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NASA NTRS1988

Measurements of several atmospheric gases above the South Pole in December 1986 from high-resolution 3- to 4-micron solar spectra

This paper reports the quantitative analysis of absorption features of a number of atmospheric gases in 0.02/cm resolution 3- to 4-micron solar spectra recorded at the Amundsen-Scott South Pole Station. The measurements were obtained shortly after the austral spring ozone minimum and define, for the first time, the ambient levels of the species immediately following the breakup of the polar vortex. The retrieved total vertical column amounts in molecules per sq cm are: 2.4 + or - 0.4 x 10 to the 21st for H2(O-16), 4.5 + or - 0.5 x 10 to the 18th for H2(O-18), 3.9 + or - 0.4 x 10 to the 17th for HDO, 2.1 + or - 0.2 x 10 to the 19th for CH4, 5.1 + or - 0.5 x 10 to the 21st for CO2, 3.9 + or - 0.4 x 10 to the 18th for N2O, and 3.7 + or - 0.5 x 10 to the 15th for C2H6. Identification of (C-12)H3D absorption features and a search for H2CO absorption in the spectral data are also described.

Goldman, Aaron↗
NASA NTRS1988

Photochemical and thermal evolution of interstellar/precometary ice analogs

The photochemical and thermal evolution of H2O-, CH3OH-, NH3-, and CO-containing ices whose relative proportions in mixture are consistent with those of interstellar ice are presently studied experimentally. The UV photolysis of these ice analogs invariably generates H2CO, CO2, CO, CH4, and HCO, largely through the photofragmentation of CH3OH. CO and CH4 leave the samples as warmup proceeds to 100 K; most of the parent ice molecules are found to have sublimed upon the reaching of 200 K, with only a mixture of more refractory substances remaining. A residue of CH2 groups remains even after warmup to 300 K.

Allamandola, L. J.↗
NASA NTRS1989

Detection of the 2(0) to 3(-1) transition of C-13H3OH at 14.8 GHz

Observations of the 2(0) to 3(-1) E-type transition of C-13H3OH at 14.78 GHz have been made toward the continuum sources Sgr A, Sgr B2, and W33. For the cases of Sgr B2 and one position in Sgr A, a C-12/C-13 abundance ratio of 40 is obtained which is much higher than the values (20-25) found for other molecules. For W33, the value 50 obtained (50) is lower than that derived from H2CO observations, but is comparable to that noted from CO observations.

Kuiper, T. B. H.↗
NASA NTRS1989

Infrared fluorescence efficiencies for the nu1 and nu5 bands of formaldehyde in the solar radiation field

Formaldehyde scattering strengths have been determined for equilibrium distributions of 100 K, 50 K, 20 K, and for the the non-LTE case of an essentially fully relaxed distribution. Integrated band g factors of 2.89 x 10 to the -4th photons/s per molecule for nu1 and 3.83 x 10 to the -4th for nu 5 are obtained. The results indicate that the most promising regions to search for cometary H2CO are at about 2782/cm, at 2794.5/cm, and at about 2835/cm.

Reuter, D. C.↗
NASA NTRS1989

Correlation of molecular valence- and K-shell photoionization resonances with bond lengths

The relationship between the interatomic distance and the positions of valence-shell and K-shell sigma(asterisk) photoionization resonances is investigated theoretically for the molecules C2, F2, N2, O2, CO, NO, C2H2, C2H4, C2H6, HCN, H2CO, N20, CO2, and C2N2. The results of molecular-orbital computations are presented in three-dimensional diagrams, which are shown to be similar to the wave functions of a particle in a cylindrical well, confirming the validity of free-electron molecular-orbital (FEMO) approximations for modeling the potential along the symmetry axis. FEMO orbital energies and resonance positions are found to be in good agreement with previous theoretical and experimental results. Also included is a Feshbach-Fano analysis of the relevance of virtual-valence orbitals to the appearance of single-channel resonances in molecular photoionization cross sections.

