Gas-Grain Modeling of Interstellar O2

Molecular oxygen(O2)is essential to human beings on the earth.Although elemental oxygen is rather abundant,O2 is rare in the interstellar medium.It was only detected in two galactic and one extra-galactic region.The inconsistency between observations and theoretical studies is a big challenge for astrochemical models.Here we report a two-phase modeling research of molecular oxygen,using the Nautilus gas-grain code.We apply the isothermal cold dense models in the interstellar medium with two typical sets of initial elemental abundances,as well as the warm-up models with various physical conditions.Under cold dense conditions,we nd that the timescales for gas-phase CO,O2 and H2O to reach peak values are dependent on the hydrogen density and are shortened when hydrogen density increases.In warm-up models,O2 abundances are in good agreement with observations at temperatures rising after 10^5 yr.In both isothermal and warm-up models,the steady-state O2 fractional abundance is independent of the hydrogen density,as long as the temperature is high enough(>30 K),at which O2 is prevented from signi cant depleting onto grain surface.In addition,low density is preferable for the formation of O2,whether molecular oxygen is under cold conditions or in warm regions.

Formation of cyanoallene(buta-2, 3-dienenitrile) in the interstellar medium: a quantum chemical and spectroscopic study

The interstellar medium,filling the vast space between stars,is a rich reservoir of molecular material ranging from simple diatomic molecules to more complex,astrobiologically important molecules such as vinylcyanide,methylcyanodiaccetylene,cyanoallene,etc.Interstellar molecular cyanoallene is one of the most stable isomers of methylcynoacetylene.An attempt has been made to explore the possibility of forming cyanoallene in interstellar space by radical-radical and radical-molecule interaction schemes in the gaseous phase.The formation of cyanoallene starting from some simple,neutral interstellar molecules and radicals has been studied using density functional theory.The reaction energies and structures of the reactants and products show that the formation of cyanoallene is possible in the gaseous phase.Both of the considered reaction paths are totally exothermic and barrierless,thus giving rise to a high probability of occurrence.Rate constants for each step in the formation process of cyanoallene in both the reaction paths are estimated.A full vibrational analysis has been attempted for cyanoallene in the harmonic and anharmonic approximations.Anharmonic spectroscopic parameters such as rotational constants,rotation-vibration coupling constants and centrifugal distortion constants have been calculated.

Volatile Depletion in Planet-Forming Disks

Newly born stars are surrounded by gas and dust with a attened axisymmetric distribution termed protoplanetary disk,in which planets are formed.Observations of these objects are necessary for understanding the formation and early evolution of stars and planets,and for revealing the composition of the raw material from which planets are made.Numerical models can extract important parameters from the observational data,including the gas and dust mass of the disk.These parameters are used as input for further modeling,e.g.,to calculate the chemical composition of the disk.A consistent thermochemical model should be able to reproduce the abundances of di erent species in the disk.However,this good wish has been challenged for many disks:models over-predict the emission line intensity of some species;namely,they are depleted(with respect to expectations from canonical models).In this review we show how this disparity indicates that dust evolution has signi cant e ects on gas chemistry,and may indicate the earliest stages of planet formation.

Methanol formation chemistry with revised reactions scheme

The aim of the presented work is to analyze the impact of experimentally evaluated reactions of hydrogen abstraction on surfaces of interstellar grains on the chemical evolution of methanol and its precursors on grains and in the gas phase under conditions of a cold dark cloud and during the collapse of a translucent cloud into a dark cloud. Analysis of simulation results shows that those reactions are highly efficient destruction channels for HCO and H2CO on grain surfaces, and significantly impact the abundances of almost all molecules participating in the formation of CH3OH. Next, in models with those reactions, maximum abundances of methanol in gas and on grain surfaces decrease by more than 2–3 orders of magnitude in comparison to models without surface abstraction reactions of hydrogen. Finally, we study the impact of binding energies of CH2OH and CH3O radicals on methanol chemistry.

GGCHEMPY: A Pure Python-based Gas-grain Chemical Code for Efficient Simulation of Interstellar Chemistry

In this paper,we present a new gas-grain chemical code for interstellar clouds written in pure Python(GGCHEMPY(GGCHEMPY is available on https://github.com/JixingGE/GGCHEMPY)).By combining with the high-performance Python compiler Numba,GGCHEMPY is as efficient as the Fortran-based version.With the Python features,flexible computational workflows and extensions become possible.As a showcase,GGCHEMPY is applied to study the general effects of three-dimensional projection on molecular distributions using a two-core system which can be easily extended for more complex cases.By comparing the molecular distribution differences between two overlapping cores and two merging cores,we summarized the typical chemical differences such as N2 H+,HC3 N,C2 S,H2 CO,HCN and C2 H,which can be used to interpret 3 D structures in molecular clouds.

