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The rates for the 17O(p,a)14N, 17O(p,?)18F and 18O(p,a)15N reactions deduced trough the Trojan Horse Method (THM) have been introduced into a state-of-the-art asymptotic giant branch (AGB) models for proton-capture nucleosynthesis and cool bottom process. The predicted abundances have been compared with isotopic compositions provided by geochemical analysis of presolar grains. As a result, an improved agreement is found between the models and the isotopic mix of oxide grains of AGB origins, whose composition is the signature of low-temperature proton-capture nucleosynthesis. [ABSTRACT FROM AUTHOR]

Among presolar materials recovered in meteorites, abundant SiC and Al2O3 grains of AGB origins were found. They showed records of C, N, O, 26Al and s-element isotopic ratios that proved invaluable in constraining the nucleosynthesis models for AGB stars [1, 2]. In particular, when these ratios are measured in SiC grains, they clearly reveal their prevalent origin in cool AGB circumstellar envelopes and provide information on both the local physics and the conditions at the nucleosynthesis site (the H- and He-burning layers deep inside the structure). Among the properties ascertained for the main part of the SiC data (the so-called mainstream ones), we mention a large range of 14N/15N ratios, extending below the solar value [3], and 12C/13C ratios ≳ 30. Other classes of grains, instead, display low carbon isotopic ratios (≳ 10) and a huge dispersion for N isotopes, with cases of large 15N excess. In the same grains, isotopes currently feeded by slow neutron captures reveal the characteristic pattern expected from this process at an efficiency slightly lower than necessary to explain the solar main s-process component. Complementary constraints can be found in oxide grains, especially Al2O3 crystals. Here, the oxygen isotopes and the content in 26Al are of a special importance for clarifying the partial mixing processes that are known to affect evolved low-mass stars. Successes in modeling the data, as well as problems in explaining some of the mentioned isotopic ratios through current nucleosynthesis models are briefly outlined. [ABSTRACT FROM AUTHOR]

We investigate the effects of grain size distribution and grain growth on molecular abundances during the chemical evolution of a cold dense interstellar cloud using a gas-grain numerical code. Dense interstellar clouds are the birth place of stellar systems like ours. Most models with grain surface chemistry have used so-called classical grains with canonical dust to gas ratio as 1:100, characterized by a radius of 0.1 μm and number density of 1.33 × 10-12 ηH, where ηH is the number density of hydrogen in all forms. We considered two different size distributions based on earliermodels and compared our findingswith classical grains. To incorporate different granular sizes, we divided the distribution of grain sizes into numbers of logarithmically equally-spaced ranges, integrated over each range to find its total granular number density, and assigned that number density to an average size in that range. Then we calculated rate coefficients for accretion, surface reactions and desorption as a function of grain size. We then followed the chemical evolution of the surface populations of these grains along with the gas phase chemistry for 10 Million years. We found that the effective surface area of a grain size (product of number density and grain cross section) is an important parameter. The fractional abundances of surface species on grains within a given distribution scale with the effective surface areas of the grain distribution components in the absence of grain growth. We found that the grain growth increases the grain size considerably which in turn increases the rate of depletion of molecules (due to higher accretion rate), such as CO, produced in the gas phase, which results in lower gas-phase abundances and higher surface abundances. For the first time, these results helps to verify the quality of the classical grain approximation for cold cloud models. Further, it also provides an important basis for future study that may require size distributions. [ABSTRACT FROM AUTHOR]

In recent years, there has seen a very rapid growth in the investigation of model interstellar ices using ultrahigh vacuum surface science methodologies. One of the areas where the application of such techniques has proved to be particularly fruitful has been the investigation of the desorption processes that icy solids may undergo in astrophysical environments. Surface science techniques have allowed us to investigate both the thermal and non-thermal desorption mechanisms that might be at play in such astrophysical environments and extract useful information on the mechanisms and rates of such process. In turn, these have provided molecular astrophysicists with a more realistic picture of the behaviour of icy grains from which to construct their models of a range of environments in which is processing and desorption plays a crucial role. This article provides a review of the literature relating UHV surface science studies of thermal and non-thermal desorption relevant to astronomy and astrophysics from the perspective of an experimental group engaged in these studies. [ABSTRACT FROM AUTHOR]