Sheehy, J. A.↗
NASA NTRS1990

Study of the polyoxymethylene and its sputtered fragments: Implications for comets

Laboratory mass spectra of sputtered polyoxymethylene (POM) reveals a fragmentation pattern consistent with observed peaks in the PICCA experiment on board the Giotto spacecraft. Both commercially available POM and radiation synthesized POM have been used in the studies. Synthesized POM was identified using infrared absorption spectra after proton irradiation of H2CO ice on silicate grains at 20 K. Laboratory results suggest that similar type sputtering is a possible mechanism for removing species from comet grains.

Moore, Marla H.↗
NASA NTRS1990

Interstellar grain chemistry and organic molecules

The detection of prominant infrared absorption bands at 3250, 2170, 2138, 1670 and 1470 cm(-1) (3.08, 4.61, 4.677, 5.99 and 6.80 micron m) associated with molecular clouds show that mixed molecular (icy) grain mantles are an important component of the interstellar dust in the dense interstellar medium. These ices, which contain many organic molecules, may also be the production site of the more complex organic grain mantles detected in the diffuse interstellar medium. Theoretical calculations employing gas phase as well as grain surface reactions predict that the ices should be dominated only by the simple molecules H2O, H2CO, N2, CO, O2, NH3, CH4, possibly CH3OH, and their deuterated counterparts. However, spectroscopic observations in the 2500 to 1250 cm(-1)(4 to 8 micron m) range show substantial variation from source reactions alone. By comparing these astronomical spectra with the spectra of laboratory-produced analogs of interstellar ices, one can determine the composition and abundance of the materials frozen on the grains in dense clouds. Experiments are described in which the chemical evolution of an interstellar ice analog is determined during irradiation and subsequent warm-up. Particular attention is paid to the types of moderately complex organic materials produced during these experiments which are likely to be present in interstellar grains and cometary ices.

Allamandola, L. J.↗
NASA NTRS1990

Laboratory simulation of interstellar grain chemistry and the production of complex organic molecules

During the past 15 years considerable progress in observational techniques has been achieved in the middle infrared (5000 to 500 cm(-1), 2 to 20 microns m), the spectral region most diagnostic of molecular vibrations. Spectra of many different astronomical infrared sources, some deeply embedded in dark molecular clouds, are now available. These spectra provide a powerful probe, not only for the identification of interstellar molecules in both the gas solid phases, but also of the physical and chemical conditions which prevail in these two very different domains. By comparing these astronomical spectra with the spectra of laboratory ices one can determine the composition and abundance of the icy materials frozen on the cold (10K) dust grains present in the interior of molecular clouds. These grains and their ice mantles may well be the building blocks from which comets are made. As an illustration of the processes which can take place as an ice is irradiated and subsequently warmed, researchers present the infrared spectra of the mixture H2O:CH3OH:CO:NH3:C6H14 (100:50:10:10:10). Apart from the last species, the ratio of these compounds is representative of the simplest ices found in interstellar clouds. The last component was incorporated into this particular experiment as a tracer of the behavior of a non-aromatic hydrocarbon. The change in the composition that results from ultraviolet photolysis of this ice mixture using a UV lamp to simulate the interstellar radiation field is shown. Photolysis produces CO, CO2, CH4, HCO, H2CO, as well as a family of moderately volatile hydrocarbons. Less volatile carbonaceous materials are also produced. The evolution of the infrared spectrum of the ice as the sample is warmed up to room temperature is illustrated. Researchers believe that the changes are similar to those which occur as ice is ejected from a comet and warmed up by solar radiation. The warm-up sequence shows that the nitrile or iso-nitrile bearing compound produced during photolysis evaporates between 200 and 250K, suggesting that it is carried by a small molecular species. These molecules could be similar to the source material in Comet Halley that is ejected in grains into the coma, freed by sublimation, and photolyzed by solar radiation to produce the observed jets.

Allamandola, L. J.↗
NASA NTRS1989

Formaldehyde absorption toward W51

Formaldehyde (H2CO) absorption toward the H II region complex W51A (G49.5 - 0.4) in the 6 cm and 2 cm wavelength rotational transitions has been measured with angular resolution of about 0.15 pc. The continuum H II region shows a large, previously undetected shell structure 5.5 pc along the major axis. The absorption, converted to optical depth, shows a higher degree of clumping throughout the map than previous maps at lower resolution; in particular, two narrow regions of enhanced opacity are observed. The absorption in the velocity range 64-67 km/s LSR extends over most of the region, with an observed velocity gradient of 5.2 km/s pc. The opacity structure largely parallels the velocity structure, with a ridge of enhanced opacity to the north of the highest velocity feature. The S/N of the maps allows accurate modeling of the spectral profiles. Nine distinct clumps in the foreground clouds have been identified and parametrized, and column densities for the 1(11) and 2(12) rotational levels of orthoformaldehyde have been derived.