The Modeling of Grain Surface Chemistry

Astrochemistry has made great progress in recent years.Especially the grain surface chemistry played important roles in the explanation of the formation of the interstellar molecules.In this review,we will discuss the progress,including the di erent numerical methods to simulate the ice mantles in the astrochemical models.We will also introduce the laboratory astrochemical experimental results,and their contributions to the grain surface chemistry in the review.

Laboratory study of the formation of fullerene(from smaller to larger, C44 to C70)/anthracene cluster cations in the gas phase

The formation and evolution mechanism of fullerenes in the planetary nebula or in the interstellar medium are still not understood.Here,we present the study on the cluster formation and the relative reactivity of fullerene cations(from smaller to larger,C44 to C70) with anthracene molecule(C14H10).The experiment is performed in an apparatus that combines a quadrupole ion trap with a time-of-flight mass spectrometer.By using a 355 nm laser beam to irradiate the trapped fullerenes cations(C60+or C70+),smaller fullerene cations C(60-2 n)+, n=1-8 or C(70-2 m)+,m=1-11 are generated,respectively.Then reacting with anthracene molecules,series of fullerene/anthracene cluster cations are newly formed(e.g.,(C14H10)C(60-2 n)+,n=1-8 and(C14H10)C(70-2 m)+,m=1-11),and slight difference of the reactivity within the smaller fullerene cations are observed.Nevertheless,smaller fullerenes show obviously higher reactivity when comparing to fullerene C60+ and C70+.A successive loss of C2 fragments mechanism is suggested to account for the formation of smaller fullerene cations,which then undergo addition reaction with anthracene molecules to form the fullerene-anthracene cluster cations.It is found that the higher laser energy and longer irradiation time are key factors that affect the formation of smaller fullerene cations.This may indicate that in the strong radiation field environment(such as photon-dominated regions) in space,fullerenes are expected to follow the top-down evolution route,and then form small grain dust(e.g.,clusters) through collision reaction with co-existing molecules,here,smaller PAHs.

A new data structure for accelerating kinetic Monte Carlo method

The kinetic Monte Carlo simulation is a rigorous numerical approach to study the chemistry on dust grains in cold dense interstellar clouds. By tracking every single reaction in chemical networks step by step, this approach produces more precise results than other approaches but takes too much computing time. Here we present a method of a new data structure, which is applicable to any physical conditions and chemical networks, to save computing time for the Monte Carlo algorithm. Using the improved structure,the calculating time is reduced by 80 percent compared with the linear structure when applied to the osu-2008 chemical network at 10K. We investigate the effect of the encounter desorption in cold cores using the kinetic Monte Carlo model with an accelerating data structure. We found that the encounter desorption remarkably decreases the abundance of grain-surface H2 but slightly influences the abundances of other species on the grain.

Theoretical approach to study the formation of C2H4O2 isomers in interstellar medium through reaction between interstellar formaldehyde molecules

The three isomers of C2H4O2,viz.,glycolaldehyde(HCOCH2 OH),acetic acid(CH3 COOH)and methyl formate(HCOOCH3),have been detected in copious amounts in the interstellar medium(ISM).The possibility for formation of these molecules through interstellar formaldehyde(HCHO)has been explored by using the quantum chemical approach described by density functional theory(DFT)and second order Moller-Plesset perturbation(MP2)theory with a 6–311 G(d,p)basis set in the gas phase as well as in icy grains.The associated molecule-molecule interactions have been discussed to study the formation of isomers of C2H4O2 in ISM.The reactions of two formaldehyde molecules exhibit a considerable potential barrier but due to quantum tunneling,these reactions could be possible in ISM.The chemical pathway is exothermic,which gives rise to a high probability for the formation of all three isomers,viz.glycolaldehyde,methyl formate and acetic acid,in interstellar space.Anharmonic rotational vibration,centrifugal distortion constants and coupling constants are also calculated and results suggest that the vibrations are harmonic in nature.

Chempl: a playable package for modeling interstellar chemistry

Astrochemical modeling is needed for understanding the formation and evolution of interstellar molecules,and for extracting physical information from spectroscopic observations of interstellar clouds.The modeling usually involves the handling of a chemical reaction network and solution of a set of coupled nonlinear ordinary differential equations,which is traditionally done using code written in compiled languages such as Fortran or C/C++.While being computationally efficient,there is room for improvement in the ease of use and interactivity for such an approach.In this work we present a new public code named CHEMPL,which emphasizes interactivity in a modern Python environment,while remaining computationally efficient.Common reaction mechanisms and a three-phase formulation of gasgrain chemistry are implemented by default.It is straightforward to run 0 D models with CHEMPL,and only a small amount of additional code is needed to construct 1 D or higher-dimensional chemical models.We demonstrate its usage with a few astrochemically relevant examples.