Keywords

Phenol, which belongs to the C₆H₆O isomeric group, is the simplest molecule in the family of alcohol of the aromatic series. Although phenol has yet to be detected in the interstellar medium, a tentative identification was reported toward the Orion KL hot core using the IRAM-30 m line survey. To explore some more species of this isomeric group, we consider ten species to study the fate of their astronomical detection. It is noticed that phenol is the most energetically favorable isomer of this group. In contrast, propargyl ether is the least favorable (having relative energy ∼103 kcal mol⁻¹ compared to phenol) species of this group. So far, the studies associated with the formation of phenol are heavily concentrated on combustion chemistry. Here, we suggest a few key reactions (C₆H₆ + OH → C₆H₅ + H₂O, C₆H₆ + O → C₆H₅OH, C₆H₆ + H → C₆H₅ + H₂, and C₆H₅ + OH → C₆H₅OH + hν) for the formation of phenol. All these pathways are included in a large gas-grain chemical network to study its formation in high mass star-forming regions and dark cloud environments. It is noticed that the phenyl (−C₆H₅) formation by the ice-phase hydrogen abstraction reaction of benzene (i.e., C₆H₆ + OH → C₆H₅ + H₂O if allowed at ∼10 K) could serve as the starting point for the formation of phenol in the gas phase by radiative association reaction C₆H₅ + OH → C₆H₅OH + hν. The gas-phase reaction C₆H₆ + O → C₆H₅OH significantly contributes to the formation of phenol, when the ice-phase reaction C₆H₆ + OH → C₆H₅ + H₂O is not considered at low temperature. Band 4 ALMA archival data of a hot molecular core, G10.47+0.03, are analyzed. It yields an upper limit on phenol abundance of 5.19 × 10⁻⁹. Our astrochemical model delivers an upper limit on phenol abundance of ∼2.20 × 10⁻⁹ in the hot molecular core, whereas its production in the dark cloud is not satisfactory.

To study the chemical evolution during the formation of molecular clouds, we model three types of clouds with different density structures: collapsing spherical, collapsing ellipsoidal, and static spherical profiles. The collapsing models are better than the static models in matching the observational characteristics in typical molecular clouds. This is mainly because the gravity can speed up the formation of some important molecules (e.g., H₂, CO, OH) by increasing the number density during collapse. The different morphologies of prolate, oblate, and spherical clouds lead to differences in chemical evolution, which are mainly due to their different evolution of number density. We also study the effect of initial chemical compositions on chemical evolution, and find that H atoms can accelerate OH formation by two major reactions: O + H → OH in gas phase and on dust grain surfaces, leading to the models in which hydrogen is mainly atomic initially better match observations than the models in which hydrogen is mainly molecular initially. Namely, to match observations, initially hydrogen must be mostly atomic. The CO molecules are able to form even without the pre-existence of H₂. We also study the influence of gas temperature, dust temperature, intensity of interstellar radiation field and cosmic-ray ionization rate on chemical evolution in static clouds. The static CO clouds with high dust temperature, strong radiation field, and intensive cosmic rays are transient due to rapid CO destruction.

The composition and structure of interstellar dust are important and complex for the study of the evolution of stars and the interstellar medium (ISM). However, there is a lack of corresponding experimental data and model theories. By theoretical calculations based on ab-initio method, we have predicted and geometry optimized the structures of Carbon-rich (C-rich) dusts, carbon (¹²C), iron carbide (FeC), silicon carbide (SiC), even silicon (²⁸Si), iron (⁵⁶Fe), and investigated the optical absorption coefficients and emission coefficients of these materials in 0D (zero-dimensional), 1D, and 2D nanostructures. Comparing the nebular spectra of the supernovae (SN) with the coefficient of dust, we find that the optical absorption coefficient of the 2D ¹²C, ²⁸Si, ⁵⁶Fe, SiC and FeC structure corresponds to the absorption peak displayed in the infrared band (5–8) μm of the spectrum at 7554 days after the SN1987A explosion. It also corresponds to the spectrum of 535 days after the explosion of SN2018bsz, when the wavelength was in the range of (0.2–0.8) and (3–10) μm. Nevertheless, 2D SiC and FeC correspond to the spectrum of 844 days after the explosion of SN2010jl, when the wavelength is within (0.08–10) μm. Therefore, FeC and SiC may be the second type of dust in SN1987A corresponding to infrared band (5–8) μm of dust and may be in the ejecta of SN2010jl and SN2018bsz. The nano-scale C-rich dust size is ∼0.1 nm in SN2018bsz, which is 3 orders of magnitude lower than the value of 0.1 μm. In addition, due to the ionization reaction in the supernova remnant (SNR), we also calculated the Infrared Radiation (IR) spectrum of dust cations. We find that the cation of the 2D layered (SiC)²⁺ has a higher IR spectrum than those of the cation (SiC)¹⁺ and neutral (SiC)⁰⁺.

