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At least 271 records · Page 15
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NASA NTRS1990

A comparison of UV surface brightness and HI surface densities for spiral galaxies

Shaya and Federman (1987) suggested that the ambient ultraviolet flux at 1000 A permeating a spiral galaxy controls the neutral hydrogen (HI) surface density in the galaxy. They found that the atomic envelopes surrounding small molecular clouds, because of their great number, provide the major contribution to the HI surface density over the stellar disk. The increase in HI surface density with later Hubble types was ascribed to the stronger UV fields from more high-mass stars in later Hubble types. These hypotheses are based on the observations of nearby diffuse interstellar clouds, which show a sharp atomic-to-molecular transition (Savage et al. 1977), and on the theoretical framework introduced by Federman, Glassgold, and Kwan (1979). Atomic envelopes around interstellar clouds in the solar neighborhood arise when a steady state is reached between photodissociation of H2 and the formation of H2 on grains. The photodissociation process involves photons with wavelengths between 912 A and 1108 A. Shaya and Federman used H-alpha flux as an approximate measure for the far UV flux and made their comparisons based on averages over Hubble type. Here, researchers compare, on an individual basis, UV data obtained with space-borne and balloon-borne instruments for galaxies with measurements of HI surface density (Warmels 1988a, b). The comparisons substantiate the conclusion of Shaya and Federman that the far UV field controls the HI content of spiral galaxies.

Federman, S. R.↗
NASA NTRS1990

A status report on the analysis of the NOAA-9 SBUV/2 sweep mode solar irradiance data

Monitoring of the near ultraviolet (UV) solar irradiance is important because the solar UV radiation is the primary energy source in the upper atmosphere. The solar irradiance at wavelengths shortward of roughly 300 nm heats the stratosphere via photodissociation of ozone in the Hartley bands. Shortward of 242 nm the solar UV flux photodissociates O2, which is then available for ozone formation. Upper stratosphere ozone variations coincident with UV solar rotational modulation have been previously reported (Gille et al., 1984). Clearly, short and long term solar irradiance observations are necessary to separate solar-forced ozone variations from anthropogenic changes. The SBUV/2 instrument onboard the NOAA-9 spacecraft has made daily measurements of the solar spectral irradiance at approximately 0.15 nm intervals in the wavelength region 160-405 nm at 1 nm resolution since March 1985. These data are not needed to determine the terrestrial ozone overburden or altitude profile, and hence are not utilized in the NOAA Operational Ozone Product System (OOPS). Therefore, assisted by the ST System Corporation, NASA has developed a scientific software system to process the solar sweep mode data from the NOAA-9 instrument. This software will also be used to process the sweep mode solar irradiance data from the NOAA-11 and later SBUV/2 instruments. An overview of the software system and a brief discussion of analysis findings to date are provided. Several outstanding concerns/problems are also presented.

Cebula, R. P.↗
NASA NTRS1991

Formation of ions and radicals from icy grains in comets

Ion and radical formation in comets are thought to occur primarily by photodissociation of gas phase molecules. Experimental evidence and theoretical calculations are presented that show that some of the radical and ions can come directly from ice grains. The experimental evidence suggest that if the frozen molecules on the surface of grains undergo direct dissociation then they may be able to release radicals directly in the gas phase. If the molecules undergo predissociation it is unlikely that they will release radicals in the gas phase since they should be quenched. Calculations of this direct photodissociation mechanism further indicate that even if the parent molecule undergoes direct dissociation, the yield will not be high enough to explain the rays structure in comets unless the radicals are stored in the grains and then released when the grain evaporates. Calculations were also performed to determine the maximum number of ions that can be stored in an icy grain's radius. This number is compared with the ratio of the ion to neutral molecular density. The comparison suggests that some of the ions observed near the nucleus of the comet could have originally been present in the cometary nucleus. It is also pointed out that the presence of these ions in icy grains could lead to radical formation via electron recombination. Finally, an avalanche process was evaluated as another means of producing ions in comets.

