Long-Fei Chen

Context . Infrared-dark clouds (IRDCs) are ideal sites for investigating the initial conditions of massive star and cluster formation. The A Lei Of the Habitat and Assembly of Infrared Dark Clouds (ALOHA IRDCs), a James Clerk Maxwell Telescope Large Program, has mapped nearby IRDCs utilising the SCUBA-2 continuum camera at the highest achievable sensitivity. Complementary molecular line observations are needed to characterise the physical, kinematic, and chemical properties of the dense gas. Aims . We aim to determine the thermal, kinematic, and chemical properties of clumps identified in the ALOHA IRDCs, and to assess their evolutionary status and level of star-forming activity. Methods . We performed single-pointing K -band and W -band observations towards 56 clumps pf ALOHA IRDCs using the Effelsberg 100 m and Yebes 40 m telescopes, respectively. We derived NH ₃ kinetic temperatures using the hyperfine group ratio method and identified infall and shock signatures from HCO ⁺ , H ¹³ CO ⁺ , SiO, and HNCO profiles. Water masers and NH ₂ D emission were used as complementary tracers of chemical evolution and star formation. Results . The clumps exhibit kinetic temperatures of 15–29 K. We detect NH ₂ D emission towards 18 sources; the NH ₂ D centroid velocities are consistent with those of NH ₃ , indicating the two species trace the same dense gas component. More than half of the clumps display blue-asymmetric HCO ⁺ profiles, identifying them as infall candidates. Water masers with prominent velocity ranges and variability are detected in 22 sources. Broad SiO emission (≳20 km s ⁻¹ ) indicates strong shocks, while narrower emission (≲6 km s ⁻¹ ) likely traces large-scale interactions or low-velocity shocks. Conclusions . The widespread infall signatures, shock tracers, masers, and NH ₂ D emission suggest that relatively quiescent, chemically young material can coexist with dynamically active gas affected by early protostellar feedback, providing insight into the coupled physical and chemical evolution of massive IRDC clumps.

We present centimeter-wave spectroscopic observations of the OH 18 cm lines in two bright comets, 12P/Pons-Brooks and C/2023 A3 (Tsuchinshan-ATLAS), conducted with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) during their 2024 apparitions. For the Halley-type comet 12P/Pons-Brooks, five epochs of OH observations were obtained. The main OH lines at 1665 and 1667 MHz were robustly detected in absorption during one preperihelion epoch, while upper limits were derived for two postperihelion epochs. For the dynamically new Oort Cloud comet C/2023 A3 (Tsuchinshan-ATLAS), six epochs of OH observations were obtained. The 1665 and 1667 MHz lines were robustly detected in absorption during two epochs immediately following the comet’s closest approach to Earth. We also reported a tentative detection of the 1721 MHz satellite line in one of these two epochs. The low OH detection rates for these two bright comets are primarily attributable to their unfavorable heliocentric radial velocity during the observations, which resulted in the anti-maser negative inversion mode of the OH excitation. We calculated the OH production rates for both comets and found that, after considering the small beam size of FAST and the collisional quenching effect, production rates are consistent with the data from literature. Specifically, this matches the power-law fit of the OH production rates for comet 12P/Pons-Brooks, as well as the production rates at comparable heliocentric distances among comet C/2023 A3 (Tsuchinshan-ATLAS) and four other dynamically new Oort Cloud comets.

Hot cores, as a stage of massive star formation, exhibit abundant line emissions of complex organic molecules (COMs). We present a deep line survey of two isomers of C$_2$H$_6$O: ethanol (C$_2$H$_5$OH; EA), and dimethyl ether (CH$_3$OCH$_3$; DE) as well as their possible precursor CH$_3$OH towards 60 hot cores by using the Atacama Large Millimeter/submillimeter Array (ALMA) 3 mm line observations. EA is detected in 40 hot cores and DE is detected in 59 hot cores. Of these, EA and DE are simultaneously detected in 39 hot cores. We calculate rotation temperatures and column densities of EA and DE by using the eXtended CASA Line Analysis Software Suite (xclass) software. The average rotation temperature of EA is higher than that of DE, whereas the average column density of EA is lower than that of DE. Combined with previous studies of hot cores and hot corinos, we find strong column density correlations among EA and DE ($\rho$ = 0.92), EA and CH$_3$OH ($\rho$ = 0.82), as well as DE and CH$_3$OH ($\rho$ = 0.80). The column density ratios of EA/DE versus the column densities of CH$_3$OH remain nearly constant with values within ~1 order of magnitude. These strong correlations and the stable ratios, suggest that EA, DE, and CH$_3$OH could be chemically linked, with CH$_3$OH potentially serving as a precursor for EA and DE. Compared with chemical models, the three different warm-up time-scale models result in the systematic overproduction of EA and the systematic underproduction of DE. Therefore, our large sample observations can provide crucial constraints on chemical models.