The origin of bistable solutions in the kinetic equations describing the chemistry of dense interstellar clouds is explained as being due to the autocatalysis and feedback of oxygen nuclei from the oxygen dimer (O₂). We identify four autocatalytic processes that can operate in dense molecular clouds, driven respectively by reactions of H⁺, He⁺, C⁺, and S⁺ with O₂. We show that these processes can produce the bistable solutions found in previous studies, as well as the dependence on various model parameters such as the helium ionization rate, the sulfur depletion and the ${{\rm{H}}}_{3}^{+}$ electron recombination rate. We also show that ion–grain neutralizations are unlikely to affect the occurrence of bistability in dense clouds. It is pointed out that many chemical models of astronomical sources should have the potential to show bistable solutions.

We present a method for deriving the electron density of ionized gas using the ratio of the intensity of the [N ii] 205 μm line to that of hydrogen radio recombination lines (RRLs). We use this method to derive electron densities of 21 velocity components in 11 lines of sight through the Galaxy, including the Galactic center. We observed, at high spectral resolution, the [N ii] 205 μm with the Herschel/HIFI and SOFIA/GREAT instruments and the RRLs with the Green Bank Telescope and the NASA Deep Space Network Deep Space Station 43 (DSS-43) telescope. We find typical electron densities between 8 and 170 cm⁻³, which are consistent with those derived at low spectral resolution using the [N ii] 205 μm/122 μm ratio with Herschel/PACS on a larger sample of sight lines in the Galactic plane. By matching the electron densities derived from the [N ii] 205 μm/RRL intensity ratio and the [N ii] 122 μm/205 μm intensity ratio, we derive the nitrogen fractional abundance for most of the velocity components. We investigate the dependence of the N/H ratio on galactocentric distance in the inner Galaxy (R_gal < 6 kpc), which is inaccessible in optical studies owing to dust extinction. We find that the distribution of nitrogen abundances in the inner Galaxy derived from our data has a slope that is consistent with that found in the outer Galaxy in optical studies. This result is inconsistent with some suggestions of a flatter distribution of the nitrogen abundance in the inner Galaxy.

We study the effects of grain surface reactions on the chemistry of protoplanetary disks where gas, ice surface layers, and icy mantles of dust grains are considered as three distinct phases. Gas-phase and grain surface chemistry is found to be mainly driven by photoreactions and dust temperature gradients. The icy disk interior has three distinct chemical regions: (i) the inner midplane with low far-UV (FUV) fluxes and warm dust (≳15 K) that lead to the formation of complex organic molecules, (ii) the outer midplane with higher FUV from the interstellar medium and cold dust where hydrogenation reactions dominate, and (iii) a molecular layer above the midplane but below the water condensation front where photodissociation of ices affects gas-phase compositions. Some common radicals, e.g., CN and C₂H, exhibit a two-layered vertical structure and are abundant near the CO photodissociation front and near the water condensation front. The three-phase approximation in general leads to lower vertical column densities than two-phase models for many gas-phase molecules owing to reduced desorption, e.g., H₂O, CO₂, HCN, and HCOOH decrease by roughly two orders of magnitude. Finally, we find that many observed gas-phase species originate near the water condensation front; photoprocesses determine their column densities, which do not vary significantly with key disk properties such as mass and dust/gas ratio.

