Liton Majumdar

Study of the chemical evolution and spectral signatures of some interstellar precursor molecules of adenine, glycine & alanine

Abstract We carry out a quantum chemical calculation to obtain the infrared and electronic absorption spectra of several complex molecules of the interstellar medium (ISM). These molecules are the precursors of adenine, glycine & alanine. They could be produced in the gas phase as well as in the ice phase. We carried out a hydro-chemical simulation to predict the abundances of these species in the gas as well as in the ice phase. Gas and grains are assumed to be interacting through the accretion of various species from the gas phase onto the grain surface and desorption (thermal evaporation and photo-evaporation) from the grain surface to the gas phase. Depending on the physical properties of the cloud, the calculated abundances varies. The influence of ice on vibrational frequencies of different pre-biotic molecules was obtained using Polarizable Continuum Model (PCM) model with the integral equation formalism variant (IEFPCM) as default SCRF method with a dielectric constant of 78.5. Time dependent density functional theory (TDDFT) is used to study the electronic absorption spectrum of complex molecules which are biologically important such as, formamide and precursors of adenine, alanine and glycine. We notice a significant difference between the spectra of the gas and ice phase (water ice). The ice could be mixed instead of simple water ice. We have varied the ice composition to find out the effects of solvent on the spectrum. We expect that our study could set the guidelines for observing the precursor of some bio-molecules in the interstellar space.

Formation of Cyanoformaldehyde in the interstellar space

Cyanoformaldehyde (HCOCN) molecule has recently been suspected towards the Sagittarius B2(N) by the Green Bank telescope, though a confirmation of this observation has not yet been made. In and around a star forming region, this molecule could be formed by the exothermic reaction between two abundant interstellar species, H2CO and CN. Till date, the reaction rate coefficient for the formation ofthis molecule is unknown. Educated guesses were used to explain the abundance of this molecule by chemical modeling. In this paper, we carried out quantum chemical calculations to find out empirical rate coefficients for the formation of HCOCN and different chemical properties during the formation of HCOCN molecules. Though HCOCN is stable against unimolecular decomposition, this gas phase molecule could be destroyed by many other means, like: ion-molecular reactions or by the effect of cosmic rays. Ionmolecular reaction rates are computed by using the capture theories. We have also included the obtained rate coefficients into our large gas-grain chemical network to study the chemical evolution of these species in various interstellar conditions. Formation of one of the isotopologue(DCOCN) of HCOCN is also studied. Our study predicts the possibility of finding HCOCN and DCOCN in the ice phase with a reasonably high abundance. In order to detect HCOCN or DCOCN in various interstellar environments, it is necessary to know the spectroscopic properties of these molecules. To this effect, we carried out quantum chemical calculations to find out different spectral parameters of HCOCN for the transition in electronic, infrared and rotational modes. We clearly show how the isotopic substitution (DCOCN) plays a part in the vibrational progressions of HCOCN.

Deuterium Enrichment of the Interstellar Medium

Abstract Despite the low elemental abundance of atomic deuterium in the interstellar medium (ISM), observational evidence suggests that several species, both in the gas phase and in ices, could be heavily fractionated. We explore various aspects of deuterium enrichment by constructing a chemical evolution model in both gaseous and granular phases. Depending on various physical parameters, gases and grains are allowed to interact with each other through the exchange of their chemical species. It is known that HCO+ and N2H+ are two abundant gas phase ions in the ISM and, their deuterium fractionation is generally used to predict the degree of ionization in the various regions of a molecular cloud. For a more accurate estimation, we consider the density profile of a collapsing cloud. The radial distributions of important interstellar molecules, along with their deuterated isotopomers, are presented. Quantum chemical simulations are computed to study the effects of isotopic substitution on the spectral properties of these interstellar species. We calculate the vibrational (harmonic) frequencies of the most important deuterated species (neutral and ions). The rotational and distortional constants of these molecules are also computed in order to predict the rotational transitions of these species. We compare vibrational (harmonic) and rotational transitions as computed by us with existing experimental and theoretical results. It is hope that our results will assist observers in detecting several hitherto unobserved deuterated species.