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.

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.

We used the Five-hundred-meter Aperture Spherical radio Telescope (FAST) to search for the molecular emissions in the L-band between 1.0 and 1.5 GHz toward four comets, C/2020 F3 (NEOWISE), C/2020 R4 (ATLAS), C/2021 A1 (Leonard), and 67P/Churyumov-Gerasimenko during or after their perihelion passages. Thousands of molecular transition lines fall in this low-frequency range, many attributed to complex organic or prebiotic molecules. We conducted a blind search for the possible molecular lines in this frequency range in those comets and could not identify clear signals of molecular emissions in the data. Although several molecules have been detected at high frequencies of greater than 100 GHz in comets, our results confirm that it is challenging to detect molecular transitions in the L-band frequency ranges. The non-detection of L-band molecular lines in the cometary environment could rule out the possibility of unusually strong lines, which could be caused by the masers or non-LTE effects. Although the line strengths are predicted to be weak, for FAST, using the ultra-wide bandwidth receiver and improving the radio frequency interference environments would enhance the detectability of those molecular transitions at low frequencies in the future.

Context. Thermal diffusion is one of the basic processes for the mobility and formation of species on cosmic dust grains. The rate of thermal diffusion is determined by the grain surface temperature, a pre-exponential factor, and an activation energy barrier for diffusion. Due to the lack of laboratory measurements on diffusion, prior astrochemical models usually assume that the diffusion pre-exponential factor is the same as that for desorption. This oversimplification may lead to an uncertainty in the model predictions. Recent laboratory measurements have found that the diffusion pre-exponential factor can differ from that for desorption by several orders of magnitude. However, the newly determined pre-exponential factor has not been tested in astrochemical models so far. Aims. We aim to evaluate the effect of the newly experimentally measured diffusion pre-exponential factor on the chemistry under cold molecular cloud conditions. Methods. We ran a set of parameters with different grain temperatures and diffusion barrier energies using the NAUTILUS astro-chemical code and compared the molecular abundance between the models with the abundance obtained using the experimentally determined pre-exponential factor for diffusion and with the abundance obtained using the values commonly adopted in prior models. Results. We found that statistically, more than half of the total gas-phase and grain surface species are not affected by the new pre-exponential factor after a chemical evolution of 10⁵ yr. The most abundant gas-phase CO and grain surface water ice are not affected by the new pre-exponential factor. For the grain surface species that are affected, compared to the commonly adopted value of the pre-exponential factor for diffusion used in the chemical models, they could be either overproduced or underproduced with the lower diffusion pre-factor used in this work. The former case applies to radicals and the species that serve as reactants, while the latter case applies to complex organic molecules (COMs) on the grain and the species that rarely react with other species. Gas-phase species could also be affected due to the desorption of the grain surface species. The abundance of some gas-phase COMs could be varied by over one order of magnitude depending on the adopted grain surface temperature and/or the ratio of diffusion to desorption energy in the model. Key species whose diffusion pre-exponential factor significantly affects the model predictions were also evaluated, and these species include CH₃OH, H₂CO, and NO. Conclusions. The results presented in this study show that the pre-exponential factor is one of the basic and important parameters in astrochemical models. It strongly affects the chemistry and should be determined carefully. More experiments to determine the diffusion of grain surface species are helpful for constraining their properties.

Cyanamide (NH₂CN) and its isomer, carbodiimide (HNCNH), may form adenine in the interstellar medium (ISM) via a series of reactions. Therefore, they are considered key prebiotic molecules in the study of the origin of life. We used the three-phase NAUTILUS chemical code, which includes the gas, the dust surface, and the icy mantle, to investigate the formation and destruction of cyanamide and carbodiimide. We added over 200 new chemical reactions of the two isomers and related species and established a relatively complete network. We applied cold core, hot corino/core, and shock models to simulate the different physical environments, and found that the two isomers are mainly produced by the free radical reactions on grain surfaces. Our simulated results suggest that cyanamide and carbodiimide molecules come from surface chemistry at early evolutionary stages. Then they are released back to the gas phase, either by thermal process (in hot cores, hot corinos) or by shock-induced desorption (in shock regions). We speculate that it is an inefficient route to form a tautomer of adenine by starting from molecules cyanoacetylene (C₃NH), cyanamide, and carbodiimide in ISM.