Kogut, A.↗
NASA NTRS1990

In situ measurement of the cosmic microwave background temperature at a distance of 7.5 kiloparsecs

Absorption of orthoformaldehyde (H2CO) toward the giant H II region W51A (G49.5 - 0.4) has been used as a remote probe of the cosmic microwave background (CMB) at a distance of 7.5 kpc. VLA observations of the 6 cm and 2 cm wavelength transitions provided sufficient resolution and sensitivity to resolve condensations within the foregound clouds. Solutions to the equations of statistical equilibrium within each condensation in the context of a large velocity gradient model yielded an estimate for the CMB temperature at 2.1 mm wavelength of 3.2 + or - 0.9 K. The uncertainty is dominated by modeling of collisional pumping by neutral hydrogen molecules (H2).

Kogut, A.↗
NASA NTRS1990

Modeling the chemistry of the dense interstellar clouds. I - Observational constraints for the chemistry

A search for correlations arising from molecular line data is made in order to place constraints on the chemical models of interstellar clouds. At 10 to the 21st H2/sq cm, N(CO) for dark clouds is a factor of six greater than the value for diffuse clouds. This implies that the strength of the UV radiation field where CO shields itself from dissociation is about one-half the strength of the average Galactic field. The dark cloud data indicate that the abundance of CO continues to increase with A(V) for directions with A(V) of 4 mag or less, although less steeply with N(H2) than for diffuse clouds. For H2CO, a quadratic relationship is obtained in plots versus H2 column density. The data suggest a possible turnover at the highest values for A(V). NH3 shows no correlation with H2, C(O-18), HC3N, or HC5N; a strong correlation is found between HC5N and HC3N, indicating a chemical link between the cyanopolyynes.

Federman, S. R.↗
NASA NTRS1989

The evolution of organic mantles on interstellar grains

By laboratory simulation of the chemical processes on dust grains it was investigated how solid organic materials can be produced in the interstellar medium. The ice mantles that accrete on grains in molecular clouds, consisting primarily of H2O, CO, H2CO, NH3, and O2, are irradiated by the internal UV field, resulting in the storage of radicals upon photodissociation of the original molecules. Transient heating events lead to the production of oxygen-rich organic species by recombination reactions. The experiments indicated that in this way the observed amount of organic material can be produced if a grain passes a few times through a molecular cloud during its life. After the destruction of the cloud the grains enter a more diffuse medium. Here they are subjected to the interstellar UV field as well as to collisions with atomic hydrogen. Experiments show that the intense photoprocessing results in the removal of small species like H2O and NH3 as well as in carbonization of the organic molecules. Contrary to this, the atomic H flux will maintain a certain hydrogen level in the mantle. These processes likely convert the original, oxygen-rich organics into an unsaturated hydrocarbon type material such as that observed towards IRS 7 and in Comet Halley grains.

Schutte, Willem A.↗
NASA NTRS1990

Simultaneous imaging of optical CN lines and radio HCN lines in comet Austin

The parent molecule of cometary CN has been the subject of speculation for a long time. When HCN was detected at 3.4 mm in Comet Kohoutek, the problem seemed to be resolved, but much more detailed work on Comet Halley raised the quantitative question of whether HCN could be the only parent. Therefore, comparative CN/HCN studies are vital for understanding the origin of cometary CN. The striking observation of CN jets in Comet Halley raised another very interesting question about the origin of CN. Traditional theory permits only dust features to remain well defined far from the nucleus. The CN jets were interpreted as arising from submicron sized dust particles, perhaps CHON particles. An estimated 10 to 50 percent of the CN in the comet was in the jets in the Halley observations. Several hypotheses can be made: (1) some of the CN originates from the dust and has nothing to do with HCN; or, (2) at least some of the HCN is also produced from the dust in the coma rather than directly from the nucleus. (In the second hypothesis, whether CHN is or is not the parent of the CN associated with the dust would need to be established.) The extended scalelengths found for CO by Eberhardt et. al. (1987) and for H2CO by Snyder et. al. (1989) also support ideas like hypothesis (2). A third hypothesis should be mentioned; contrary to the usual theory, the gas flow does not become isotropic (Combi 1987). For all of the reasons mentioned, it is essential to make a detailed comparison of the spatial distributions of CN and HCN. Furthermore, because of the variability of cometary emissions, it is necessary to make measurements simultaneously.