It is now confirmed that observed hydrogen molecules in the interstellar medium are formed on the grain surfaces. Dust grains behave like a catalyst for the formation of the hydrogen molecule. After accretion from the gas phase H atoms randomly hopped around the surface to find out the reactant partner. When two H atoms combine a single hydrogen molecule is formed. So far, the average recombination time to produce a hydrogen molecule used to be computed by a crude assumption about the diffusion of the H atom on a grain surface. Here, we perform a Monte-Carlo simulation to exactly quantify the recombination rates for the formation of hydrogen molecule on the grain surface. During our simulation, it is noticed that the production rate is highly dependent upon the grain size and the accretion rate. Since molecular hydrogen is the most abundant species in the ISM, changes in its production rate could affect the other species also. We mainly carried our our simulation for two kind of grains namely, Amorphous Carbon and Olivine because they are very abundant in the ISM. [ABSTRACT FROM AUTHOR]

The main aim of this paper is to study the fomation of various complex molecules in the Interstellar Medium (ISM) via gas-grain interaction by taking into account more realistic physical conditions in or around the molecular cloud. Due to this reason, we carry out a two dimensional hydrodynamical simulation of the collapsing phase of a proto-star. Total variation diminishing scheme (TVD) is used to study the properties of the Interstellar collapsing cloud. Our hydrodynamic model is capable of mimicing evolution of the physical properties during the formation of a proto-star. Arm with this sophisticated hydrodynamical model we couple our reasonably large chemical network for gas and grain, to study the chemical evolution during these processes. Our model confirms that the chemical composition are highly sensitive to the dynamic behavior of the collapsing cloud, specifically on the density and temperature distribution. [ABSTRACT FROM AUTHOR]

Diffuse Interstellar Bands (DIBs) are optical absorption features on the interstellar extinction curve. Ultra-high resolution spectroscopic observations suggest that at least some of these features are due to large molecules. Observational results also reveal that the strengths of the DIBs are not strongly correlated with each other, implying that there must be several carriers. Considering the wide range of interstellar species and the cost and duration of experimental work that is involved to determine the carriers of DIBs, Density Functional Theory (DFT) and Time Dependent Density Functional Theory (TDDFT) calculations offer opportunities to investigate which molecules are suitable for laboratory studies. The widespread presence of Polycyclic Aromatic Hydrocarbons (PAHs) in astrophysical environments is known from observations of the Unidentified Infrared (UIR) emission bands. Since PAHs are stable enough to be present under interstellar conditions, they are good candidates to be the carriers of the DIBs. We report DFT and TDDFT calculations to predict electronic transitions of neutral as well as protonated-deuteronated PAHs with various sites of protonation and deuteronation. The PAH molecules considered for calculation include pyrene, perylene, coronene and heptacene. Compared to their neutral forms, these charged isoelectronic forms of PAHs are predicted to have active transitions in the visible region, which means they are suitable candidates as carriers for some of the DIBs. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbon (PAH) molecules are responsible for the mid-infrared emission features. Their ubiquitous presence in almost all types of astrophysical environments and related variations in their spectral profilesmake them an important tool to understand the physics and chemistry of the interstellar medium. The observed spectrum is generally a composite superposition of all different types of PAHs possible in the region. In the era of space telescopes the spectral richness of the emission features has enhanced their importance as probe and also the need to understand the variations with respect to PAH size, type and ionic state. Quantum computational studies of PAHs have proved useful in elucidating the profile variations and put constraints on the possible types of PAHs in different environments. The study of PAHs has also significantly contributed to the problems of diffuse interstellar bands (DIBs), UV extinction and understanding the chemistry of the formation of complex organics in space. The review highlights the results of various computational models for the understanding of infrared emission features, the PAH-DIB relation, formation of prebiotics and possible impact in the understanding of far-infrared features. [ABSTRACT FROM AUTHOR]

