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1. Spectroscopic Properties of the Astrochemical Molecules [Al, O, Si] x (x = 0, +1)

   https://doi.org/10.3847/1538-4357/ac92fb  

   Friskey, Jacqueline M. ; Esposito, Vincent J. ; Trabelsi, Tarek ; Francisco, Joseph S. ( October 2022 , The Astrophysical Journal)

   Aluminum and silicon are present in large quantities in the interstellar medium, making the triatomic species consisting of both elements intriguing with regard to the foundations of astrochemistry. Spectroscopic parameters have been calculated via high-level ab initio methods to assist with laboratory and observational detection of [Al, O, Si]^x(x= 0,+1). All [Al, O, Si]^x(x= 0,+1) isomers exist in the linear geometry, with linear AlOSi (X²Π) and linear AlOSi⁺(X¹Δ) being the most stable neutral and cationic species, respectively. Formation of the neutral species most likely occurs via reaction of AlO/SiO on an Si/Al dust grain surface, respectively. The cation molecules may form via ion–neutral reaction or as a consequence of photoionization. The rotational frequencies of linear AlOSi (X²Π) have been calculated using vibrationally corrected rotational constants and centrifugal distortion to lead experimental and observational radio detection. The rotational frequencies are discussed for three temperatures indicative of various astronomical environments: the central circumstellar envelope (CSE) (100 K), outer CSE (30 K), and the interstellar medium (3 K). At 100 K, the lines originating fromJ′ > 30 are the best candidates for detection via ground-based telescope. Anharmonic vibrational analysis revealed various Fermi resonances that may complicate the vibrational spectrum of linear AlOSi (X²Π). Finally, electronic spectroscopy may be the best means for laboratory detection of linear AlOSi (X²Π), due to the presence of two overlapping electronic transitions with large oscillator strengths occurring at approximately 250 nm.

    

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2. Reaction Pathway and Rovibrational Analysis of Aluminum Nitride Species as Potential Dust Grain Nucleation Agents

   https://doi.org/10.3847/1538-4357/ad182e  

   Palmer, C. Zachary ; Fortenberry, Ryan C. ( February 2024 , The Astrophysical Journal)

   A dust nucleating agent may be present in interstellar or circumstellar media that has gone seemingly undetected and unstudied for decades. Some analyses of the Murchison CM2 meteorite suggest that at least some of the aluminum present within condensed as aluminum nitrides instead of the long-studied, but heretofore undetected suite of aluminum oxides. The present theoretical study utilizes explicitly correlated coupled cluster theory and density functional theory to provide a formation pathway from alane (AlH₃) and ammonia to the cyclic structure Al₂N₂H₄, which has the proper Al/N ratio expected of bulk aluminum nitrides. Novel rovibrational spectroscopic constants are computed for alane and the first two formed structures, AlNH₆and AlNH₄, along the reaction pathway for use as reference in possible laboratory or observational studies. Theν₈bending frequency for AlNH₆at 755.7 cm⁻¹(13.23μm) presents a vibrational transition intensity of 515 km mol⁻¹, more intense than the antisymmetric C−O stretch of carbon dioxide, and contains a dipole moment of 5.40 D, which is ∼3× larger than that of water. Thus, the present reaction pathway and rovibrational spectroscopic analysis may potentially assist in the astrophysical detection of novel, inorganic species which may be indicative of larger dust grain nucleation.

    

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3. Toward the IR Detection of Carbonic Acid: Absorption and Emission Spectra

   https://doi.org/10.3847/1538-4357/ad0f16  

   Fortenberry, Ryan C. ; Esposito, Vincent J. ( January 2024 , The Astrophysical Journal)

   With the recent radioastronomical detection ofcis-trans-carbonic acid (H₂CO₃) in a molecular cloud toward the Galactic center, the more stable but currently unobservedcis-cisconformer is shown here to have strong IR features. While the higher-energycis-trans-carbonic acid was detected at millimeter and centimeter wavelengths, owing to its larger dipole moment, the vibrational structure ofcis-cis-carbonic acid is more amenable to its observation at micron wavelengths. Even so, both conformers have relatively large IR intensities, and some of these fall in regions not dominated by polycyclic aromatic hydrocarbons. Water features may inhibit observation near the 2.75μm hydride stretches, but other vibrational fundamentals and even overtones in the 5.5–6.0μm range may be discernible with JWST data. This work has employed high-level, accurately benchmarked quantum chemical anharmonic procedures to compute exceptionally accurate rotational spectroscopic data compared to experiment. Such performance implies that the IR absorption and even cascade emission spectral features computed in this work should be accurate and will provide the needed reference for observation of either carbonic acid conformer in various astronomical environments.