Jackson, William M.↗
NASA NTRS1991

A determination of Mg(+)-ligand binding energies

Theoretical calculations employing large basis sets and including correlation are carried out for Mg(+) with methanol, water, and formaldehyde. For Mg(+) with ethanol and acetaldehyde, the trends in the binding energies are studied at the self-consistent-field level. The predictions for the binding energy of Mg(+) to methanol and water of 41 + or - 5 and 36 + or - 5 kcal/mol, respectively, are much less than the experimental upper bounds, of 61 + or - 5 and 60 + or - 5 kcal mol, determined by using photodissociation techniques. The theoretical results are inconsistent with the onset of Mg(+) production observed in the photodissociation experiments, as the smallest absorptions are calculated at about 80 kcal/mol for both Mg(+)-CH3OH and Mg(+)-H2O, and these transitions are to bound excited states. The binding energy for Mg(+) with formaldehyde is predicted to be similar to Mg(+)-H2O. The relative binding energies are in reasonable agreement with experiment. The binding energy of a second water molecule to Mg(+) is predicted to be similar to the first. This suggests that the reduced reaction rate observed for the second ligand is not a consequence of a significantly smaller binding energy, at least for the smaller ligards such as those considered in this work.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1991

Neutral matter in planetary nebulae

A review of current studies of neutral envelopes is presented with particular attention given to the use of the envelopes as test cases for understanding the ionization and thermal structure of photodissociation regions. The study of near-IR H2 emission is discussed with detailed spectra given for a few planetary nebulae, and airborne observations of far-IR atomic lines are discussed. These two methods can discern photodissociation regions with warm gas and UV flux is fairly prominent. The use of resonance-absorption-line spectroscopy is also reviewed with respect to the analysis of the Na D lines, and thereby allows the measurement of integrated columns of material through the shell. The methods provide evidence for the notion that planetary nebulae consist of more than just ionized material; large amounts of neutral and molecular material are being confirmed, which has important implications for the mass-loss episode of the nebulae.

Dinerstein, Harriet L.↗
NASA NTRS1991

Production of N(+)-asterisk from N2 + hnu - Effective EUV emission yields from laboratory and dayglow data

Photodissociative ionization of N2 into the unbound N2(+) H 2Sigma-g(+) state is identified as the parentage of a number of N(+) and N-excited states that contribute to the u.v. dayglow. Yields have been obtained for the production of extreme ultraviolet emission lines of N(+) and N from a laboratory experiment using a broadband synchrotron radiation source with fluorescence spectroscopy. The yields are termed 'effective', in that they are constant percentages of the H state cross section. These are compared wih effective yields needed to reproduce N II 1085 and 916 A lines from four dayglow observations using a model of solar energy deposition and photoelectron production and loss in the earth's thermosphere. The 1085 A effective yield measured in the laboratory (18 percent) agrees with that from the dayglow data (average of 17 percent) to well within experimental uncertainties. Thus, it is concluded that photodissociative ionization of N2 is the primary source of the N II 1085 A dayglow. However, there is an order of magnitude discrepancy among the various dayglow observations of the 1085/916 intensity ratio, only one of which is consistent with the laboratory observation of 4.4. Neither contamination by other dayglow features nor atmospheric extinction can account for the disparities. Laboratory measurements of N2 and O2 absorption cross sections at these wavelengths are also reported.

Meier, R. R.↗
NASA NTRS1993

The case of missing CO-13 in mergers

We present a comparison of the CO-13 and CO-12 emissions of six systems of merging galaxies: NGC 828, NGC 3256, NGC 4194, NGC 6240, Arp 220, and Arp 299. The observations were made in both J=1-0 and J=2-1 transitions with the IRAM 30 m and SEST 15 m telescopes. In all galaxies but NGC 828, the CO-13 is much weaker than in spiral galaxies. The average emissivity ratios measured at the few kiloparsec scale are: CO-12(1-0)/CO-13(1-0) approx. equals 30, CO-12(2-1)/CO-13(2-1) approx. equals 40. These values are significantly larger than those usually measured in normal spiral galaxies, which are always between 5 and 15 for the J=1-0 line. We show that such a peculiar behavior cannot be interpreted as due to the dominant presence of diffuse gas and it cannot be attributed to optically thin CO emission either. The faint CO-13 emission of mergers must result from either an underabundance of CO-13 or an overabundance of CO-12. They may be accounted for by different mechanisms: CO-13 molecules are more easily photodissociated than CO-12 ones - however our observations seem to rule out physical conditions characteristic of photodissociation regions; in the course of the merging, large amounts of unprocessed gas, with a high (CO-12/CO-13) abundance ratio, are driven from the external regions of the progenitor galaxies to the new nucleus - this could increase the (CO-12/CO-13) ratio by a factor of 2; the interstellar medium can also be quickly enriched in C-12 (thus in CO-12) by a factor of 2, due to selective nucleosynthesis of this isotope (vs. C-13) in the massive stars born during the starburst. The last two processes can thus significantly contribute to the weakness of the CO-13 lines. The high CO-13-to-CO-12 line ratios that we have measured are due to the deep transformations that take place in the interstellar medium during the merging and the starburst: indeed, the only object with nearly normal line ratios, NGC 828, appears less perturbed and active in star formation than the other sources in the sample. In most mergers, the faintness of the CO-13 emission may indicate that the conversion factor from CO-12 emissivities to H2 column densities could differ substantially from the standard galactic value.