In this paper, we discuss the chemistry that occurs in the cold dense interstellar medium and as temperatures rise above standard interstellar cloud values due to star formation or other energetic processes. During the warm up to the hot core stage (100-300 K), a radical photochemistry on grain surfaces leads to the formation of complex organic molecules (COMs), which then desorb into the gas, where they are detected. At still higher temperatures, it is necessary to consider a large number of additional gas-phase reactions that become important. We emphasize a new high-temperature gas-phase network that is capable of being used in simulations of the chemistry that occurs at temperatures up to 800-1000 K. Examples are given. It is also possible that a very high temperature surface chemistry, in which species other than atomic hydrogen stick to grains via strong chemisorption forces, occurs. [ABSTRACT FROM AUTHOR]

We report interstellar Si depletion and dust-phase column densities of Si along 131 Galactic sight lines using previously reported gas-phase Si II column densities, after correcting for the differences in oscillator strengths. With our large sample, we could reproduce the previously reported correlations between depletion of Si and average density of hydrogen along the line of sight () as well as molecular fraction of hydrogen (f(H2). We have also studied the variation of amount of Si incorporated in dust with respect to different extinction parameters. With the limitations we have with the quality of data, we could find a strong relation between the Si dust and extinction. While we cannot predict the density dependent distribution of size of Si grains, we discuss about the large depletion fraction of Si and the bigger size of the silicate grains. [ABSTRACT FROM AUTHOR]

The formation of molecules in the interstellar medium is an important topic of research nowadays because molecules play important roles in astrophysics and astrochemistry. Molecular hydrogen, the most abundant molecule, helps the cooling of collapsing clouds and through the study of its transitions, it gives information on the conditions of the ISM (Interstellar Medium) environment it finds itself in. While neutral or ionized, it intervenes in reactions to form most molecules that have been detected in the ISM. Water is the main component of ices coating dust grain; ices are the place of the formation and storage of molecules of biogenic interest. Furthermore, how water interacts with grains is of importance in understanding the delivery of water to planetary bodies. The formation of molecular hydrogen and water occurs largely on dust grains. Here we review the most important experimental discoveries about the formation of molecular hydrogen, and we present new work on the formation of water on ices and on grain analogs. [ABSTRACT FROM AUTHOR]

We studied the formation of H2 on the interstellar dust grain using continuous-time random-walk (CTRW) Monte Carlo technique. Both physisorption and chemisorption sites were considered for the adsorption of H atoms on the surface of olivine. In our model, an incoming H atom was first given a physisorption site to adsorb to the surface and then it was allowed to move to a chemisorption site depending on the probability of reaction. We considered hydrogen atom mobility due to both thermal hopping and quantum mechanical tunneling. We considered a wide range of temperature ranging from 5 K to 825 K representing different interstellar regions, we also considered various values of incoming H flux which corresponds to different hydrogen number density (0.1 cm-3 to 100 cm-3). We also used rough surface with multiple binding sites. It is found that tunneling dominates the surface chemistry at low temperature for all the cases, but as the temperature increases, the thermal hopping starts dominating. Surface with chemisorption sites can produce H2 at very high temperature also (as high as 700 K depending upon the depth of the chemisorption well) which is not possible when only physisorption sites are considered. [ABSTRACT FROM AUTHOR]

This work reports the spectral information of some interstellar complex molecules which could be treated as the precursor molecules for the formation of some bio-molecules in the interstellar medium (ISM). We carry out quantum chemical simulation to consider the Core correlation and vibrational corrections to the rotational constants and centrifugal distortion constants which are computed from harmonic and anharmonic force fields obtained at MP2/6-311G(d,p) level of theory. These precursor molecules could be produced in the gas phase as well as in the ice phase. This prompted use to couple the hydrodynamics of the collapsing phase of the protostar with our reasonably large chemical network including the gas phase as well as the grain phase chemical network, to study the chemical evolution of these species during the collapsing phase of a proto-star. We have noticed the Significant differences between spectroscopy of these species in the gas as well as in ice (water ice) phase due to the solute-solvent elctrostatic interactions. Time dependent density functional theory (TDDFT) is used to study the UV-VIS spectrum of these complexmolecules which are biologically important. Interstellar grain mantle around the dense cloud (> 104 cm-3) mainly composed by 60-70% Water, 5-30% Methanol and 2-20% CO2. So in reality, the ice could be mixed instead of simple water ice. To have an idea about the real spectra, we carry out our simulations for the mixed ice also by considering the actual composition of the above species in interstellar grain mantle. Spectral signatures are found to be significantly shifted with change of the solvent which confirms that the polarization of the solute by the continum has important effects on the absolute and relative solvation energies. Our simulated spectrum are in good agreement with some of the recent experimental result. We expect that our quantum chemical approach along with the hydro-chemical study might be useful for the observer to predict the abundances of some bio-molecules based on the chemical abundances of their precursor molecules. [ABSTRACT FROM AUTHOR]