    

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4. Where have all the interstellar silicon carbides gone?

   https://doi.org/10.1093/mnras/stab3175  

   Chen, Tao ; Xiao, C Y ; Li, Aigen ; Zhou, C T ( December 2021 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The detection of the 11.3$\, {\rm \mu m}$ emission feature characteristic of the Si–C stretch in carbon-rich evolved stars reveals that silicon carbide (SiC) dust grains are condensed in the outflows of carbon stars. SiC dust could be a significant constituent of interstellar dust since it is generally believed that carbon stars inject a considerable amount of dust into the interstellar medium (ISM). The presence of SiC dust in the ISM is also supported by the identification of pre-solar SiC grains of stellar origin in primitive meteorites. However, the 11.3$\,\mu {\rm m}$ absorption feature of SiC has never been seen in the ISM, and oxidative destruction of SiC is often invoked. In this work, we quantitatively explore the destruction of interstellar SiC dust through oxidation based on molecular dynamics simulations and density functional theory calculations. We find that the reaction of an oxygen atom with SiC molecules and clusters is exothermic and could cause CO-loss. Nevertheless, even if this is extrapolable to bulk SiC dust, the destruction rate of SiC dust through oxidation could still be considerably smaller than the (currently believed) injection rate from carbon stars. Therefore, the lack of the 11.3$\,\mu{\rm m}$ absorption feature of SiC dust in the ISM remains a mystery. A possible solution may lie in the currently believed stellar injection rate of SiC (which may have been overestimated) and/or the size of SiC dust (which may actually be considerably smaller than submicron in size). 

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5. Unveiling the warm and dense ISM in z  > 6 quasar host galaxies via water vapor emission

   https://doi.org/10.1051/0004-6361/202243406  

   Pensabene, A. ; van der Werf, P. ; Decarli, R. ; Bañados, E. ; Meyer, R. A. ; Riechers, D. ; Venemans, B. ; Walter, F. ; Weiß, A. ; Brusa, M. ; et al ( November 2022 , Astronomy & Astrophysics)

   Water vapor (H₂O) is one of the brightest molecular emitters after carbon monoxide (CO) in galaxies with high infrared (IR) luminosity, allowing us to investigate the warm and dense phase of the interstellar medium (ISM) where star formation occurs. However, due to the complexity of its radiative spectrum, H₂O is not frequently exploited as an ISM tracer in distant galaxies. Therefore, H₂O studies of the warm and dense gas at high-zremain largely unexplored. In this work, we present observations conducted with the Northern Extended Millimeter Array (NOEMA) toward threez > 6 IR-bright quasarsJ2310+1855,J1148+5251, andJ0439+1634targeted in their multiple para- and ortho-H₂O transitions (3₁₂ − 3₀₃, 1₁₁ − 0₀₀, 2₂₀ − 2₁₁, and 4₂₂ − 4₁₃), as well as their far-IR (FIR) dust continuum. By combining our data with previous measurements from the literature, we estimated the dust masses and temperatures, continuum optical depths, IR luminosities, and star formation rates (SFR) from the FIR continuum. We modeled the H₂O lines using the MOLPOP-CEP radiative transfer code, finding that water vapor lines in our quasar host galaxies are primarily excited in the warm, dense (with a gas kinetic temperature and density ofT_kin = 50 K,n_H2 ∼ 10^4.5 − 10⁵ cm⁻³) molecular medium with a water vapor column density ofN_H2O ∼ 2 × 10¹⁷ − 3 × 10¹⁸ cm⁻³. High-JH₂O lines are mainly radiatively pumped by the intense optically-thin far-IR radiation field associated with a warm dust component at temperatures ofT_dust ∼ 80 − 190 K that account for < 5 − 10% of the total dust mass. In the case of J2310+1855, our analysis points to a relatively high value of the continuum optical depth at 100 μm (τ₁₀₀ ∼ 1). Our results are in agreement with expectations based on the H₂O spectral line energy distribution of local and high-zultra-luminous IR galaxies and active galactic nuclei (AGN). The analysis of the Boltzmann diagrams highlights the interplay between collisions and IR pumping in populating the high H₂O energy levels and it allows us to directly compare the excitation conditions in the targeted quasar host galaxies. In addition, the observations enable us to sample the high-luminosity part of the H₂O–total-IR (TIR) luminosity relations (L_H2O − L_TIR). Overall, our results point to supralinear trends that suggest H₂O–TIR relations are likely driven by IR pumping, rather than the mere co-spatiality between the FIR continuum- and line-emitting regions. The observedL_H2O/L_TIRratios in ourz > 6 quasars do not show any strong deviations with respect to those measured in star-forming galaxies and AGN at lower redshifts. This supports the notion that H₂O can be likely used to trace the star formation activity buried deep within the dense molecular clouds.

    

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