Casoli, Fabienne↗
NASA NTRS1993

Molecular processes in comets

A major effort was expended in developing procedures for calculating by quantum-mechanical methods the cross sections for the photodissociation of the hydroxyl radical OH. The molecule OH is the primary dissociation product of water and observations of OH when combined with a determination of the lifetime of OH in the solar radiation field can be used to calculate the rate of release of water from comets. There are many possible photodissociation channels whose efficiency depends upon the details of the spectrum of the radiation. We have calculated the potential energy curves of all the states of OH that can be reached by electric dipole absorption from the ground X(exp 2)(Pi) state by photons with energies up to about 12 eV.

Dalgarno, A.↗
NASA NTRS1992

Direct observation of ClO from chlorine nitrate photolysis

Chlorine nitrate photolysis has been investigated with the use of a molecular beam technique. Excitation at both 248 and 193 nanometers led to photodissociation by two pathways, ClONO2 yields ClO + NO2 and ClONO2 yields Cl + NO3, with comparable yields. This experiment provides a direct measurement of the ClO product channel and consequently raises the possibility of an analogous channel in ClO dimer photolysis. Photodissociation of the ClO dimer is a critical step in the catalytic cycle that is presumed to dominate polar stratospheric ozone destruction. A substantial yield of ClO would reduce the efficiency of this cycle.

Minton, Timothy K.↗
NASA NTRS1994

Temperature dependence of ultraviolet absorption cross-sections of alternative hydrochlorofluorocarbons

Ultraviolet absorption cross-section of six alternative hydrochlorofluorocarbons (HCFC-21 HCFC-22, HCFC-123, HCFC-124, HCFC-141b and HCFC-142b1) have been measured between 170 and 260 nm for temperature ranging from 210 to 295 K. These data are compared with other available determinations performed at room temperature and their temperature dependence is discussed. Photodissociation coefficients are estimated and their temperature dependence is discussed. Impact of the photodissociation on the total atmospheric destruction of these compounds is illustrated.

Gillotay, D.↗
NASA NTRS1994

Ultraviolet absorption cross-sections of some carbonyl compounds and their temperature dependence

Ultraviolet absorption cross-section of phosgene (CCl2O), trichloroacetylchloride (CCl3-CClO) and trichloroacetaldehyde (CCl3-CHO) have been measured between 170 and 320 nm for temperature ranging from 210 to 295 K with classical double beam equipment. These data are compared with other available determinations performed at room temperature. Photodissociation coefficients are estimated and their temperature dependence is discussed. Impact of the photodissociation on the total atmospheric destruction of these compounds is illustrated.

Gillotay, D.↗
NASA NTRS1993

The (C2) 158 micrometer emission from the Horsehead nebula

The C II 158 micrometer line and the several rotational lines of CO, CO-13, and CS toward selected positions in the Horsehead extinction region in IC 434 are mapped. The observations show that the region has a gas density of about 10,000 cu cm and an external UV (Ultraviolet) flux to 20 to 100 times the average interstellar UV field. Although this is a regime where the C+ emission varies rapidly with UV intensity, fine structure line emission from gas with this range of physical conditions were not investigated previously. Comparisons of results with models of photodissociation regions show that existing plane parallel photodissociation region models are in general agreement with the observed intensity. It is not necessary to invoke a clumpy structure in the boundary layer to explain the observations, but the overall geometry of the cloud is important in determining the distribution of C+ emission.

Zhou, S.↗
NASA NTRS1994

Evolution of macromolecular dust: Far-ultraviolet spectral dust extinction and gas absorption of stellar light as measured with the Hopkins Ultraviolet Telescope

