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

The organic component of interstellar grains

The 3.4 micron absorption feature observed in the spectrum of a number of Galactic Center (GC) sources indicates the presence of organic molecules in the interstellar medium. It is ascribed to the C-H stretch vibration of tetrahedrally bonded carbon. From the observed features due to the interstellar organic material, an estimate was made of its composition and abundance. The ratio of the number of C-H groups of tetrahedrally to those of trigonally bonded carbon was 1.5, the cosmic abundance of carbon was .00037, and the depth of the silicate absorption toward the GC was taken equal to 3.6

New insights in the photochemistry of grain mantles: The identification of the 4.62 and 6.87 micron bands

The mid-IR spectral region of molecular clouds is known to show the fingerprints of molecules frozen in the icy mantles of the interstellar grains. To study the complex chemical and physical interactions on the ice mantles accreted on grains in molecular clouds numerous UV irradiation and diffusion experiments were performed. The irradiation of binary ices was studied. Using isotopic labelling on NH3/CO and NH3/O2 ices numerous compounds were identified, of which OCN(-), NO2(-), NO3(-), and NH4(+) ions reveal a new type of chemical reactions. It appeared that these compounds were formed by proton transfer reactions induced by the interaction between an acid (HNCO, HNO2, HNO3) and a base (NH3) through a hydrogen bond. This mechanism was confirmed by a study of photolyzed diluted argon mixtures. The main astrophysically relevant data from the overall study are presented. The 4.62 micron band in W33A can be reproduced with NH3/CO containing irradiated ices and was identified with OCN(-). The 6.87 micron band in W33A and other photostellar objects is reproduced with NH3/O2 containing ices and is identified with NH4(+)

Laboratory and observational study of the interrelation of the carbonaceous component of interstellar dust and solar system materials

By studying the chemical and isotopic composition of interstellar ice and dust, one gains insight into the composition and chemical evolution of the solid bodies in the solar nebula and the nature of the material subsequently brought into the inner part of the solar system by comets and meteorites. It is now possible to spectroscopically probe the composition of interstellar ice and dust in the mid-infrared, the spectral range which is most diagnostic of fundamental molecular vibrations. We can compare these spectra of various astronomical objects (including the diffuse and dense interstellar medium, comets, and the icy outer planets and their satellites) with the spectra of analogs we produce in the laboratory under conditions which mimic those in these different objects. In this way one can determine the composition and abundances of the major constituents of the various ices and place general constraints on the types of organics coating the grains in the diffuse interstellar medium. In particular we have shown the ices in the dense clouds contain H2O, CH3OH, CO, perhaps some NH3 and H2CO, we well as nitriles and ketones or esters. Furthermore, by studying the photochemistry of these ice analogs in the laboratory, one gains insight into the chemistry which takes place in interstellar/precometary ices. Chemical and spectroscopic studies of photolyzed analogs (including deuterated species) are now underway. The results of some of these studies will be presented and implications for the evolution of the biogenic elements in interstellar dust and comets will be discussed

SALT Spectropolarimetry and Self-Consistent SED and Polarization Modeling of Blazars

We report on recent results from a target-of-opportunity program to obtain spectropolarimetry observations with the Southern African Large Telescope (SALT) on flaring gamma-ray blazars. SALT spectropolarimetry and contemporaneous multi-wavelength spectral energy distribution (SED) data are being modelled self-consistently with a leptonic single-zone model. Such modeling provides an accurate estimate of the degree of order of the magnetic field in the emission region and the thermal contributions (from the host galaxy and the accretion disk) to the SED, thus putting strong constraints on the physical parameters of the gamma-ray emitting region. For the specific case of the $\gamma$-ray blazar 4C+01.02, we demonstrate that the combined SED and spectropolarimetry modeling constrains the mass of the central black hole in this blazar to $M_{\rm BH} \sim 10^9 \, M_{\odot}$.Comment: Submitted to Galaxies - Proceedings of "Polarized Emission from Astrophysical Jets", Ierapetra, Crete, June 12 - 16, 201

Methanol masers : Reliable tracers of the early stages of high-mass star formation

