Synthesis of CO and CO2 Molecules by UV Irradiation of Water Ice-covered Hydrogenated Carbon Grains

We present the results of UV irradiation with Lyα photons of carbon grains with a water ice cap at 11 K. Formation of CO and CO2 molecules takes place during irradiation. An estimation of the formation cross section of these molecules by Lyα photons has been obtained from the intensity increase of their infrared stretching bands as a function of the photon fluence. The fraction of carbon in the grains converted to CO and CO2 by UV photons is 0.06 and 0.05, respectively. The spectral profile of the CO stretching feature and that of the CO2 bending mode indicate a polar environment for these molecules. On the basis of the present laboratory results and those obtained in previous work on ion irradiation of similar samples, it has been possible to estimate the contribution of polar CO and CO2 produced on carbon grains by energetic processing to the observed column densities of these molecules for dense clouds whose visual extinction is known. A significant amount of polar CO and CO2 is produced through the mechanism we have studied. Furthermore, we have found that the laboratory profile of the bending band of CO2 produced on carbon grains is compatible with that observed toward the field star Elias 16.

[1]  G. Strazzulla,et al.  CO2 production by ion irradiation of H2O ice on top of carbonaceous materials and its relevance to the Galilean satellites , 2005 .

[2]  E. Bergin,et al.  Spitzer Observations of CO2 Ice toward Field Stars in the Taurus Molecular Cloud , 2005, astro-ph/0505345.

[3]  V. Mennella,et al.  Formation of CO and CO2 Molecules by Ion Irradiation of Water Ice-covered Hydrogenated Carbon Grains , 2004 .

[4]  D. Padgett,et al.  Spitzer Space Telescope Spectroscopy of Ices toward Low-Mass Embedded Protostars , 2004, astro-ph/0406298.

[5]  A. Tielens,et al.  A 3-5 mu m VLT spectroscopic survey of embedded young low mass stars I - Structure of the CO ice , 2003, astro-ph/0307097.

[6]  M. Moore,et al.  Photodestruction of Relevant Interstellar Molecules in Ice Mixtures , 2003 .

[7]  Y. Pendleton,et al.  The Effects of Ion Irradiation on the Evolution of the Carrier of the 3.4 Micron Interstellar Absorption Band , 2003 .

[8]  J. Brucato,et al.  C-H Bond Formation in Carbon Grains by Exposure to Atomic Hydrogen: The Evolution of the Carrier of the Interstellar 3.4 Micron Band , 2002 .

[9]  G. Leto,et al.  A comparison of ion irradiation and UV photolysis of CH4 and CH3OH , 2002 .

[10]  A. Kouchi,et al.  Measurements of Conversion Rates of CO to CO2 in Ultraviolet-induced Reaction of D2O(H2O)/CO Amorphous Ice , 2002 .

[11]  F. Motte,et al.  The Environment and Nature of the Class I Protostar Elias 29: Molecular Gas Observations and the Location of Ices , 2002, astro-ph/0201317.

[12]  Y. Pendleton,et al.  The Organic Refractory Material in the Diffuse Interstellar Medium: Mid-Infrared Spectroscopic Constraints , 2002 .

[13]  B. Kasemo,et al.  Photoreactions of water and carbon at 90 K , 2001 .

[14]  J. Chiar,et al.  Solid Carbon Dioxide in Regions of Low-Mass Star Formation , 2001 .

[15]  V. Pirronello,et al.  Formation of Carbon Dioxide by Surface Reactions on Ices in the Interstellar Medium , 2001 .

[16]  J. Greenberg,et al.  UV photodestruction of CH bonds and the evolution of the 3.4 mu m feature carrier. I. The case of aliphatic and aromatic molecular species , 2001 .

[17]  T. Kerr,et al.  Solid-State CO toward ρ Ophiuchi Elias 25: Ice Thresholds Revisited , 2000 .

[18]  J. Brucato,et al.  Activation of the 3.4 Micron Band in Carbon Grains by Exposure to Atomic Hydrogen , 1999 .

[19]  T. Prusti,et al.  Erratum: "Observations of Solid Carbon Dioxide in Molecular Clouds with the Infrared Space Observatory" (ApJ, 522, 357 [1999]) , 1999 .

[20]  T. Prusti,et al.  Detection of Abundant CO2 Ice in the Quiescent Dark Cloud Medium toward Elias 16 , 1998 .

[21]  P. Roche,et al.  Interstellar Dust Absorption Features in the Infrared Spectrum of HH 100-IR: Searching for the Nitrogen Component of the Ices , 1996 .

[22]  T. Kerr,et al.  High-Resolution Studies of Solid CO in the Taurus Dark Cloud: Characterizing the Ices in Quiescent Clouds , 1995 .

[23]  Robert E. Johnson,et al.  Ultraviolet photodesorption from water ice , 1995 .

[24]  T. Kerr,et al.  Solid Carbon Monoxide in the Serpens Dark Cloud (Grad. Stud. Prize) 30 Min , 1993 .

[25]  T. Kerr,et al.  Infrared spectroscopy of solid CO: the ρ Ophiuchi molecular cloud , 1993 .

[26]  D. Hall.  Energetic Charged-Particle Interactions with Atmospheres and Surfaces. Vol. 19—Physics and Chemistry in Space R. E. Johnson, eds L. J. Lanzerotti and D. Stöffler, Springer Verlag, Berlin, 1990, 230 pp, hard cover ISBN 3 540 51908 4, DM 178 , 1991 .

[27]  S. Sandford,et al.  The volume- and surface-binding energies of ice systems containing CO, CO2, and H2O. , 1990, Icarus.

[28]  Robert E. Johnson.  Energetic Charged-Particle Interactions with Atmospheres and Surfaces , 1990 .

[29]  M. Moore,et al.  Studies of proton irradiated H2O + CO2 and H2O + CO ices and analysis of synthesized molecules , 1990 .

[30]  Scott A. Sandford,et al.  Photochemical and thermal evolution of interstellar/precometary ice analogs , 1988 .

[31]  S. Sandford,et al.  Laboratory studies of the infrared spectral properties of CO in astrophysical ices. , 1988, The Astrophysical journal.

[32]  J. Fournier,et al.  Fluorescence and thermoluminescence of N2O, CO, and CO2 in an argon matrix at low temperature , 1979 .

[33]  K. G. Brown,et al.  Absolute infrared intensities and band shapes in pure solid CO and CO in some solid matrices , 1975 .

[34]  R. Cairns,et al.  Metal Photocathodes as Secondary Standards for Absolute Intensity Measurements in the Vacuum Ultraviolet , 1966 .