The Photolysis of Aromatic Hydrocarbons Adsorbed on the Surfaces of Cosmic Dust Grains

The results of laboratory mass-spectrometer studies of the laser-induced dissociation of molecules of simple aromatic hydrocarbons adsorbed on a quartz substrate under the conditions of deep vacuum and low temperatures are adapted to the physical and chemical conditions in regions of active star formation in molecular clouds. The main properties of the photolysis of physically adsorbed molecules compared to the photodissociation of isolated molecules in the gas phase are identified. The relevance of molecular photolytic desorption to the real conditions in the interstellar medium is analyzed, in particular, to the conditions in photodissociation regions. It is shown that the photodissociation of adsorbed benzene occurs along other channels and with appreciably lower efficiency than does the corresponding process in the gas phase. The photodissociation of aromatic hydrocarbons adsorbed on the surfaces of interstellar grains cannot make a large contribution to the abundance of hydrocarbons with small numbers of atoms observed in the interstellar medium.

     

Determining the parameters of massive protostellar clouds via radiative transfer modeling

A one-dimensional method for reconstructing the structure of prestellar and protostellar clouds is presented. The method is based on radiative-transfer computations and a comparison of theoretical and observed intensity distributions at both millimeter and infrared wavelengths. The radiative transfer of dust emission is modeled for specified parameters of the density distribution, central star, and external background, and the theoretical distribution of the dust temperature inside the cloud is determined. The intensity distributions at millimeter and IR wavelengths are computed and quantitatively compared with observational data. The best-fit model parameters are determined using a genetic minimization algorithm, which makes it possible to reveal the ranges of parameter degeneracy as well. The method is illustrated by modeling the structure of two infrared dark clouds IRDC-320.27+029 (P2) and IRDC-321.73+005 (P2). The derived density and temperature distributions can be used to model the chemical structure and spectral maps in molecular lines.     

Relationships of soil,plant, and microbial characteristics in silt-clay and sand,tall-formSpartina alterniflora marshes

The relationships between soil texture, plant growth, and anaerobic microbial activity in two tall-formSpartina alterniflora marshes on Sapelo Island, Georgia, were compared. The soil of one marsh was composed of typical silt-clay-sized particles; the soil of the other marsh consisted of >90% sand-sized particles. The two soils supported similar biomasses ofS. alterniflora, however, plants were taller and more robust in the silt-clay-soil than in the sand soil. Total microbial adenosine triphosphate concentrations in the silt-clay and sand soils averaged 5.71 and 1.64 μg per cm³, respectively. Seawater slurries of both soils exhibited potential for microbial sulfate reduction, methanogenesis, and glucose fermentation; rates for the processes averaged 2.03 and 0.33 nmol S-cm³ per h; 1.20 and 0.87 μmol CH₄ per cm³ per h; and 0.04 and 0.12 per min (rate constant) for the sand and silt-clay soils, respectively.     

Achieving environmentally relevant organochlorine pesticide concentrations in eggs through maternal exposure in Alligator mississippiensis

Alligator mississippiensis eggs from organochlorine pesticide (OCP) contaminated sites in Florida exhibit high rates of embryonic mortality compared to reference sites (P < 0.05). The objective of the present study was to use captive adult alligators to test the hypotheses that maternal exposure to OCPs results in increased OCP concentrations in eggs, and that increased exposure is associated with increased embryonic mortality. A total of 24 adult alligators (8 males and 16 females) were housed in eight pens. Eight females in four pens were dosed with a mixture of p,p'-DDE, toxaphene, dieldrin, and chlordane at a rate of 0.2+/-0.01 mg/kg/day for 274+/-8 days. Treated females produced eggs containing higher OCP concentrations (12,814+/-813 ng/g yolk) than controls (38+/-4 ng/g yolk). Eggs of treated females exhibited decreased viability (13+/-22%) as compared to controls (45+/-20%). Results indicated that 0.6% of administered OCPs were maternally transferred to the eggs of American alligators, and that maternal exposure is associated with decreased egg/embryo viability in this species.     

Origin of quartz clusters in Vinalhaven granite and porphyry,coastal Maine

We use the crystallographic orientations of quartz crystals, as determined with EBSD, to provide new evidence for the formation of clustered quartz crystals during magma crystallization. Vinalhaven is dominated by granite, with minor porphyry that formed when granite remelted during input of coeval basalt. CL zoning suggests that most quartz clusters in granite and porphyry formed by synneusis, the “swimming together” of preformed crystals. In granite, most quartz pairs in clusters have random orientations—only about 10% have parallel or Esterel twin orientations. Porphyry has fewer quartz clusters, and all pairs have approximately parallel or Esterel twin orientations. CL zoning of quartz pairs in porphyry indicates that they attached prior to a major remelting event. Interpretation of the Vinalhaven quartz clusters leads us to propose that oriented synneusis occurs during crystal accumulation on a magma chamber floor. During hindered settling, some quartz crystals should have come into contact along their dipyramidal faces. Once in contact, continued settling and loss of interstitial melt may have rotated some quartz crystals such that lattices on their dipyramidal faces matched—producing parallel and Esterel twin orientations and creating strong bonds between pairs. Only a small proportion of pairs with matched dipyramidal faces formed in the granite and, during rejuvenation to produce porphyry, only these oriented pairs survived. Hence, the presence of oriented synneusis in a plutonic rock may demonstrate a history of crystal accumulation.     

Studies of Star-forming Complexes in the Galaxies NGC 628, NGC 2976, and NGC 3351

The parameters of the radiation of interstellar matter in star-forming complexes in the high-metallicity galaxies NGC 628, NGC 2976, and NGC 3351, which have different morphological types, are analyzed. The relationship between the emission in Hα and in lines of CO and HI is considered, as well as the relationship between Hα and the emission of dust in the infrared range (IR). The fluxes and surface brightnesses in the UV and IR correlate well with the Hα emission. The HI emission also correlates well with Hα, while the correlation between the CO and Hα emission is much weaker. The ratio of the fluxes at 8 and 24 µm decreases with increasing Hα flux. This may be due to changes in the properties of the dust ensemble (a decrease in the mass fraction of polycyclic aromatic hydrocarbons) or to changing excitation conditions. Analysis of the kinematics of the CO lines shows that the CO flux grows with increasing velocity scatter ΔV when ΔV ? 70 km/s. Preliminary evidence for the existence of star-forming complexes with higher values of ΔV is provided, when the increase in the velocity scatter is accompanied by a decrease in the CO luminosity of the complex.

     

Problems of star-formation theory and prospects for submillimeter observations

We consider the current state of star-formation theory and requirements for millimeter and submillimeter observations that would enable the resolution of the most pressing problems in the physics of star formation. Two key features of star-forming regions that define these observational requirements are the relatively low energy of processes taking place there and the smallness of the corresponding spatial scales. This is especially true for objects in the latest stages of “pre-stellar” evolution, that is, hot cores, hypercompact and ultracompact HII regions, and protoplanetary disks. The angular resolution, sensitivity, and spectral coverage in existing ground-based and space submillimeter and millimeter telescopes are not fully adequate to the observtional requirements. To obtain detailed information on star-forming regions, as well as individual protostars, it is necessary to use a space-based interferometer.