Qiang Chang

Ethanolamine (NH₂CH₂CH₂OH) can form not only glycine directly under the conditions of proposed Archean alkaline hydrothermal vents, a possible environment for the origin of life, but also the polar hydrophilic head of phosphatidylethanolamine, the second most abundant phospholipid in cell membranes, under possible conditions of the primitive Earth. Recently, NH₂CH₂CH₂OH was detected toward the G+0.693-0.027 molecular cloud. We construct the chemical network for NH₂CH₂CH₂OH and its related species (HOCH₂CN, CH₃NH₂, NH₂OH, CH₃OH, C₂H₅OH, and C₂H₃OH) via quantum chemical calculations, simulate their abundance evolutions with the pnautilus code, and then acquire the best-fitting shock-wave model for G+0.693: the combination of isothermal model I₅ (T _g = T _d = 16 K, n _H = 4 × 10³ cm⁻³, A _V = 6 mag, and ζ = 1.3 × 10⁻¹⁵ s⁻¹) and continuous shock model S₅ (V _s = 20 km s⁻¹). We find NH₂CH₂CH₂OH mainly comes from the thermal desorption produced by shock-induced heating and the photodesorption generated by cosmic-ray-induced UV photons; overall, its ice-phase species is mostly produced by four sequential ice-phase addition pathways, in which the final reactions are J-H + J-NH₂CH₂CHOH → J-NH₂CH₂CH₂OH, J-H + J-NHCH₂CH₂OH → J-NH₂CH₂CH₂OH, J-H + J-NH₂CH₂CH₂O → J-NH₂CH₂CH₂OH, and J-CH₂OH + J-CH₂NH₂ → J-NH₂CH₂CH₂OH. We also discuss the formation of HOCH₂CN, CH₃OH, C₂H₅OH, and C₂H₃OH in the best-fitting shock-wave model, as well as the formation of NH₂CH₂CH₂OH in a typical hot-core model. Moreover, we predict NH₂CH₂CH₂OH may be detected toward the newly formed hot core; the undiscovered species CH₂CH₂OH and HOCCNH are potentially detectable toward G+0.693.

In recent years, a significant number of oxygen-bearing complex organic molecules (COMs) have been detected in the gas phase of cold dark clouds such as TMC-1. The formation of these COMs cannot be explained by diffusive mechanisms on grains and gas-phase reactions. This study investigates the formation of oxygen-bearing COMs in cold dark clouds using multiphase gas–grain models that incorporate cosmic ray-induced nondiffusive radiation chemistry and nonthermal sputtering desorption mechanisms. Additionally, we present the effects of varying elemental C/O ratio and different sputtering rates. We utilized an accelerated Gillespie algorithm, based on the regular Gillespie algorithm. The results of our models for dimethyl ether (CH₃OCH₃), methyl formate (HCOOCH₃), acetaldehyde (CH₃CHO), ethanol (C₂H₅OH), and methanol (CH₃OH) show reasonable agreement with observations toward TMC-1, within a factor of 3. Out of the 94 species compared with observations, 63 show agreement within 1 order of magnitude, accounting for 67.02%. Overall inclusion of nonthermal mechanisms in multiphase models shows notable improvement of modeling on oxygen-bearing COMs in the interstellar medium.