From far-ultraviolet (FUV) spectra of B stars taken with the Hopkins Ultraviolet Telescope (HUT) and Voyager space observations, we determine the far-ultraviolet extinction by Galactic dust and hydrogen absorption in the local spiral-arm clouds towards HD 25443 (B0.5 III), HD 37903 (B1.5 V), and HD 200775 (B3 Ve). We find that the (n (H)) = 1.3 cu cm and number fraction (f = 0.31) of H atoms in molecular hydrogen relative to the total hydrogen indicates a mostly diffuse medium toward HD 25443. We also determine f = 0.56 for the gas in the photodissociation region (PDR) in front of HD 37903 and f approximately equals 0.36 for the HD 200775 PDR. The inferred ratios of N(H2)/I(CO) = 2.4 to 2.8 x 10(exp 20)/K(km/s)/sq cm for cool gas in dark clouds agree with previous canonical estimates of N(H2)/I(CO) = 2 to 3 x 10(exp 20)/K(km/s)/sq cm and support the use of the ratio for determining masses of molecular clouds. We find that the shape and strength of the HD 25443 FUV extinction is the same as a diffuse-medium mean extinction (R(sub nu) = 2.95) extrapolated from mid-UV wavelengths (normal). The HD 37903 FUV extinction through a bright H2 photodissociation region is higher than a mean FUV extinction(R(sub nu) = 4.11), as is the HD 200775 extinction. Another star from the literature rho Oph, probes the dense medium and exhibits an FUV extinction of normal mean strength (R(sub nu) = 4.55) but steeper shape. The normal FUV -extinction of HD 25443 implies that the small FUV-extinction dust in the diffuse medium forms in p art from larger grains as the grains shatter under shocks. The normal strength of the rho Oph FUV-extinction indicates that the minute dust condenses onto or coagulates into larger grains in the dense medium. The high extinction deviation of the HD 37903 PDR (and HD 200775 PDR) shows that FUV radiation from hot stars with T(sub eff) approximately equals 20,000 to 23,000 K in PDRs can evaporate some of the FUV-extinction dust from grain surfaces. In contrast, previously measured extinctions in Galactic H(+) (H II) regions shows that FUV radiation at T(sub eff) greater than 25,000 K can destroy some of the dust in H(+) regions. Thus, some of the FUV-extinction dust is highly volatile. We find that the dust carrier of the variable FUV-extinction is correlated (r = 0.99) with the mid-IR (12 micrometers) radiation from hydrocarbon dust. The volatile FUV-extinction component could thus be due to minute hydrocarbon dust particles or macromolecules, probably PAHs, which are known to condense onto larger grains in the dense medium. Larger grains compsed of PAHs clustered into a solid could thus produce the steep FUV extinction of the rho Oph dense medium. After exposure to FUV photons wihtin approximately 1 pc of HD 37903, PAHs with about 30 carbon atoms would evaporate off the larger grains, increasing gaseous PAH abundance by approximately 14%. Also, after receiving shocks in the diffuse medium toward HD 25443, the larger hydrocarbon grains would shatter into PAH molecules. In summary, as the Galactic medium cycles betwee n the diffuse and dense medium, hydrocarbon dust cycles into and out of larger grains under the influence of shocks, radiation, and condensation.

Buss, Richard H., Jr.↗
NASA NTRS1994

An extended source for CN jets in Comet P/Halley

We examined radial intensity profiles of CN jets in comparison with the diffuse, isotropic component of the CN coma of Comet P/Halley. All images were bias-subtracted, flat-fielded, and continuum-subtracted. We calculated the diffuse profiles by finding the azimuthal mean of the coma least contaminated by jets yielding profiles similar to those of vectorial and Haser models of simple photodissociation. We found the jet profiles by calculating a mean around a Gaussian-fitted center in r-theta space. There is an unmistakable difference between the profiles of the CN jets and the profiles of the diffuse CN. Spatial derivatives of these profiles, corrected for geometrical expansion, show that the diffuse component is consistent with a simple photodissociation process, but the jet component is not. The peak production of the jet profile occurs 6000 km from the nucleus at a heliocentric distance of 1.4 AU. Modeling of both components of the coma indicate results that are consistent with the diffuse CN photochemically produced, but the CN jets need an additional extended source. We found that about one-half of the CN in the coma of Comet P/Halley originated from the jets, the rest from the diffuse component. These features, along with the width of the jet being approximately constant, are consistent with a CHON grain origin for the jets.

Klavetter, James Jay↗
NASA NTRS1994

The gas distribution of comet Halley and its relation to the nucleus rotation

We used a set of spatially resolved spectra of comet Halley to explore whether the gas distribution profile could be inverted to yield information on the rotation of the comet. The data were obtained both pre- and post-perihelion. The pre-perihelion data showed reasonable symmetry and so were used to define the lifetimes against photodissociation of the various molecules. These lifetimes were then used to define the lifetimes against photodissociation of the various molecules. These lifetimes were then used along with a nonsteady-state vectorial model to fit the post-perihelion gas distribution profiles. The resulting molecular lightcurves are compared with the photometric lightcurves of Schlicher et al. (1990) to show that the rotational information is encoded in the observed gas distribution within the coma. The molecular lightcurves can differentiate between the preferred Schlicher et al. average period and the period they find for the same time interval as the spectra.

