On Water Formation in the Interstellar Medium: Laboratory Study of the O + D Reaction on Surfaces

The Astrophysical Journal Letters, 741:L9 (5pp), 2011 November 1 doi:10.1088/2041-8205/741/1/L9 C 2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A. ON WATER FORMATION IN THE INTERSTELLAR MEDIUM: LABORATORY STUDY OF THE O + D REACTION ON SURFACES Dapeng Jing1,JiaoHe1, John Brucato2, Antonio De Sio3, Lorenzo Tozzetti2, and Gianfranco Vidali1 1 Physics Department, Syracuse University, Syracuse, NY 13244, USA 2 INAF, Astrophysical Observatory of Arcetri, Florence, Italy 3 Department of Astronomy & Space Science, University of Florence, Florence, Italy Received 2011 July 18; accepted 2011 September 12; published 2011 October 10 ABSTRACT In the interstellar medium (ISM), an important channel of water formation is the reaction of atoms on the surface of dust grains. Here, we report on a laboratory study of the formation of water via the O+D reaction network. While prior studies were done on ices, as appropriate to the formation of water in dense clouds, we explored how water formation occurs on bare surfaces, i.e., in conditions mimicking the transition from diffuse to dense clouds (Av ∼ 1–5). Reaction products were detected during deposition and afterward when the sample is brought to a high temperature. We quantified the formation of water and intermediary products, such as D2O2, over a range of surface temperatures (15–25 K). The detection of OD on the surface signals the importance of this reactant in the overall scheme of water formation in the ISM. Key words: ISM: abundances – ISM: atoms – ISM: molecules Online-only material: color figures 1. INTRODUCTION (2009) calculated that if H chemisorbs on the crystalline surface of forsterite, then an oxygen atom impinging nearby would Interstellar water, first detected in the microwave spectral form OH. Another H atom could react with OH and form water region by Cheung et al. (1969), is found in diffuse clouds and or become chemisorbed next to OH forming a dissociated wa- is the main component of ices that coat dust grains in denser ter complex. The exothermicity of the reaction can lead to the regions (Av > 5) of the interstellar medium (ISM). Besides its formation of H2O. obvious role in biology, water has other important functions. In Following Tielens & Hagen (1982), water-forming reactions the gas phase, its abundance relative to hydrogen varies widely on surfaces can be grouped in three main branches: reactions of depending on the physical conditions of the cloud, from 10−8 H with molecular oxygen, ozone, and atomic oxygen. Miyauchi in cold dense regions, where most is accreted on grains, to et al. (2008), Ioppolo et al. (2008), Matar et al. (2008), Oba et al. 10−4 in warm gas and shocked regions, where it evaporates (2009), and Ioppolo et al. (2010) did experiments to study the or is sputtered from grains (Van Dishoeck & Helmich 1996; formation of water on surfaces via the following reactions. Melnick & Bergin 2005; Bjerkeli et al. 2009). When accreted → on grains, it provides a medium for a rich chemistry that leads to H+O2 HO2 the formation of molecules of biogenic relevance (Watanabe & H+HO → H O Kouchi 2008; Herbst & van Dishoeck 2009); where abundant in 2 2 2 the gas phase, it is a powerful coolant (Nisini 2000). Except for H+H O → H O+OH maser emissions, observations from the ground are difficult due 2 2 2 to atmospheric water vapor. The launch of the Herschel Space H+OH→ H2O. Observatory permits to view how ubiquitous water vapor is. For example, water was detected in several envelopes of carbon In these experiments, the surface is an oxygen ice laid on a stars in the asymptotic giant branch (Neufeld et al. 2011) where metal substrate or amorphous water ice. Hydrogen atoms are little was expected. Closer to home, there is much interest in made to impinge on the ice. Water as well as other products, understanding the source of water on planets and Kuiper Belt such as H2O2, are detected by reflection absorption infrared objects (Genda & Ikoma 2008; Encrenaz 2008). spectroscopy (RAIRS; Cuppen et al. 2010). Another channel It is now recognized that gas-phase chemistry cannot gen- first proposed by Tielens & Hagen (1982) involves ozone: erate the amount of water that has been detected in space (d’Hendecourt et al. 1985; Hasegawa et al. 1992). It was sug- O+O2 → O3 gested that dust grains play a role in water formation (Tielens & Hagen 1982). Although we are interested here in studying H+O3 → O2 +OH reactions involving neutral species, there are experiments that studied the formation of water by the impact of UV or cosmic H2 +OH→ H2O+H. ray analogs onto model ices (Ennis et al. 2011). Spurred by renewed interest in surface catalyzed reactions, there has been a Mokrane et al. (2009) and Romanzin et al. (2011) looked at the flurry of theoretical and experimental works. Cuppen & Herbst formation of H2O via this channel. Ozone was first deposited on (2007) and Cazaux et al. (2010) theoretically studied reaction a water ice surface, then H or D atoms were sent on this surface networks leading to the formation of water. These papers also and the products were measured either by thermal programmed contain activation energies for key reactions. Goumans et al. desorption (TPD; Mokrane et al. 2009) or RAIRS (Romanzin 1 The Astrophysical Journal Letters, 741:L9 (5pp), 2011 November 1 Jing et al. et al. 2011). A third channel, the formation of water through Table 1 successive hydrogenation of atomic oxygen, Formation Slopes and Efficiencies of Various Products Slope and Efficiency ε HDO D ODO O H+O→ OH 2 2 2 3 15 K slope (cm−2 minute−1)1.5× 1013 4.2 × 1013 7.6 × 1013 2.6 × 1012 −2 −1 × 13 × 13 × 13 × 12 OH + H → H O, 25 K slope (cm minute )1.2 10 3.0 10 8.9 10 4.9 10 2 ε at 15 K 0.043 0.12 0.22 0.007 ε at 25 K 0.034 0.087 0.26 0.014 was explored by Dulieu et al. (2010). They sent H and O beams on the surface of an amorphous water ice and detected the 12 −2 −1 12 −2 −1 formation of water using TPD. 3.7 × 10 cm s , O flux is 2.8 × 10 cm s , and O2 In prior experiments, water formation was studied either on flux is 1.5 × 1012 cm−2 s−1. We also measure traces of various model ices (such as O2 ice) or on water ice, i.e., in simulated masses to determine the composition of the beams. Except for 16 ISM conditions where ices are already present (Av > 4–5) and H2 O from the background of the beamline chambers, no other 16 most hydrogen is in molecular form, while oxygen is poorly contamination species (N2, O2, NO, and CO2) are found. constrained (Caselli et al. 2010). In this study, we look at the initial stages of formation of water on grains, i.e., when atomic 3. RESULTS AND DISCUSSION hydrogen and oxygen are still abundant in the gas phase, i.e., in not-so-dense regions of the ISM (Av ∼ 1–3). In this section, we present our experimental results in detail. First, we expose the sample to D and O atoms simultaneously 2. EXPERIMENTAL at 15 K and 25 K. TPD results clearly show the formation of water (HDO and D2O) as well as D2O2 and O3. The presence of The experiments are performed in an ultra high vacuum HDO is not surprising although only D is used. It is known that (UHV) setup which consists of a main chamber and two during heating, an isotope exchange between water molecules molecular/atomic beamlines. Details of the setup can be found will occur (Smith et al. 1997). This is also observed by Dulieu elsewhere (He et al. 2011). The sample used in the experiments et al. (2010). is a 1 μm thick amorphous thin film of silicate deposited on As shown in Figure 1, mass spectra of HDO show a typical a 0.5 inch diameter gold coated copper disk. The sample was zeroth-order desorption kinetics with a shared leading edge and prepared at the University of Florence by using a 9 kV electron highly asymmetric peaks leaning to the right. This corresponds beam impinging on a target consisting of MgO, FeO, and SiO4 to the sublimation of physisorbed HDO molecules weakly mixed to olivine (MgFe)2SiO4 stoichiometric ratio. The vapors bounded to the substrate. D O peaks also share the leading edge. − 2 from the target condensed on the disk at a rate of a few Å s 1. However, the peak shape is not as asymmetric as HDO peaks. Due to the fragility of the thin film, no ex situ cleaning of the This could arise from the lateral interaction through hydrogen sample is used. Instead, the sample is cleaned by repeatedly bonding between D2O molecules (Sanchez 1996). Intermediary heating to 380 K in vacuum during bakeout and prior to each products, such as D2O2 and O3, are also observed as indicated experiment. A triple-pass Hiden quadrupole mass spectrometer by the peaks of mass 40 and 54 at around 180 K and 60 K, (QMS) is mounted on a rotary platform and is used to quantify respectively. The TPD data are then applied to a simple rate the reactants entering the chamber and the products evolving equation model (Perets et al. 2007). The number of molecules from the sample surface. The two beamlines are equipped with of a certain species on the surface N can be expressed by the radio-frequency (RF) dissociation sources that can generate following formula: deuterium and oxygen atoms from their parent molecules.

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