Palmer, Patrick↗
NASA NTRS1990

The near ultraviolet spectra of comets P/Brorsen-Metcalf and Austin

Results are reported on spectrophotometric observations of comets P/Brorsen-Metcalf and Austin from 3000 to 3600 A at a spectral resolution of about 1.8 A. The strongest features are the OH(A-X) 0-0 and 1-1 bands, and the NH(A-X) 0-0 bands. For the first time, the OH(A-X) 0-1 band was clearly found. The existence of the CN(B-X) 2-1 and 3-2 bands were verified and measured. A feature at 3258 A that was first seen in uncalibrated spectra was detected, and was identified as the NH singlet (c-a) 0-0 transition. The CO2(+) features at 3378, 3504, and 3512 A were also firmly identified. This ion was reported as being present in the tail of Comet Bester (1984 I) by Swings and Page (1950). The identification of a weak feature at 3547 A was proposed as the fundamental transition of H2CO, which would make this the first optical cometary detection of this molecule which is very abundant in giant molecular clouds.

Cochran, William D.↗
NASA NTRS1990

Comet Brorsen-Metcalf in the 3.5 micron region

Comet Brorsen-Metcalf was observed on UT 21 to 24 Aug. 1989 using the CRSP spectrometer and the 1.3 meter telescope at Kitt Peak National Observatory. The cometary continuum was detected on all nights. The data are very noisy, due to the short observation window and the untried nature of the instrument. Low resolution (0.0227 micron) spectra show the 3.4 micron C-H stretch feature having a contrast of at most a factor of two to the neighboring continuum. High resolution (0.0031 micron) spectra between 3.4 and 3.6 microns show 1 sigma features that might be attributed to the nu 5 band of formaldehyde (H2CO). Similar spectra of the region between 3.2 and 3.4 microns show one 3 sigma line at 3.34 microns, which is as yet unidentified. Although the cometary spectra were more spatially extended than the spectra of standard stars, no extension of the line emission beyond the continuum was observed.

Storrs, A. D.↗
NASA NTRS1990

Submillimeter astronomy and low mass star formation

Surveys made at wavelengths shorter than 100 microns are criticized in that they are biased against finding the coldest, most deeply obscured, and possibly youngest protostars. Large scale mapping of star forming regions in the submillimeter continuum and in lines such as CS H2CO and NH3, that are excited at high density, is identified as crucial in order to find clouds in the earliest stages of collapse into stars. The use of photometry from 1 to 1000 microns in characterizing the disks around embedded stars and T Tauri stars is outlined.

Beichman, Charles A.↗
NASA NTRS1990

Mass spectra of sputtered polyoxymethylene - Implications for comets

Laboratory mass spectra of sputtered polyoxymethylene, POM, reveal a mass pattern similar to that detected by the PICCA experiment on board the Giotto spacecraft. Both commercially available POM and radiation-synthesized POM have been used in the studies. Synthesized POM was identified by its infrared absorption spectra after proton irradiation of H2CO condensed on silicate grains at 20 K. Laboratory results suggest that a similar type of sputtering is a possible mechanism for removing species from cometary grains.

Moore, Marla H.↗
NASA NTRS1991

Dust and molecular properties of the low-opacity cloud Lynds 1563

Optical, molecular, and far-infrared data are analyzed for L1563, estimated peak Ab 2.5 mag. The cloud is detected by IRAS at 12, 25, 60, and 100 microns, and with CO, (C-13)O, and H2CO molecules. A column density comparison yields an estimate of the temperature of the classical dust grains of 15.6 + or - 1 K, while the color temperature derived from the ratio I(60)/I(100) is 26 K. Both dust and color temperatures decrease toward the cloud center.

Clark, Frank O.↗