195 research outputs found
Formation of Hydrogen, Oxygen, and Hydrogen Peroxide in Electron Irradiated Crystalline Water Ice
Water ice is abundant both astrophysically, for example in molecular clouds,
and in planetary systems. The Kuiper belt objects, many satellites of the outer
solar system, the nuclei of comets and some planetary rings are all known to be
water-rich. Processing of water ice by energetic particles and ultraviolet
photons plays an important role in astrochemistry. To explore the detailed
nature of this processing, we have conducted a systematic laboratory study of
the irradiation of crystalline water ice in an ultrahigh vacuum setup by
energetic electrons holding a linear energy transfer of 4.3 +/- 0.1 keV mm-1.
The irradiated samples were monitored during the experiment both on line and in
situ via mass spectrometry (gas phase) and Fourier transform infrared
spectroscopy (solid state). We observed the production of hydrogen and oxygen,
both molecular and atomic, and of hydrogen peroxide. The likely reaction
mechanisms responsible for these species are discussed. Additional formation
routes were derived from the sublimation profiles of molecular hydrogen (90-140
K), molecular oxygen (147 -151 K) and hydrogen peroxide (170 K). We also
present evidence on the involvement of hydroxyl radicals and possibly oxygen
atoms as building blocks to yield hydrogen peroxide at low temperatures (12 K)
and via a diffusion-controlled mechanism in the warming up phase of the
irradiated sample.Comment: ApJ, March 2006, v639 issue, 43 pages, 7 figure
Spitzer Observations of CO2 Ice Towards Field Stars in the Taurus Molecular Cloud
We present the first Spitzer Infrared Spectrograph observations of the 15.2
micron bending mode of CO2 ice towards field stars behind a quiescent dark
cloud. CO2 ice is detected towards 2 field stars (Elias 16, Elias 3) and a
single protostar (HL Tau) with anabundance of ~15-20% relative to water ice.
CO2 ice is not detected towards the source with lowest extinction in our
sample, Tamura 17 (A_V = 3.9m). A comparison of the Elias 16 spectrum with
laboratory data demonstrates that the majority of CO2 ice is embedded in a
polar H2O-rich ice component, with ~15% of CO2 residing in an apolar H2O-poor
mantle. This is the first detection of apolar CO2 towards a field star. We find
that the CO2 extinction threshold is A_V = 4m +/- 1m, comparable to the
threshold for water ice, but significantly less than the threshold for CO ice,
the likely precursor of CO2. Our results confirm CO2 ice forms in tandem with
H2O ice along quiescent lines of sight. This argues for CO2 ice formation via a
mechanism similar to that responsible for H2O ice formation, viz. simple
catalytic reactions on grain surfaces.Comment: Accepted by Astrophysical Journal Letter
The Thermal Evolution of Ices in the Environments of Newly Formed Stars: The CO_2 Diagnostic
Archival data from the Infrared Spectrometer of the Spitzer Space Telescope are used to study the 15 ÎĽm absorption feature of solid CO_2 toward 28 young stellar objects (YSOs) of approximately solar mass. Fits to the absorption profile using laboratory spectra enable categorization according to the degree of thermal processing of the ice matrix that contains the CO_2. The majority of YSOs in our sample (20 out of 28) are found to be consistent with a combination of polar (H_2O-rich) and nonpolar (CO-rich) ices at low temperature; the remainder exhibit profile structure consistent with partial crystallization as the result of significant heating. Ice-phase column densities of CO_2 are determined and compared with those of other species. Lines of sight with crystallization signatures in their spectra are found to be systematically deficient in solid-phase CO, as expected if CO is being sublimated in regions where the ices are heated to crystallization temperatures. Significant variation is found in the CO2 abundance with respect to both H_2O (the dominant ice constituent) and total dust column (quantified by the extinction, AV ). YSOs in our sample display typically higher CO_2 concentrations (independent of evidence for thermal processing) in comparison to quiescent regions of the prototypical cold molecular cloud. This suggests that enhanced CO_2 production is driven by photochemical reactions in proximity to some YSOs, and that photoprocessing and thermal processing may occur independently
Fully Sampled Maps of Ices and Silicates in Front of Cepheus A East with Spitzer
We report the first fully sampled maps of the distribution of interstellar
CO2 ices, H2O ices and total hydrogen nuclei, as inferred from the 9.7 micron
silicate feature, toward the star-forming region Cepheus A East with the IRS
instrument onboard the Spitzer Space Telescope. We find that the column density
distributions for these solid state features all peak at, and are distributed
around, the location of HW2, the protostar believed to power one of the
outflows observed in this star-forming region. A correlation between the column
density distributions of CO2 and water ice with that of total hydrogen
indicates that the solid state features we mapped mostly arise from the same
molecular clumps along the probed sight lines. We therefore derive average CO2
ice and water ice abundances with respect to the total hydrogen column density
of X(CO2)_ice~1.9x10^-5 and X(H2O)_ice~7.5x10^-5. Within errors, the abundances
for both ices are relatively constant over the mapped region exhibiting both
ice absorptions. The fraction of CO2 ice with respect to H2O ice is also
relatively constant at a value of 22% over that mapped region. A clear
triple-peaked structure is seen in the CO2 ice profiles. Fits to those profiles
using current laboratory ice analogs suggest the presence of both a
low-temperature polar ice mixture and a high-temperature methanol-rich ice
mixture along the probed sightlines. Our results further indicate that thermal
processing of these ices occurred throughout the sampled region.Comment: 26 pages, 8 figures, accepted for publication in Ap
The Infrared Band Strengths of H2o, Co and Co2 in Laboratory Simulations of Astrophysical Ice Mixtures
Infrared spectroscopic observations toward objects obscured by dense cloud
material show that HO, CO and, likely, CO are important constituents of
interstellar ice mantles. In order to accurately calculate the column densities
of these molecules, it is important to have good measurements of their infrared
band strengths in astrophysical ice analogs. We present the results of
laboratory experiments to determine these band strengths. Improved experimental
methods, relying on simultaneous independent depositions of the molecule to be
studied and of the dominating ice component, have led to accuracies better than
a few percent. Furthermore, the temperature behavior of the infrared band
strengths of CO and HO are studied. In contrast with previous work, the
strengths of the CO, CO, and HO infrared features are found to depend
only weakly on the composition of the ice matrix, and the reversible
temperature dependence of the CO band is found to be weaker than previously
measured for a mixture of CO in HO.Comment: 17 pages uuencoded compressed Postscript file-- includes all 6
figures (replaces most recent posting with only figs 2-5
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