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 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 des for atomic oxygen from dust grain analogs. The values of E des are 1660±60 K and 1850±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 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 far-ultraviolet 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 .

A SEARCH FOR O2 IN CO-DEPLETED MOLECULAR CLOUD CORES WITH HERSCHEL

“…This is almost twice the value previously considered in chemical models (800 K, Tielens & Allamandola 1987) and was in fact suggested by Hollenbach et al (2009). Model calculations confirm that, in the case of PDRs, higher O atom sticking efficiencies lead to a significant reduction in the gas phase O 2 abundance (He et al 2015). Nevertheless, the low O 2 abundances inferred in CO depletion cores can only be explained by these models if the gas-phase atomic oxygen abundance is significantly lower than 10 −5 .…”

Grain Surface Models and Data for Astrochemistry

“…Binding energies for radicals can only be obtained in an indirect manner, usually involving the simulation of experimental data, and an exploration of the possible parameter space, using stochastic chemical models. However, there are recent experimental results reporting the experimental determination of the binding energy of atomic oxygen on a range of surfaces (Dulieu et al 2013;He et al 2014He et al , 2015, showing that for some species, direct measurements are possible.…”

How chemistry influences cloud structure, star formation, and the IMF

“…, b Collings et al (2003), c Cazaux & Tielens (2004), d ; O 2 (ice) = 1000 K, O 3 (ice) = 1800 K, OH(ice) = 3500 K, e ; H(bare) = 550 K, f Fraser et al (2001), g Garrod & Pauly (2011); H(bare) = 450 K, CO(bare) = 1150 K, h Noble et al (2012a), i Noble et al (2012b), j Dulieu et al (2013); O 3 (bare) = 2100 K, k Karssemeijer et al (2014); Karssemeijer & Cuppen (2014); CO(ice) = 1000-1600 K, l associated with H 2 CO binding energies, m Garrod & Herbst (2006), n Al-Halabi & van Dishoeck (2007), o He et al (2015); O(bare,ice) = (1850 K,1660 K), p Minissale (2014); H 2 CO(bare) = 3400 K, q Karssemeijer, de Wijs & Cuppen (2014); CO 2 (ice) = 2670 K, r Sandford & Allamandola (1990); CO 2 (bare,ice) = (2690 K,2860 K), s Pirronello et al (1997).…”