The GLIMPSE and MSX surveys have been used to examine the mid-infrared properties of a statistically complete sample of 6.7 GHz methanol masers. The GLIMPSE point sources associated with methanol masers are clearly distinguished from the majority, typically having extremely red mid-infrared colors, similar to those expected of low-mass class 0 young stellar objects. The intensity of the GLIMPSE sources associated with methanol masers is typically 4 magnitudes brighter at 8.0 micron than at 3.6 micron. Targeted searches towards GLIMPSE point sources with [3.6]-[4.5] > 1.3 and an 8.0 micron magnitude less than 10 will detect more than 80% of class II methanol masers. Many of the methanol masers are associated with sources within infrared dark clouds (IRDC) which are believed to mark regions where high-mass star formation is in its very early stages. The presence of class II methanol masers in a significant fraction of IRDC suggests that high-mass star formation is common in these regions. Different maser species are thought to trace different evolutionary phases of the high-mass star formation process. Comparison of the properties of the GLIMPSE sources associated with class II methanol masers and other maser species shows interesting trends, consistent with class I methanol masers tracing a generally earlier evolutionary phase and OH masers tracing a later evolutionary phase.Comment: 45 pages, 19 figures, accepted for publication in Ap

Molecular and mass spectroscopic analysis of isotopically labeled organic residues

Experimental studies aimed at understanding the evolution of complex organic molecules on interstellar grains were performed. The photolysis of frozen gas mixtures of various compositions containing H2O, CO, NH3, and CH4 was studied. These species were chosen because of their astrophysical importance as deducted from observational as well as theoretical studies of ice mantles on interstellar grains. These ultraviolet photolyzed ices were warmed up in order to produce refractory organic molecules like the ones formed in molecular clouds when the icy mantles are being irradiated and warmed up either by a nearby stellar source or impulsive heating. The laboratory studies give estimates of the efficiency of production of such organic material under interstellar conditions. It is shown that the gradual carbonization of organic mantles in the diffuse cloud phase leads to higher and higher visual absorptivity - yellow residues become brown in the laboratory. The obtained results can be applied to explaining the organic components of comets and their relevance to the origin of life

Implementation of digital pheromones in PSO accelerated by commodity Graphics Hardware

In this paper, a model for Graphics Processing Unit (GPU) implementation of Particle Swarm Optimization (PSO) using digital pheromones to coordinate swarms within ndimensional design spaces is presented. Previous work by the authors demonstrated the capability of digital pheromones within PSO for searching n-dimensional design spaces with improved accuracy, efficiency and reliability in both serial and parallel computing environments using traditional CPUs. Modern GPUs have proven to outperform the number of floating point operations when compared to CPUs through inherent data parallel architecture and higher bandwidth capabilities. The advent of programmable graphics hardware in the recent times further provided a suitable platform for scientific computing particularly in the field of design optimization. However, the data parallel architecture of GPUs requires a specialized formulation for leveraging its computational capabilities. When the objective function computations are appropriately formulated for GPUs, it is theorized that the solution efficiency (speed) can be significantly increased while maintaining solution accuracy. The development of this method together with a number of multi-modal unconstrained test problems are tested and presented in this paper

Laboratory simulation of the photoprocessing and warm-up of cometary and pre-cometary ices: Production of complex organic molecules

The recent missions to Comet Halley detected large quantities of organic material on grains as well as organic molecules in the gas phase. A possible origin of these materials is the energetic processing of ice mantles on the grains prior to comet formation, either in the pre-solar nebula or the interstellar medium. This process was simulated in the laboratory by depositing interstellar ice analogs (H2O/CH3OH/CO/NH3) on a cold (10 K) substrate with simultaneous UV irradiation. The material evaporating during warm-up of the photolyzed ice as well as the residue remaining at room temperature was analyzed by a number of techniques. It was found that a large number of organic molecules of various complexity are synthesized during the simulation process, stressing the possible significance of UV photolysis for producing the organic Comet material

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 H$_2$O, CO and, likely, CO$_2$ 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 H$_2$O are studied. In contrast with previous work, the strengths of the CO, CO$_2$, and H$_2$O 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 H$_2$O.Comment: 17 pages uuencoded compressed Postscript file-- includes all 6 figures (replaces most recent posting with only figs 2-5