Progress in the ultra high vacuum surface experiments allows us to simulate environments that exist in the dense interstellar medium. This enable us to study various physical and chemical parameters which are needed to explain the formation of complex molecules on the interstellar dust grains. Dense clouds have densities between 104 and 107 cm-3 and temperature as low as 8 K and impinging flux of molecular and atomic reactants. In this proceeding, we discussed on a new laboratory set-up which was recently built at S N Bose National Centre for Basic Sciences and presented thermal desorption study of air on olivine substrate which is a close analog of interstellar dust grains. [ABSTRACT FROM AUTHOR]

We investigated the dust properties and magnetic field orientation towards the star cluster NGC 1931. Cluster members have been identified using both optical color-color as well as QV - UV stokes plane diagrams. Thus identified probable cluster members have been used to estimate the distance to the cluster NGC 1931 which is found to be 2.3±0.3 kpc. Few young stellar objects with possible intrinsic polarization have been identified. One-dimensional surface stellar density contours suggest two clustering in the NGC 1931 region. The reddening E(B - V) is found to be variable between 0.50 to 0.90 mag. Polarimetric and photometric results indicate for the presence of slightly bigger dust grains within the intra-cluster medium. Although our previous work (Eswaraiah et al. 2011) reveals uniform dust grain alignment in the foreground medium towards the direction of NGC 1931, the polarization efficiency is found to be less than that of the general diffuse interstellar medium. This could be due to the normal extinction law in the foreground medium and anomalous reddening law in the intra-cluster medium. [ABSTRACT FROM AUTHOR]

To mimic the exact interstellar condition, gas grain interactions via accretion from the gas phase and desorption from the grain surface are considered. Chemical composition of the grain mantle is noticed to be highly dependent on the physical parameters associated with a molecular cloud. Around the lower visual extinction (Aν ≤ 6), Interstellar photons are seen to play an important role via photo-dissociation and photo-evaporation towards the chemical composition of the interstellar grain mantle. We have studied the effects of photo-reactions by varying the number of layers which could be affected by it. Model calculations are carried out for a realistic time dependent case (i.e., extinction parameter and number density of the cloud both are changing with time) to follow the chemical evolution during the collapsing phase of a proto-star. Different routes to the formation of water molecules are extensively studied and it is noticed that around the dense region, production of water molecules via O3 and H2O2 contributes significantly. [ABSTRACT FROM AUTHOR]

Comets provide a wonderful laboratory for studying the ion pickup process. As a comet nears the Sun, neutral atoms and molecules sublime from the nucleus and from ice grains in the coma, providing an extended water-rich region. Photoionisation and charge exchange provide a source of new pickup ions, which immediately interact with the electric and magnetic field in the surrounding plasma. The missions to comets Halley, Giacobini-Zinner, Grigg-Skjellerup and Borrelly provided a wealth of data on pickup ions and their interaction with plasma waves, including observations of ring and bispherical shell distributions and mass loading over significant regions of space. At comet Grigg-Skjellerup, non-gyrotropic pickup ions were also seen. Here, we review the observations and interpretation of pickup ions at comets, illustrating what was learned from the cometary missions and following the evolution of the ion distribution. We also discuss the international Rosetta mission which will orbit rendezvous with and then orbit comet Churyumov-Gerasimenko as it nears the Sun and as activity develops. The interaction will change significantly as this occurs in 2014-15. We also compare the pickup process at comets with that observed at other unmagnetized objects, including Titan, Enceladus, Venus and Mars, using data from Cassini, Venus Express and Mars Express. [ABSTRACT FROM AUTHOR]

Theoretical scenarios for dark matter searches are briefly reviewed. In particular, neutralinos and sneutrinos in R-parity conserving models, as well as gravitinos in R-parity violating models, are discussed. [ABSTRACT FROM AUTHOR]