Hydro-chemical study of the evolution of interstellar pre-biotic molecules during the collapse of molecular clouds

One of the stumbling blocks for studying the evolution of interstellar molecules is the lack of adequate knowledge about the rate coefficients of various reactions which take place in the interstellar medium and molecular clouds. Some theoretical models of rate coefficients do exist in the literature for computing abundances of complex pre-biotic molecules. So far these have been used to study the abundances of these molecules in space. However, in order to obtain more accurate final compositions in these media, we have calculated the rate coefficients for the formation of some of the most important interstellar pre-biotic molecules by using quantum chemical theory. We use these rates inside our hydro-chemical model to examine the chemical evolution and final abundances of pre-biotic species during the collapsing phase of a proto-star. We find that a significant amount of various pre-biotic molecules could be produced during the collapse phase of a proto-star. We thoroughly study the formation of these molecules via successive neutral-neutral and radical-radical/radical-molecular reactions. We present the time evolution of the chemical species with an emphasis on how the production of these molecules varies with the depth of a cloud. We compare the formation of adenine in interstellar space using our rate-coefficients and using those obtained from existing theoretical models. Formation routes of the pre-biotic molecules are found to be highly dependent on the abundances of the reactive species and the rate coefficients involved in the reactions. The presence of grains strongly affects the abundances of the gas phase species. We also carry out a comparative study between different pathways available for the synthesis of adenine, alanine, glycine and other molecules considered in our network. Despite the huge abundances of the neutral reactive species, production of adenine is found to be strongly dominated by the radical-radical/radical-molecular reaction pathways. If all the reactions considered here contribute to the production of alanine and glycine, then neutral-neutral and radical-radical/radical-molecular pathways are both found to have a significant part in the production of alanine. Moreover, radical-radical/radical-molecular pathways also play a major role in the production of glycine.

Detection of CH3SH in protostar IRAS 16293-2422

The nature of the main sulphur reservoir in star forming regions is a long standing mystery. The observed abundance of sulphur-bearing species in dense clouds is only about 0.1 per cent of the same quantity in diffuse clouds. Therefore, the main sulphur species in star forming regions of the interstellar medium are still unknown. IRAS 16293-2422 is one of the regions where production of S-bearing species is favourable due to its conditions which allows the evaporation of ice mantles. We carried out observations in the 3 mm band towards the solar type protostar IRAS 16293-2422 with the IRAM 30m telescope. We observed a single frequency setup with the EMIR heterodyne 3 mm receiver with an Lower Inner (LI) tuning frequency of 89.98 GHz. Several lines of the complex sulphur species CH3SH were detected. Observed abundances are compared with simulations using the NAUTILUS gas-grain chemical model. Modelling results suggest that CH3SH has the constant abundance of 4e-9 (compared to H2) for radii lower than 200 AU and is mostly formed on the surfaces. Detection of CH3SH indicates that there may be several new families of S-bearing molecules (which could form starting from CH3SH) which have not been detected or looked for yet.

METHYL ACETATE AND ITS SINGLY DEUTERATED ISOTOPOMERS IN THE INTERSTELLAR MEDIUM

Methyl acetate (CH_3COOCH_3) has been recently observed by IRAM 30 m radio telescope in Orion though the presence of its deuterated isotopomers is yet to be confirmed. We therefore study the properties of various forms of methyl acetate, namely, CH_3COOCH_3, CH_2DCOOCH_3 and CH_3COOCH_2D. Our simulation reveals that these species could be produced efficiently both in gas as well as in ice phases. Production of methyl acetate could follow radical-radical reaction between acetyl (CH_3CO) and methoxy (CH_3O) radicals. To predict abundances of CH_3COOCH_3 along with its two singly deuterated isotopomers and its two isomers (ethyl formate and hydroxyacetone), we prepare a gas-grain chemical network to study chemical evolution of these molecules. Since gas phase rate coefficients for methyl acetate and its related species were unknown, either we consider similar rate coefficients for similar types of reactions (by following existing data bases) or we carry out quantum chemical calculations to estimate the unknown rate coefficients. For the surface reactions, we use adsorption energies of reactants from some earlier studies. Moreover, we perform quantum chemical calculations to obtain spectral properties of methyl acetate in infrared and sub-millimeter regions. We prepare two catalog files for the rotational transitions of CH_2DCOOCH_3 and CH_3COOCH_2D in JPL format, which could be useful for their detection in regions of interstellar media where CH_3COOCH_3 has already been observed.

Chemical evolution during the process of proto-star formation by considering a two dimensional hydrodynamic model

Abstract Chemical composition of a molecular cloud is highly sensitive to the physical properties of the cloud. In order to obtain the chemical composition around a star forming region, we carry out a two dimensional hydrodynamical simulation of the collapsing phase of a proto-star. A total variation diminishing scheme (TVD) is used to solve the set of equations governing hydrodynamics. This hydrodynamic code is capable of mimicking evolution of the physical properties during the formation of a proto-star. We couple our reasonably large gas-grain chemical network to study the chemical evolution during the collapsing phase of a proto-star. To have a realistic estimate of the abundances of bio-molecules in the interstellar medium, we include the recently calculated rate coefficients for the formation of several interstellar bio-molecules into our gas phase network. Chemical evolution is studied in detail by keeping grain at the constant temperature throughout the simulation as well as by using the temperature variation obtained from the hydrodynamical model. By considering a large gas-grain network with the sophisticated hydrodynamic model more realistic abundances are predicted. We find that the chemical composition are highly sensitive to the dynamic behavior of the collapsing cloud, specifically on the density and temperature distribution.