Astrochemical reaction networks are one of the tools to study the synthesis and evolution of interstellar molecules. By simulating astrochemical reaction networks under physical conditions that pertain to specific astronomical sources, the abundance of interstellar molecules and their temporal evolution can be predicted. This helps to understand the fundamental laws of interstellar molecular evolution. Deuterated molecules are an important component of the interstellar medium. The detection and study of deuterated molecules provide critical observational evidence and theoretical constraints for the development of astrochemical reaction networks. In recent years, as astrochemical research advanced, an increasing number of deuterated molecules have been discovered in interstellar space, sparking greater interest in the formation and evolution of these molecules in the universe. These discoveries have driven further development of the chemical reaction networks involving deuterated molecules, enabling a better understanding of the synthesis, transformation, and evolution of deuterated molecules in various interstellar environments. The formation of deuterated molecules is closely linked to low-temperature and high-density physical conditions. In such environments, the frequency of molecular collisions increases, and chemical reactions occur more frequently and efficiently, providing the necessary impetus for deuteration reactions. The realization of deuteration is a complex process, involving multiple reaction pathways, including the incorporation and substitution of deuterium atoms. Deuteration reactions are not limited to gas-phase reactions; they also include surface reactions on dust grains. On the surfaces of interstellar dust, molecular processes such as molecular adsorption, synthesis, and desorption significantly influence the efficiency of deuteration reactions. To better understand the formation mechanisms of deuterated molecules in different interstellar environments, astrochemical reaction networks must fully account for these complex reaction processes. The KIDA database, one of the most widely used gas-phase chemical reaction networks, already includes a significant number of reactions involving deuterated molecules. The chemical reaction networks for deuterated molecules in this database mainly focus on gas-phase reactions, so they lack a comprehensive description of surface reactions on dust grains. Dust grains play an important role in the interstellar medium, acting not only as catalysts for molecular reactions but also as sites where processes such as adsorption, desorption, and surface chemical reactions can take place. Surface reactions on dust are crucial for the synthesis of deuterated molecules, making it essential to improve the reaction networks in this area. To address this gap, we have extended the KIDA gas-phase deuteration reaction network by incorporating key surface reactions that occur on dust grains. These reactions mainly include molecular adsorption, desorption, synthesis, and decomposition on the surfaces of dust particles. The updated reaction network allows for a more comprehensive simulation of the evolution of deuterated molecules in various interstellar environments, providing a more accurate and complete theoretical foundation for future astrochemical simulations. Under the classical physical conditions of cold molecular clouds, we used this extended network to simulate the synthesis of deuterated molecules. Preliminary simulation results indicate that the newly expanded chemical reaction network involving deuterated molecules can be used for subsequent modeling studies of deuterated species.

In recent years, a significant number of oxygen-bearing complex organic molecules (COMs) have been detected in the gas phase of cold dark clouds such as TMC-1. The formation of these COMs cannot be explained by diffusive mechanisms on grains and gas phase reactions. This study investigates the formation of oxygen-bearing COMs in cold dark clouds using multiphase gas-grain models that incorporate cosmic ray-induced non-diffusive radiation chemistry and non-thermal sputtering desorption mechanisms. Additionally, we present the effects of varying elemental C/O ratio and different sputtering rates. We utilized an accelerated Gillespie algorithm, based on the regular Gillespie algorithm. The results of our models for dimethyl ether (CH3OCH3), methyl formate (HCOOCH3), acetaldehyde (CH3CHO), ethanol (C2H5OH), and methanol (CH3OH) show reasonable agreement with observations toward TMC-1, within a factor of 3. Out of the 94 species compared with observations, 63 show agreement within 1 order of magnitude, accounting for 67.02%. Overall inclusion of non-thermal mechanisms in multi-phase models shows notable improvement of modeling on oxygen-bearing COMs in the interstellar medium.

Context. Recent studies show that multiphase models trap too many volatile species such as CH₄ inside ice mantles, so they usually underestimate the abundances of long carbon-chain species observed toward warm carbon-chain chemistry (WCCC) sources. Aims. We propose a new multiphase model that allows more volatile species to diffuse out of the ice mantle upon warming. The new multiphase model is used to study the synthesis of long carbon-chain molecules in WCCC sources. Methods. We included porous structure in the ice mantles. The porous structure can enlarge the active layers of ice mantles so that fewer volatile species are trapped. The porous multiphase models were simulated using an accelerated Gillespie algorithm. Results. The abundances of long carbon-chain species predicted by the porous multiphase models can be more than one order of magnitude higher than those predicted by the multiphase model at temperatures relevant to WCCC sources. Moreover, the porous multiphase models predict more abundant long carbon-chain species as the porosity of the ice mantles increases. On the other hand, the two-phase model still estimates higher long carbon-chain species abundances than the porous multiphase models do. The abundances of long carbon-chain species predicted by our porous multiphase models agree reasonably well with observations toward three WCCC sources, L483, L1527, and B228. Conclusions. Our porous multiphase model solves the problem of too many volatile species being trapped in ice mantles in the multiphase models.

We theoretically demonstrate a scheme to generate circularly polarized (CP) isolated attosecond pulses (IAPs) with tunable helicity using a polarization gating laser field interacting with the CO molecule. The results show that a broadband CP supercontinuum is produced from the oriented CO molecule, which supports the generation of an IAP with an ellipticity of 0.98 and a duration of 90 as. Furthermore, the helicity of the generated harmonics and IAP can be effectively controlled by modulating the laser field and the orientation angle of the CO molecule. Our method will advance research on chiral-specific dynamics and magnetic circular dichroism on the attosecond timescale.