The reaction of cold H and D atoms with solid formaldehyde (D2CO and H2CO) deposited on amorphous H2O ice at 15 K was studied experimentally by in-situ FTIR spectroscopy. New routes of formation of deuterated formaldehyde (HDCO and D2CO) were identified. In the reaction of D atoms with H2CO, H-D substitution was observed, H2CO → HDCO → D2CO, and for the reaction of H atoms with D2CO, the analogous D-H substitution was observed, D2CO → HDCO → H2CO. In the reaction of D atoms with H2CO, the rate of H-D substitution was greater than that of formation of deuterated methanol-d2–4 by addition of 2D atoms to formaldehyde-d0–2. However, in the reaction of H atoms with D2CO, the rate of D-H substitution was comparable to that of addition of 2H atoms to D2CO. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

A first step, before studying the formation of molecules on surfaces at low temperature by recombination of atoms, is a good understanding of the adsorption/desorption mechanisms of the molecules themselves and a correct interpretation of the results of TPD (Thermally Programmed Desorption) experiments. Experimental studies (performed on our newly built experimental setup “FORMOLISM”) of adsorption and desorption of molecular hydrogen and deuterium on an amorphous porous solid water (ASW) ice surface between 10 and 35 K reveal a very efficient isotopic segregation process. The slight difference in the maximum desorption temperature between the two isotopes leads to a preferential coverage of the sites binding more tightly of the ice surface by D2, independent of the gas deposition process history. A statistical model, which takes into account thermodynamic aspects of adsorption sites as well as isotopic competition, is proposed to understand the enhancement of deuterium fractionation. Model results are in very good agreement with the experimental ones. This mechanism could play a key role in chemistry at the surface of interstellar dust grains. It could in particular explain the isotopic enrichment observed in some dark clouds. The next step will be to measure the rovibrational energy of the molecular hydrogen formed on such surfaces using a REMPI-TOF detection which allows for selective quantum state identification. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The ability of high resolution X-ray spectroscopy to directly probe the grain composition of the ISM is discussed. Using experimental data taken at the National Synchrotron Light Source at Brookhaven National Laboratory and the Advanced Photon Source at Argonne National Laboratory, current and near-future mission prospects for (1) distinguishing gas from dust phase absorption, and (2) determining the chemical composition of interstellar grains, through the details of X-ray absorption fine structure is assessed. Details, in-depth discussions and complete references can be found in Lee & Ravel 2005. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Our current understanding of the absorption and emission properties of interstellar grains are reviewed. The constraints placed by the Kramers-Kronig relation on the wavelength-dependence and the maximum allowable quantity of the dust absorption are discussed. Comparisons of the opacities (mass absorption coefficients) derived from interstellar dust models with those directly estimated from observations are presented. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

This paper summarizes the evidence of the ultraviolet properties of dust grains found in starburst galaxies. Observations of starburst galaxies clearly show that the 2175 Å feature is weak or absent. This can be the result of radiative transfer effects (mixing the dust and stars) or due to dust grains which do not have this feature. Spherical DIRTY radiative transfer models imply that it is not radiative transfer effects, but other radiative transfer models with disk/bulge geometries have found cases where it could be radiative transfer effects. Recent work on the extinction curves in the Magellanic Clouds and Milky Way has revealed that the traditional explanation of low metallicity for the absence of the 2175 Å feature in the Small Magellanic Cloud is likely incorrect. The SMC has one sightline with a 2175 Å feature and the Milky Way has sightlines without this feature. In addition, where the 2175 Å feature is found to be weak or absent in both Magellanic Clouds and the Milky Way, there is evidence for recent star formation. Taking the sum of the radiative transfer modeling of starburst galaxies and the behavior of Local Group extinction curves, it is likely that the dust grains in starburst galaxies intrinsically lack the 2175 Å feature. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

