WATER, O2, AND ICE IN MOLECULAR CLOUDS

By D. Hollenbach, M. Kaufman, E. Bergin, G. Melnick. 2008

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We model the temperature and chemical structure of molecular clouds as a function of depth into the cloud, assuming a cloud of constant density n illuminated by an external far-ultraviolet (FUV; 6 eV <hν < 13.6 eV) flux G0 (scaling factor in multiples of the local interstellar field). Extending previous photodissociation region (PDR) models, we include the freezing of species, simple grain surface chemistry, and desorption (including FUV photodesorption) of ices. We also treat the opaque cloud interior with time-dependent chemistry. Here, under certain conditions, gas-phase elemental oxygen freezes out as water ice and the elemental C/O abundance ratio can exceed unity, leading to complex carbon chemistry. Gas-phase H2O and O2 peak in abundance at intermediate depth into the cloud, roughly AV∼ 3–8 from the surface, the depth proportional to ln(G0/n). Closer to the surface, molecules are photodissociated. Deeper into the cloud, molecules freeze to grain surfaces. At intermediate depths, PDRs are attenuated by dust extinction, but photodesorption prevents total freeze-out. For G0 < 500, abundances of H2O and O2 peak at values ∼10−7, producing columns ∼1015 cm−2, independent of G0 and n. The peak abundances depend primarily on the product of the photodesorption yield of water ice and the grain surface area per H nucleus. At higher values of G0, thermal desorption of O atoms from grains slightly enhances the gas-phase H2O peak abundance and column, whereas the gas-phase O2 peak abundance rises to ∼10−5 and the column to ∼2 × 1016 cm−2. We present simple analytical equations for the abundances as a function of depth, which clarify the dependence on parameters. The models are applied to observations of H2O, O2, and water ice in a number of sources, including B68, NGC 2024, and ρ Oph.

Subfields: Astronomy and Astrophysics, Atmospheric Science, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/48571d0a400f5e1485d2992c856a29a095bc323c

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Citations in the corpus

The warm gas atmosphere of the HD 100546 disk seen by Herschel - Evidence of a gas-rich, carbon-poor atmosphere? (2012, methodology): “We follow the approach of Visser et al. (2011) and Hollenbach et al. (2009).”
Observations of the Icy Universe (2015, background): “Mantle growth is expected to proceed at a rate independent of the grain size, although the smallest grains ( 0.002 µm) will be ice-less as they become too hot upon stochastic photon heating (Hollenbach et al., 2009).”
THE ORIGIN OF COMPLEX ORGANIC MOLECULES IN PRESTELLAR CORES (2014, background): “Farther away from the center (∼ 10 4 AU), where the density is low enough (≤ 105 cm−3) for the photodesorption rate not to be overcome by the freeze-out rate, the gaseous H2O abundance reaches ∼ 3× 10−7, as predicted by previous models (Dominik et al. 2005; Hollenbach et al. 2009).”
H$_2$ formation on interstellar dust grains: the viewpoints of theory, experiments, models and observations (2017, background): “Models indicate that the grains remain bare until a visual exinction of 3 ( Esplugues et al., 2016; Hollenbach et al., 2009 ), so hat bare surfaces are most relevant for understanding H 2 forma- m c 4 P H r l r r p b L a b t t l e l t t P e p i 2 F c g 4.2.3.”
THE DARK MOLECULAR GAS (2010, background): “However subsequent, work by Hollenbach et al. (2009) showed that OH is formed at greater abundances than previously thought when grain surface chemistry is included.”
Water in Star-forming Regions with the Herschel Space Observatory (WISH). I. Overview of Key Program and First Results (2010, background): “Pre-Herschel model predictions ranged from peak antenna temperatures of below 10 mK (Hollenbach et al. 2009) to several hundred degrees milliKelvin.”
Our astrochemical heritage (2012, background): “Thirdly, they catalyze the formation of important species, in particular H2O, with such high efficiency that more than 30 % of oxygen atoms are locked into water ice as soon as the visual extinction reaches values ≥3 mag (e.g., Murakawa et al. 2000; Hollenbach et al. 2009; Whittet 2010; Chiar et al.”
Physical Processes in the Interstellar Medium (2014, background): “This is, because most of the water molecules that form rapidly freeze out onto the surface of dust grains, forming a significant part of the ice mantles that surround these grains (Bergin et al., 2000; Hollenbach et al., 2009).”
FIRST DETECTION OF WATER VAPOR IN A PRE-STELLAR CORE (2012, methodology): “Current state-of-the-art chemical models (Hollenbach et al. 2009) posit that small amounts of water vapor exist (containing less than a percent of the available cosmic oxygen) in dense regions that are well below the evaporation temperature, when exposed to the FUV starlight pervading interstellar…”
Chemical enrichment of giant planets and discs due to pebble drift (2017, background): “Following Hollenbach et al. (2009) and Visser (2009) the thermal desorption rate per grain is given by 4πs20Ns,iRd,ifs,i, where Ns,i = 1.5×1015 cm−2 is the number of binding sites per unit area on the grain, where we have assumed spherical grains and s0 is their mean size by area. fs,i is the…”
Astrochemistry and compositions of planetary systems (2020, background): “Thermal desorption tends to dominate the grain-gas balance in protostellar envelopes and other heated regions (Léger et al. 1985), while photodesorption regulates the gas-grain balance in UV exposed regions (Hollenbach et al. 2009).”
THE EFFECTS OF SNOWLINES ON C/O IN PLANETARY ATMOSPHERES (2011, background): “For all other molecules, we calculate the density-dependent sublimation temperatures following the prescription of Hollenbach et al. (2009) using binding energies of H2O, CO2 and CO of 5800 K, 2000 K and 850 K (Collings et al. 2004; Fraser et al. 2001; Aikawa et al. 1996).”
The Disk Substructures at High Angular Resolution Project (DSHARP). II. Characteristics of Annular Substructures (2018, methodology): “To examine whether the DSHARP substructures are connected to snowlines, we shade in the freezeout temperature ranges for CO2, N2, and CO, following the approach in Hollenbach et al. (2009) and using the binding energies and abundances listed in Table 3.”