PHOTODESORPTION OF ICES. II. H2O AND D2O

By K. Oberg, H. Linnartz, R. Visser, E. V. van Dishoeck. 2008

224 citations recorded by the source corpus.

Gaseous H2O has been detected in several cold astrophysical environments, where the observed abundances cannot be explained by thermal desorption of H2O ice or by H2O gas-phase formation. These observations hence suggest an efficient nonthermal ice desorption mechanism. Here, we present experimentally determined UV photodesorption yields of H2O and D2O ices and deduce their photodesorption mechanism. The ice photodesorption is studied under ultrahigh vacuum conditions and at astrochemically relevant temperatures (18–100 K) using a hydrogen discharge lamp (7–10.5 eV), which simulates the interstellar UV field. The ice desorption during irradiation is monitored using reflection absorption infrared spectroscopy of the ice and simultaneous mass spectrometry of the desorbed species. The photodesorption yield per incident photon, Ypd(T, x), is identical for H2O and D2O and its dependence on ice thickness and temperature is described empirically by Ypd(T, x) = Ypd(T, x > 8)(1 − e−x/l(T)), where x is the ice thickness in monolayers (MLs) and l(T) is a temperature-dependent ice diffusion parameter that varies between ∼1.3 ML at 30 K and 3.0 ML at 100 K. For thick ices, the yield is linearly dependent on temperature due to increased diffusion of ice species such that Ypd(T, x > 8) = 10−3(1.3 + 0.032 × T) UV photon−1, with a 60% uncertainty for the absolute yield. The increased diffusion also results in an increasing H2O:OH desorption product ratio with temperature from 0.7:1.0 at 20 K to 2.0:1.2 at 100 K. The yield does not depend on the substrate, the UV photon flux, or the UV fluence. The yield is also independent of the initial ice structure since UV photons efficiently amorphize H2O ice. The results are consistent with theoretical predictions of H2O photodesorption at low temperatures and partly in agreement with a previous experimental study. Applying the experimentally determined yield to a Herbig Ae/Be star+disk model provides an estimate of the amount of gas-phase H2O that may be observed by, e.g., Herschel in an example astrophysical environment. The model shows that UV photodesorption of ices increases the H2O content by orders of magnitude in the disk surface region compared to models where nonthermal desorption is ignored.

Subfields: Astronomy and Astrophysics, Atmospheric Science, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/653bb84653a583179eabcfc968d68f3eb29d87a6

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In Astronomy and Astrophysics

1. R. Garrod
2. P. Caselli
3. E. Herbst
4. E. Dishoeck
5. Cecilia Ceccarelli
6. V. Wakelam
7. E. Bergin
8. Simon Bruderer

In Atmospheric Science

1. R. Garrod
2. E. Herbst
3. P. Caselli
4. V. Wakelam
5. E. Dishoeck
6. A. Tielens
7. E. Bergin
8. Cecilia Ceccarelli

In Spectroscopy

1. R. Garrod
2. P. Caselli
3. Cecilia Ceccarelli
4. E. Herbst
5. V. Wakelam
6. E. Dishoeck
7. K. Oberg
8. E. Bergin

Citations in the corpus

• FIRST DETECTION OF WATER VAPOR IN A PRE-STELLAR CORE (2012, background): “When present, UV photons photodesorb water molecules and other species from the icy mantles (Öberg et al. 2009).”
• Formation rates of complex organics in UV irradiated CH3OH-rich ices I: Experiments (2009, background): “These yields agree with previous photodesorption studies of other molecules (Westley et al. 1995;Öberg et al. 2007, 2009a,b) and confirms the assumption in several observational and model papers that most ice molecules have similar photodesorption yields, around 10−3 per incident UV photon.”
• The UMIST database for astrochemistry 2012 (2012, methodology): “…(1963), Nahar & Pradhan (1997), Nejad & Millar (1988), Neufeld & Wolfire (2009), Nguyen et al. (2006), Novotný et al. (2010), Öberg et al. (2005), Öberg et al. (2009a), Öjekull et al. (2004), Osamura et al. (1999), Pequignot & Aldrovandi (1986), Petrie et al. (1991), Petrie et al. (2003),…”
• ON THE FORMATION OF CO2 AND OTHER INTERSTELLAR ICES (2011, background): “Öberg et al. (2009c) considered the segregation of ice mixtures in protostellar envelopes, caused by the preferentially higher binding of molecules to each other than to different species.”
• Gas and grain chemical composition in cold cores as predicted by the Nautilus three-phase model (2016, methodology): “…the last years, quantitative photo-desorption yields have been measured experimentally for pure ices sample using hydrogen discharge lamps: CO (Öberg et al. 2007; Muñoz Caro et al. 2010), H2O (Öberg et al. 2009c), N2, CO2 (Öberg et al. 2009a; Yuan & Yates 2013) and CH3OH (Öberg et al. 2009b).”
• Complex organic molecules in protoplanetary disks (2013, methodology): “This is supported by recent estimates of the grain-surface photodestruction of pure methanol ice which also show that photochemistry occurs deep within the bulk ice (Öberg et al. 2009c).”
• A THREE-PHASE CHEMICAL MODEL OF HOT CORES: THE FORMATION OF GLYCINE (2013, methodology): “Other authors have assumed a swapping mechanism tobe active in laboratory ice experiments (Öberg et al. 2009; Fayolle et al. 2011), which is also implicitly assumed in this model.”
• Water in Star-forming Regions with the Herschel Space Observatory (WISH). I. Overview of Key Program and First Results (2010, unknown): “The efficiency of the latter process has been quantified by laboratory experiments to be ∼10 3 per incident UV photon (Westley et al. 1995; Öberg et al. 2009).”
• Grain Surface Models and Data for Astrochemistry (2017, methodology): “Within this framework, it is also possible to include branching ratios of the expected products, for those species for which photodissociation is the primary process (e.g., H2O and CO2).”
• Reaction Networks for Interstellar Chemical Modelling: Improvements and Challenges (2010, background): “As photodesorption proceeds, photodissociation also occurs and new spectral features appear that correspond with reaction products (Öberg et al. 2009a).”
• The warm gas atmosphere of the HD 100546 disk seen by Herschel - Evidence of a gas-rich, carbon-poor atmosphere? (2012, methodology): “However, some “mild” photochemistry could lead to more complex, less volatile organics (Öberg et al. 2009a).”
• The molecular composition of the planet-forming regions of protoplanetary disks across the luminosity regime (2015, methodology): “We use experimentally determined photodesorption yields where available (see, e.g., Öberg et al. 2009a,b,c).”
• Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores (2009, methodology): “Higher temperatures allow the radicals to diffuse rapidly enough to form large molecules such as methyl formate and dimethyl ether (Garrod et al. 2008; Öberg et al. 2009d).”
• The Molecular Universe (2011, background): “Recent laboratory studies have focused on putting these ideas on a firm footing identifying the chemical routes involved (Öberg et al. 2009; Hudson et al. 2008).”
• Our astrochemical heritage (2012, methodology): “…are experimentally found to promote the formation of organic species more complex than CH3OH (Gerakines et al. 1996; Bennett and Kaiser 2007; Öberg et al. 2009a, 2010b) up to amino acids (e.g., Bernstein et al. 2002; Muñoz Caro et al. 2002, 2004) and allow solid species to return into the…”