We present an observational study of the sulfur (S)-bearing species toward Orion KL at 1.3 mm by combining ALMA and IRAM-30 m single-dish data. At a linear resolution of ∼800 au and a velocity resolution of 1 km s⁻¹, we have identified 79 molecular lines from six S-bearing species. In these S-bearing species, we found a clear dichotomy between carbon–sulfur compounds and carbon-free S-bearing species for various characteristics, e.g., of line profiles, spatial morphology, and molecular abundances with respect to H₂. Lines from the carbon–sulfur compounds (i.e., OCS, ¹³CS, and H₂CS) exhibit spatial distributions concentrated around the continuum peaks and extended to the south ridge. The full width at half maximum (FWHM) linewidth of these molecular lines is in the range of 2 ∼11 km s⁻¹. The molecular abundances of OCS and H₂CS decrease slightly from the cold (∼68 K) to the hot (∼176 K) regions. In contrast, lines from the carbon-free S-bearing species (i.e., SO₂, ³⁴SO, and H₂S) are spatially more extended to the northeast of mm4, exhibiting broader FWHM line widths (15 ∼ 26 km s⁻¹). The molecular abundances of carbon-free S-bearing species increase by over an order of magnitude as the temperature increase from 50 to 100 K. In particular, ³⁴SO/³⁴SO₂ and OCS/SO₂ are enhanced from the warmer regions (>100 K) to the colder regions (∼50 K). Such enhancements are consistent with the transformation of SO₂ at warmer regions and the influence of shocks.

We report an astrochemical study on the evolution of interstellar molecular clouds and consequent star formation in the center of the barred spiral galaxy M83. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to image molecular species indicative of shocks (SiO and CH₃OH), dense cores (N₂H⁺), and photodissociation regions (CN and CCH), as well as a radio recombination line (H41α) tracing active star-forming regions. M83 has a circumnuclear gas ring that is joined at two intersections by gas streams from the leading-edge gas lanes on the bar. We found elevated abundances of the shock and dense-core tracers in one of the orbit-intersecting areas, and found peaks of CN and H41α downstream. In the other orbit-intersection area, we found a similar enhancement of the shock tracers, but less variation of other tracers, and no sign of active star formation in the stream. We propose that the observed chemical variation or lack of it is due to the presence or absence of collision-induced evolution of molecular clouds and induced star formation. This work presents the clearest case of the chemical evolution in the circumnuclear rings of barred galaxies thanks to the ALMA resolution and sensitivity.

The chemical structure of high-mass star nurseries is important for a general understanding of star formation. Deuteration is a key chemical process in the earliest stages of star formation because its efficiency is sensitive to the environment. Using the IRAM-30 m telescope at 1.3–4.3 mm wavelengths, we have imaged two parsec-scale high-mass protostellar clumps (P1 and S) that show different evolutionary stages but are located in the same giant filamentary infrared dark cloud G28.34+0.06. Deep spectral images at subparsec resolution reveal the dust and gas physical structures of both clumps. We find that (1) the low-J lines of N₂H⁺, HCN, HNC, and HCO⁺ isotopologues are subthermally excited; and (2) the deuteration of N₂H⁺ is more efficient than that of HCO⁺, HCN, and HNC by an order of magnitude. The deuterations of these species are enriched toward the chemically younger clump S compared with P1, indicating that this process favors the colder and denser environment (T_kin ∼ 14 K, N(NH₃) ∼ 9 × 10¹⁵ cm⁻²). In contrast, single deuteration of NH₃ is insensitive to the environmental difference between P1 and S; and (3) single deuteration of CH₃OH (>10%) is detected toward the location where CO shows a depletion of ∼10. This comparative chemical study between P1 and S links the chemical variations to the environmental differences and shows chemical similarities between the early phases of high- and low-mass star-forming regions.

Supernova remnant (SNR) shock waves are the main place where interstellar dust grains are destroyed. However, the dust destruction efficiency in nonradiative shocks is still not well known. One way to estimate the fraction of dust destroyed is to compare the difference between postshock gas abundances and preshock medium total abundances when the preshock elemental depletion factors are known. We compare the postshock gas abundances of 16 SNRs in the Large Magellanic Cloud (LMC) with the LMC interstellar medium abundances that we derived based on 69 slow-rotating early B-type stars. We find that, on average, ∼61% of Si-rich dust grains are destroyed in the shock, while the fraction of dust destroyed is only ∼40% for Fe-rich dust grains. This result supports the idea that the high depletion of Fe in the diffuse neutral medium is not caused by the resilience of Fe-rich grains but because of faster growth rate. This work also presents a potential way to constrain the chemical composition of interstellar dust.