Formation of Different Isotopomers of Chloronium in the Interstellar Medium

The main focus of this paper is to explore the possibility of finding two deuterated isotopomers of H2Cl+ (chloronium) in and around the interstellar medium. The presence of a chloronium ion has recently been confirmed by the Herschel Space Observatorys Heterodyne Instrument for the far-infrared. It observed para-chloronium toward six sources in the Galaxy. To date the existence of its deuterated isotopomers (HDCl+ and D2Cl+) have not been discussed in the literature. We find that these deuterated gas phase ions could be destroyed by various ion-molecular reactions, dissociative recombination (DR), and cosmic rays (CRs). We compute all of the ion-molecular (polar) reaction rates by using the parameterized trajectory theory and the ion-molecular (non-polar) reaction rates by using the Langevin theory. For DR- and CR-induced reactions, we adopt two well-behaved rate formulas. We also include these rate coefficients in our large gas-grain chemical network to study the chemical evolution of these species around the outer edge of the cold, dense cloud. In order to study spectral properties of the chloronium ion and its two deuterated isotopomers, we have carried out quantum chemical simulations. We calculated ground-state properties of these species by employing second-order Moller-Plesset perturbation theory (MP2) along with quadruple-zeta correlation consistent (aug-cc-pVQZ) basis set. Infrared and electronic absorption spectra of these species are calculated by using the same level of theory. The MP2/aug-cc-pVQZ level of theory is used to report the different spectroscopic constants of these gas phase species. These spectroscopic constants are essential to predict the rotational transitions of these species. Our predicted column densities of D2Cl+, HDCl+, along with spectral information may enable their future identification around outer edges of cold, dark clouds.

Deuterium enrichment of the interstellar grain mantle

We carry out Monte-Carlo simulation to study deuterium enrichment of interstellar grain mantles under various physical conditions. Based on the physical properties, various types of clouds are considered. We find that in diffuse cloud regions, very strong radiation fields persists and hardly a few layers of surface species are formed. In translucent cloud regions with a moderate radiation field, significant number of layers would be produced and surface coverage is mainly dominated by photo-dissociation products such as, C,CH_3,CH_2D,OH and OD. In the intermediate dense cloud regions (having number density of total hydrogen nuclei in all forms ~ 2 x 10^4 cm^-3), water and methanol along with their deuterated derivatives are efficiently formed. For much higher density regions (~ 10^6 cm^-3), water and methanol productions are suppressed but surface coverage of CO,CO_2,O_2,O_3 are dramatically increased. We find a very high degree of fractionation of water and methanol. Observational results support a high fractionation of methanol but surprisingly water fractionation is found to be low. This is in contradiction with our model results indicating alternative routes for de-fractionation of water. Effects of various types of energy barriers are also studied. Moreover, we allow grain mantles to interact with various charged particles (such as H^+, Fe^+,S^+ and C^+) to study the stopping power and projected range of these charged particles on various target ices.

Monte Carlo simulation to investigate the formation of molecular hydrogen and its deuterated forms

Abstract H 2 is the most abundant interstellar species, and its deuterated forms (HD and D 2 ) are also present in high abundance. The high abundance of these molecules could be explained by considering the chemistry that occurs on interstellar dust. Because of its simplicity, the rate equation method is widely used to study the formation of grain-surface species. However, because the recombination efficiency for the formation of any surface species is highly dependent on various physical and chemical parameters, the Monte Carlo method is best suited for addressing the randomness of the processes. We perform Monte Carlo simulations to study the formation of H 2 , HD and D 2 on interstellar ice. The adsorption energies of surface species are the key inputs for the formation of any species on interstellar dusts, but the binding energies of deuterated species have yet to be determined with certainty. A zero-point energy correction exists between hydrogenated and deuterated species, which should be considered during modeling of the chemistry on interstellar dusts. Following some previous studies, we consider various sets of adsorption energies to investigate the formation of these species under diverse physical conditions. As expected, notable differences in these two approaches (rate equation method and Monte Carlo method) are observed for the production of these simple molecules on interstellar ice. We introduce two factors, namely, S f and β , to explain these discrepancies: S f is a scaling factor, which can be used to correlate the discrepancies between the rate equation and Monte Carlo methods, and β indicates the formation efficiency under various conditions. Higher values of β indicate a lower production efficiency. We observed that β increases with a decrease in the rate of accretion from the gas phase to the grain phase.