Cochran, Anita L.↗
NASA NTRS1998

The Mid-Infrared Spectrum of the Galactic Center: A Starburst Nucleus

Using the Michelson interferometer on the Midcourse Space Experiment (MSX), we have taken spectra of many positions in the central 25 min of the Galactic Center (GC) with a 6 min x 9 min FOV. The spectral coverage was 380 to 1700/ cm (6 to 26 microns) and the resolution was approx. 21/cm. The spectra exhibit strong UIR/PAH features at 6.2, 7.7, 8.6 and 11.3 microns, in addition to the ionic lines of (Ne II), at 12.8 microns, (S III) 18.7 microns, and (Ar II) 6.98 microns. There are deep silicate absorption features at 10 and 18 microns and a cold continuum increasing at the longest wavelengths. Additional weak features are present in the spectra. We discuss the variation in the extinction at 10 microns as a function of location in the GC. Compared to the MSX spectrum of the Orion nebula, smoothed to the same resolution and multiplied by the estimated GC extinction, the GC spectra have similar PAH features, but the Orion Nebula also has strong lines of (He III) 15.6 microns, (S IV) 10.5 microns, and (Ar III) 8.99 microns and its 25 microns continuum is stronger (colder). Thus, the GC exhibits the mid-IR spectrum of a low excitation H II region and a nearby molecular cloud with a surface photodissociation region (PDR). This is in excellent agreement with the canonical model of a starburst nucleus in which the hot stars and molecular clouds are randomly distributed. The outer surfaces of the clouds are photodissociated and ionized by the photons from the stars located outside the clouds. The PAH molecules are transiently heated by the stellar photons. Since the exciting stars are located well outside the clouds, the radiation field is dilute compared to a newly-formed blister H II region like Orion; this dilute radiation field causes the relatively low excitation of the ionic lines.

Simpson, J. P.↗
NASA NTRS1999

KWIC: A Widefield Mid-Infrared Array Camera/Spectrometer for the KAO

This grant covered a one year data analysis period for the data we obtained with the Kuiper Widefield Infrared Camera (KWIC) on the KAO during CY94 and CY95. A fairly complete list of scientific papers produced, or soon to be produced under this award is contained at the end of this report. Below we summarize some of the highlights of the work we did under this grant. KWIC Imaging of the Orion Nebula. KWIC was successfully developed under the KAO grants program (NASA grant NAG2-800). First funding arrived in November of 1992, and we flew our first two flights in February of 1994 -just 15 months later. These flights were very successful. We imaged the Orion Nebula in the 37.7 micron continuum and [SiII] 35 micron line and imaged M82 and Arp299 in the 37.7 micron continuum. Our Orion image demonstrates that the 37.7 micron continuum arises in the warm dust associated with the photodissociated surfaces (photodissociation regions, or PDRs) of molecular clouds. We use the brightness and color temperature distribution to ascertain the morphology of the Orion PDR. The [SiII] image of Orion encompassed the entire Orion A HII region and its enveloping PDR. Most of the emission in the PDR regions of the map appears to coincide very well with our 37.7 micron continuum map indicating a PDR origin for the [SiII] in agreement with theoretical predictions. The [SiII] line emission is very clumpy in the PDR directly imaging the clump spectrum indirectly ascertained by examining the distribution and flux ratios of [CII] and [0I] far-IR fine structure line, and high J CO emission. We also detected very strong [SiII] line emission from the embedded BN-KL star formation region tracing the morphology and physical conditions of the high velocity shock from these very young stars.

Stacey, Gordon J.↗
NASA NTRS2000

Photochemistry of Methane in Model Atmospheres of Jupiter and Titan

The two central findings were 1) hydrogen atoms and hydrogen molecules photodissociated from methane are relatively richer in H than in D in other words deuterium atoms have a greater probability of remaining attached to the carbon atom. Titan, a moon of Saturn has an atmosphere which is largely nitrogen but also contains about 3% methane as well as smaller amounts of C2 and C3 hydrocarbons. If all these hydrocarbons are of biological origin, the isotopic scrambling occurring in living organisms would result in equal D atom abundances. On the other hand, if the higher hydrocarbons are derived from methane by photodissociation of methane, they should be richer in D than methane. Precise values for the enrichment were derived from our photochemical data. 2) When methane is dissociated by vuv light, methylene is produced in a singlet state. This explains why the higher hydrocarbons are sparse on Jupiter but relatively rich on Titan.

Bersohn, Richard↗