An experimental study of the binding energy of NH3 on different types of ice and its effect on the ice line of NH3 and H2O

NH3-H2O co-deposition experiments. All experiments were performed on gold substrate. TPDs have a slope of 0.2K/s. Solid lines represent decomposition of NH3, dash-dotted lines represent decomposition of water. Lines of the same color belong to the same set of experiments. Input: TPD of NH3 from the gold surface is used to calibrate all resulting experiments. – astro-ph.SR

N-bearing molecules (such as N2H+ or NH3) are excellent tracers of high-density, low-temperature regions such as dense cloud nuclei and can shed light on the chemical evolution of ices and comets in protoplanetary disks.

However, there are uncertainties about the grain surface chemistry of these molecules—which may play an important role in their formation and evolution. This study experimentally investigates the behavior of NH3 along with other major interstellar ice constituents (ie. H2O, CO, CO2) on surfaces reflecting grains under interstellar conditions.

of NH3 using the Ultra High Vacuum (UHV) system VENUS (VErs des Nouvelles Syntheses) for other adsorbates (here, H2O, 13CO and CO2) and temperature programmed desorption (TPD) and temperature time programmed exposure desorption (TP-DED) experiments. We obtained the binding energy (BE) distribution of NH3 in crystalline ice(CI) and compact-amorphous solid water (c-ASW) by analysis of TPD profiles of NH3 in substrates.

When H2O is introduced into the co-deposited mixture of NH3-13Co or NH3-CO2 without H2O, we observe a significant delay in corrosion and a decrease in the decomposition rate of NH3. Second, H2O traps nearly 5–9 percent of the co-deposited NH3 released during the amorphous-crystalline phase transition of water.

Third, for CI, we obtained the BE distribution between 3780K-4080K and c-ASW between 3780K-5280K – using the pre-exponential factor A = 1.94e15 s-1. Consistent with quantum calculations, we conclude that NH3 behavior is significantly affected by the presence of H2O due to the formation of hydrogen bonds. This interaction preserves NH3 on grain surfaces at high temperatures, making it available to central protostars in protoplanetary disks. This also explains why freezing of NH3 is efficient in pre-stellar cores.

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S. Gakkanpara Suresh, F. Tullew, J. Vitorino, B. Caselli

Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)
Citation: arXiv:2311.18619 [astro-ph.GA] (or arXiv:2311.18619v1 [astro-ph.GA] for this version)
Submission history
Posted by: Shreya Kakanpara Suresh
[v1] Thu, 30 Nov 2023 15:20:39 UTC (3,856 KB)

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