Astrophysical Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1903.09720 [astro-ph.EP] (or arXiv:1903.09720v1 [astro-ph.EP] for this version)
Submission history
From: Aida Behmard
[v1] Fri, 22 Mar 2019 22:09:25 UTC (566 KB)
Astrobiology, Astrochemistry

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