We present an unbiased λ 3 mm spectral line survey (between 84.5 and 115.8 GHz), conducted by the Purple Mountain Observatory 13.7 m radio telescope, together with updated modeling results, toward the carbon-rich asymptotic giant branch star IRC+10216 (CW Leo). A total of 75 spectral lines (96 transitions) are detected, and identified to arise from 19 molecules: C₂H, l-C₃H, C₄H, CN, C₃N, HC₃N, HC₅N, HCN, HNC, CH₃CN, MgNC, CO, c-C₃H₂, SiC₂, SiO, SiS, CS, C₂S, C₃S, and their isotopologues. Among them, one molecular emission line (H¹³CCCN J = 13–12) is discovered in IRC+10216 for the first time. The excitation temperature, column density, and fractional abundance of the detected species are deduced by assuming they are in local thermodynamic equilibrium. In addition, the isotopic ratios of [¹²C]/[¹³C], [³²S]/[³⁴S], [²⁸Si]/[²⁹Si], and [¹²C³⁴S]/[¹³C³²S] are obtained and found to be consistent with previous studies. Finally, we summarize all of the 106 species detected in IRC+10216 to date with their observed and modeled column densities for the convenience of future studies.

Recent experiments show that the desorption energy of H₂ on a diamond-like carbon (DLC) surface depends on the H₂ coverage of the surface. We aim to quantitatively explain the coverage dependent H₂ desorption energy measured by the experiments. We derive a math formula to calculate an effective H₂ desorption energy based on the encounter desorption mechanism. The effective H₂ desorption energy depends on two key parameters, the desorption energy of H₂ on H₂ substrate and the ratio of H₂ diffusion barrier to its desorption energy. The calculated effective H₂ desorption energy qualitatively agrees with the coverage dependent H₂ desorption energy measured by the experiments if the values of these two parameters in literature are used in the calculations. We argue that the difference between the effective H₂ desorption energy and the experimental results is due to the lacking of knowledge about these two parameters. So, we recalculate these two parameters based on experimental data. Good agreement between theoretical and experimental results can be achieved if these two updated parameters are used in the calculations.

Recent experiments show that the desorption energy of H$_2$ on a diamond-like carbon (DLC) surface depends on the H$_2$ coverage of the surface. We aim to quantitatively explain the coverage dependent H$_2$ desorption energy measured by the experiments. We derive a math formula to calculate an effective H$_2$ desorption energy based on the encounter desorption mechanism. The effective H$_2$ desorption energy depends on two key parameters, the desorption energy of H$_2$ on H$_2$ substrate and the ratio of H$_2$ diffusion barrier to its desorption energy. The calculated effective H$_2$ desorption energy qualitatively agrees with the coverage dependent H$_2$ desorption energy measured by the experiments if the values of these two parameters in literature are used in the calculations. We argue that the difference between the effective H$_2$ desorption energy and the experimental results is due to the lacking of knowledge about these two parameters. So, we recalculate these two parameters based on experimental data. Good agreement between theoretical and experimental results can be achieved if these two updated parameters are used in the calculations.

Hydroxylamine (NH₂OH) is a key precursor of both the formation of amino acids and the concurrent synthesis of pyrimidine and purine nucleosides. Recently, NH₂OH was detected towards the quiescent molecular cloud G+0.693-0.027, which is located in the Galactic Center. In this study, we construct the chemical network for NH₂OH and its related species via quantum chemical calculations, and simulate their chemistry using the astrochemical code Nautilus in isothermal models and shock models. According to the disagreement distance formula and molecular abundance ratios, we acquire the best-fit isothermal model T18 and the best-fit C-shock model S2, respectively. Of these two models, S2 is more suitable for G+0.693-0.027 due to its reproduction of abundances of all considered molecules. From the model, we found that NH₂OH and CH₃NH₂ are mainly formed on grain surfaces, while HNCN and CH₂NH, which are the post-shock tracers, are produced by both gas-phase and grain-surface reactions. Our chemical modelling also agrees with the observed abundances or upper limits of NH₂OH and its related species towards IRAS 16293-2422 B and Sgr B2(N). Moreover, we predict that NH₂OH and the undiscovered species H₂NO and HNOH are of high abundance, thus are detectable in cold clouds and pre-stellar cores, and in G+0.693-0.027, respectively.