We use the radiation transfer simulation of Xilouris et al. to constrain the quantity of dust in three nearby spiral galaxies (NGC 4013, NGC 5907 and NGC 891). The predicted visual optical depth from the model is compared with the thermal continuum radiation detected from these galaxies at 850 μm. This comparison yields the emissivity of dust grains in the submillimeter waveband which is a factor 4 higher than the benchmark, semi-empirical model of Draine & Lee. Our estimates are more closely aligned with recent measurements carried out in the laboratory on amorphous carbon and silicate particulates. A comparison between the distribution of 850 μm surface brightness and the intensity levels in the 12CO(1-0) and 21 cm lines underlines the spatial association between dust detected in the submillimeter waveband and molecular gas clouds. We suggest that the relatively high emissivity values that we derive may be attributable to amorphous, fluffy grains situated in denser gas environments. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

A number of phenomena connected with dust grains within the solar system can be explained by their electric charging. In the presented paper, we discuss the influence of the grain diameter on the changes of equilibrium potential of spherical grains from glass and pure SiO[sub 2] in a laboratory experiment which is based on the levitation of a single dust grain in the AC electric field of a quadrupole. The yield of secondary emission derived from the measurements exhibits a strong dependence on the grain diameter. For larger grains, it reveals the properties of planar surfaces, whereas it can be higher for one micron grains and 10 keV of the primary beam energy. The rise of the secondary emission yield was attributed to the emission from the "back side" of the grain which starts when the penetration depth of primary electrons becomes comparable with the grain size. The results are discussed in view of present models of this process and several correction of the models are suggested. [ABSTRACT FROM AUTHOR]

The dust grains or their clusters can be frequently found in many space environments — interstellar clouds, tails of comets or planetary tings are only typical examples. Being surrounded by plasma, dust grains are charged by various processes as photoemission, collection of electrons and/or ions, secondary or field emissions. The determination of a charge, which can the grain or cluster of grains gain by these processes, is very important for an estimation of the dusty plasma properties because electrostatic forces highly dominate the gravitation. However, our knowledge of charging processes is inferred from measurements on large planar samples and thus it can be applied on micron-sized grains only with a care and their application on the clusters is under question. Our experimental set-up allows us to trap a single dust grain or cluster of grains in an electrodynamic quadrupole and to expose it by the electron and/or ion beams. Precise measurements of a stepwise change of charge-to-mass ratio induced by the elementary charge allow us to determine the charge and mass of the investigated grain. We have investigated SiO[sub 2] spherical grains as well as their clusters. A systematic study reveals that the dependence of the charge acquired by an investigated object (grain or cluster of grains) by the particle beam of given energy depends not only on grain macroscopic characteristics but on the configuration of the grains in a particular cluster. [ABSTRACT FROM AUTHOR]

Five silicon carbide and two graphite presolar grains, whose isotopic signatures point toward a nova origin, are described. In addition, theoretical isotopic ratios of several nuclear species, ranging from C to Si, computed with hydrodynamic models of nova outbursts, are shown. Such theoretical information can be useful for future identification of nova candidate grains. [ABSTRACT FROM AUTHOR]

Micron sized, presolar grains of SiC and graphite (as well as other phases not discussed here) found in mineral separates of primitive meteorites constitute a new source of cosmic material. The grains are identified by anomalous isotopic ratios in both major and minor elements. In some cases, these differ from average solar system values by a factor >10[sup 3]. The SiC grains can be separated into distinct isotopic families representing different stellar sources. Only two types are discussed here—“mainstream” SiC grains, constituting ∼98% of the total and less abundant X-grains that constitute <1% of the total. The former have isotopic ratios characteristic of S-process nucleosynthesis and are believed to be condensates formed in the atmospheres of AGB stars. In contrast, X grains have isotopic patterns characteristic of R-process nucleosynthesis and are believed to be supernovae condensates. In addition to giving a wealth of new detail on the nuclear processes in different stars, the grains open up entirely new avenues of research. For example, electron microscope mineralogical/petrographic studies of microtomed grains give insight on the processes of grain growth. Molecules of polycyclic aromatic hydrocarbons (PAHs) are common in the graphite grains. Isotopic measurements demonstrate that some of the PAHs formed from the same suite of atoms as the parent grains and are thus indigenous. The relationship of presolar grains to cosmic ray physics is briefly considered. Overall, the grain work serves to deepen the mystery of the similarity of isotopic ratios measured in cosmic rays to those of Solar System materials. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]