J/ApJ/743/131 Infrared absorbance of water H_2_O/H_2_O_2_ ice (Smith+, 2011) ================================================================================ On the formation of interstellar water ice: constraints from a search for hydrogen peroxide ice in molecular clouds. Smith R.G., Charnley S.B., Pendleton Y.J., Wright C.M., Maldoni M.M., Robinson G. =2011ApJ...743..131S ================================================================================ ADC_Keywords: Abundances ; Interstellar medium ; Spectra, infrared Keywords: infrared: ISM - ISM: abundances - ISM: clouds - ISM: lines and bands - ISM: molecules - molecular processes Abstract: Recent surface chemistry experiments have shown that the hydrogenation of molecular oxygen on interstellar dust grains is a plausible formation mechanism, via hydrogen peroxide (H_2_O_2_), for the production of water (H_2_O) ice mantles in the dense interstellar medium. Theoretical chemistry models also predict the formation of a significant abundance of H_2_O_2_ ice in grain mantles by this route. At their upper limits, the predicted and experimental abundances are sufficiently high that H_2_O_2_ should be detectable in molecular cloud ice spectra. To investigate this further, laboratory spectra have been obtained for H_2_O_2_/H_2_O ice films between 2.5 and 200um, from 10 to 180K, containing 3%, 30%, and 97% H_2_O_2_ ice. Integrated absorbances for all the absorption features in low-temperature H_2_O_2_ ice have been derived from these spectra. For identifying H_2_O_2_ ice, the key results are the presence of unique features near 3.5, 7.0, and 11.3um. Comparing the laboratory spectra with the spectra of a group of 24 protostars and field stars, all of which have strong H_2_O ice absorption bands, no absorption features are found that can definitely be identified with H_2_O_2_ ice. In the absence of definite H_2_O_2_ features, the H_2_O_2_ abundance is constrained by its possible contribution to the weak absorption feature near 3.47um found on the long-wavelength wing of the 3um H_2_O ice band. This gives an average upper limit for H_2_O_2_, as a percentage of H_2_O, of 9%+/-4%. This is a strong constraint on parameters for surface chemistry experiments and dense cloud chemistry models. Description: The laboratory setup used to make the infrared spectral measurements of thin H_2_O_2_/H_2_O ice films has already been described by Smith et al. (1994MNRAS.271..481S) and Maldoni et al. (1998MNRAS.298..251M). File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table2.dat 120 1849 Absorbance data for the 3% near-infrared 2.5-25um (4000-400cm^-1^) H_2_O_2_/H_2_O ice films table3.dat 131 1849 Absorbance data for the 30% near-infrared 2.5-25um (4000-400cm^-1^) H_2_O_2_/H_2_O ice films table4.dat 153 1849 Absorbance data for the 97% near-infrared 2.5-25um (4000-400cm^-1^) H_2_O_2_/H_2_O ice films table5.dat 120 235 Absorbance data for the 3% far-infrared 20-200um (500-50cm^-1^) H_2_O_2_/H_2_O ice films table6.dat 120 235 Absorbance data for the 30% far-infrared 20-200um (500-50cm^-1^) H_2_O_2_/H_2_O ice films table7.dat 142 235 Absorbance data for the 97% far-infrared 20-200um (500-50cm^-1^) H_2_O_2_/H_2_O ice films table10.dat 46 24 Upper limits to the H_2_O_2_ ice abundances, relative to H_2_O ice, for a selection of protostars and field stars based on the optical depth at 3.50um within the 3.47um feature -------------------------------------------------------------------------------- See also: J/ApJ/701/1347 : Optical constants of H_2_O-ice (Mastrapa+, 2009) J/ApJ/678/985 : c2d Spitzer survey of interstellar ices. I. (Boogert+, 2008) J/ApJ/684/1240 : Prestellar cores in Perseus, Serpens & Oph (Enoch+, 2008) J/ApJS/86/713 : IR spectroscopy of ices (Hudgins+, 1993) Byte-by-byte Description of file: table[234567].dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 10 E10.5 cm-1 sigma Wavenumber 12- 21 E10.3 --- Abs10 Absorbance at 10K 23- 32 E10.3 --- Abs30 Absorbance at 30K 34- 43 E10.3 --- Abs50 Absorbance at 50K 45- 54 E10.3 --- Abs70 Absorbance at 70K 56- 65 E10.3 --- Abs90 Absorbance at 90K 67- 76 E10.3 --- Abs110 Absorbance at 110K 78- 87 E10.3 --- Abs120 Absorbance at 120K 89- 98 E10.3 --- Abs130 Absorbance at 130K 100-109 E10.3 --- Abs140 Absorbance at 140K 111-120 E10.3 --- Abs150 Absorbance at 150K 122-131 E10.3 --- Abs160 ? Absorbance at 160K (for tables 3, 4 and 7) 133-142 E10.3 --- Abs170 ? Absorbance at 170 K (only for tables 4 and 7) 144-153 E10.3 --- Abs180 ? Absorbance at 180 K (only for table 4) -------------------------------------------------------------------------------- Byte-by-byte Description of file: table10.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 11 A11 --- Group "Processed" or "Unprocessed" (1) 13- 28 A16 --- Name Source name 30- 33 F4.2 --- tau3.08 Optical depth {tau} at 3.08um 35- 38 F4.2 --- tau3.50 Optical depth {tau} at 3.50um 40- 41 I2 % H2O2 H_2_O_2_ ice abundance relative to H_2_O ice 43- 46 A4 --- Ref Reference(s) (2) -------------------------------------------------------------------------------- Note (1): Sources have been divided into two groups on the basis of the appearance of the 3.08um H_2_O ice absorption feature in their spectra which reflects different levels of thermal processing of the ice (see, e.g., the discussion in Smith et al. 1989ApJ...344..413S). For protostars in the "unprocessed" group their 3.08um H_2_O ice absorption features are typical of 10-20K amorphous ice while the "processed" group have 3.08um features which are narrower and contain substructure consistent with exposure to higher temperatures. See section 2.4 for further explanations. Note (2): Reference as follows: 1 = Willner et al. 1982ApJ...253..174W 2 = Smith et al. 1989ApJ...344..413S 3 = Brooke et al. 1996ApJ...459..209B 4 = Whittet et al. 1988MNRAS.233..321W 5 = Sato et al. 1990ApJ...359..192S 6 = Chiar et al. 1996ApJ...472..665C 7 = Brooke et al. 1999ApJ...517..883B 8 = Sellgren et al. 1994ApJ...433..179S 9 = Boogert et al. 2000A&A...360..683B 10 = Tanaka et al. 1990ApJ...352..724T 11 = Kastner & Weintraub 1996ApJ...466L.103K 12 = Eiroa & Hodapp 1989A&A...210..345E 13 = Whittet et al. 1996ApJ...458..363W 14 = Tanaka et al. 1994ApJ...430..779T -------------------------------------------------------------------------------- History: From electronic version of the journal ================================================================================ (End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 07-May-2013