A New Determination of the Binding Energy of Atomic Oxygen on Dust Grain Surfaces: Experimental Results and Simulations

Abstract

The energy to desorb atomic oxygen from an interstellar dust grain surface, $E_{\mathrm{des}}$, is an important controlling parameter in gas-grain models; its value impacts the temperature range over which oxygen resides on a dust grain. However, no prior measurement has been done of the desorption energy. We report the first direct measurement of $E_{\mathrm{des}}$ for atomic oxygen from dust grain analogs. The values of $E_{\mathrm{des}}$ are $1660\pm 60$~K and $1850\pm 90$~K for porous amorphous water ice and for a bare amorphous silicate film, respectively, or about twice the value previously adopted in simulations of the chemical evolution of a cloud. We use the new values to study oxygen chemistry as a function of depth in a molecular cloud. For $n=10^4$ cm${}^{-3}$ and $G_0$=10${}^2$ ($G_0$=1 is the average local interstellar radiation field), the main result of the adoption of the higher oxygen binding energy is that H${}_2$O can form on grains at lower visual extinction $A_{\mathrm{V}}$, closer to the cloud surface. A higher binding energy of O results in more formation of OH and H${}_2$O on grains, which are subsequently desorbed by FUV radiation, with consequences for gas-phase chemistry. For higher values of $n$ and $G_0$, the higher binding energy can lead to a large increase in the column of H${}_2$O but a decrease in the column of O${}_2$.

7 pages, 8 figures. Accepted by ApJ

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• Molecular oxygen in the rho Ophiuchi cloud
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