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1. Expanding the Inventory of Molecule-rich Planetary Nebulae: New Observations of M4-17, Hu 1-1, M1-59, and Na 2

   https://doi.org/10.3847/1538-4357/ad83be  

   Gold, K_R; Schmidt, D_R; Ziurys, L_M (November 2024, The Astrophysical Journal)

   Abstract Molecular observations of four planetary nebulae (PNe), M4-17, Hu 1-1, M1-59, and Na 2, were conducted at 1–3 mm using the Arizona Radio Observatory’s 12 m antenna and Submillimeter Telescope, and the Institut de Radioastronomie Millimétrique 30 m Telescope. Toward M4-17, HNC (J= 3 → 2), CCH (N= 2 → 1,N= 3 → 2), CN (N= 1 → 0,N= 2 → 1), H₂CO (J_Ka,Kc= 2_1,2→ 1_1,1,J_Ka,Kc= 2_0,2→ 1_0,1,J_Ka,Kc= 2_1,1→ 1_1,0), CS (J= 3 → 2,J= 5 → 4), and H¹³CN (J= 2 → 1) were detected. An almost identical set of transitions was identified toward Hu 1-1. Moreover, c–C₃H₂was detected in Hu 1-1 via three 2 mm lines:J_Ka,Kc= 3_1,2→ 2_2,1,J_Ka,Kc= 4_1,4→ 3_0,3, andJ_Ka,Kc= 3_{2, 2}→ 2_1,1. HNC, CCH, CN, CS, and H¹³CN were found in M1-59, as well as H₂S via itsJ_Ka,Kc= 1_1,0→ 1_0,1line—the first detection of this key sulfur species in PNe. In addition, CCH and CN were identified in the 27,000 yr old Na 2. Among these four sources, CN and CCH were the most prevalent molecules (after CO and H₂) with fractional abundances, relative to H₂, off∼ 0.9–7.5 × 10⁻⁷and 0.8–7.5 × 10⁻⁷, respectively. CS and HNC have abundances in the rangef∼ 0.5–5 × 10⁻⁸, the latter resulting in HCN/HNC ∼ 3 across all three PNe. The unusual species H₂CO, c–C₃H₂, and H₂S hadf∼ 3–4 × 10⁻⁷, 10⁻⁸, and 6 × 10⁻⁸. This study suggests that elliptical PNe such as Hu 1-1 can have a diverse molecular composition. The presence of CN, CCH, and HCO⁺in Na 2, with comparable abundances to younger PNe, demonstrates that molecular content is maintained into the late PN stage. 

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2. An imaging line survey of OMC-1 to OMC-3: Averaged spectra of template regions

   https://doi.org/10.1051/0004-6361/201936885  

   Brinkmann, N.; Wyrowski, F.; Kauffmann, J.; Colombo, D.; Menten, K. M.; Tang, X. D.; Güsten, R. (April 2020, Astronomy & Astrophysics)

   Context.Recently, sensitive wide-bandwidth receivers in the millimetre regime have enabled us to combine large spatial and spectral coverage for observations of molecular clouds. The resulting capability to map the distributions of lines from many molecules simultaneously yields unbiased coverage of the various environments within star-forming regions. Aims.Our aim is to identify the dominant molecular cooling lines and characteristic emission features in the 1.3 mm window of distinct regions in the northern part of the Orion A molecular cloud. By defining and analysing template regions, we also intend to help with the interpretation of observations from more distant sources which cannot be easily spatially resolved. Methods.We analyse an imaging line survey covering the area of OMC-1 to OMC-3 from 200.2 to 281.8 GHz obtained with the PI230 receiver at the APEX telescope. Masks are used to define regions with distinct properties (e.g. column density or temperature ranges) from which we obtain averaged spectra. Lines of 29 molecular species (55 isotopologues) are fitted for each region to obtain the respective total intensity. Results.We find that strong sources like Orion KL have a clear impact on the emission on larger scales. Although not spatially extended, their line emission contributes substantially to spectra averaged over large regions. Conversely, the emission signatures of dense, cold regions like OMC-2 and OMC-3 (e.g. enhanced N₂H⁺emission and low HCN/HNC ratio) seem to be difficult to pick up on larger scales, where they are eclipsed by signatures of stronger sources. In all regions, HCO⁺appears to contribute between 3 and 6% to the total intensity, the most stable value for all bright species. N₂H⁺shows the strongest correlation with column density, but not with typical high-density tracers like HCN, HCO⁺, H₂CO, or HNC. Common line ratios associated with UV illumination, CN/HNC and CN/HCO⁺, show ambiguous results on larger scales, suggesting that the identification of UV illuminated material may be more challenging. The HCN/HNC ratio may be related to temperature over varying scales. 

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3. Sub-kiloparsec empirical relations and excitation conditions of HCN and HCO ⁺ J = 3–2 in nearby star-forming galaxies

   https://doi.org/10.1051/0004-6361/202244317  

   García-Rodríguez, A; Usero, A; Leroy, A K; Bigiel, F; Jiménez-Donaire, M J; Liu, D; Querejeta, M; Saito, T; Schinnerer, E; Barnes, A; et al (April 2023, Astronomy & Astrophysics)

   We present new HCN and HCO⁺(J= 3–2) images of the nearby star-forming galaxies (SFGs) NGC 3351, NGC 3627, and NGC 4321. The observations, obtained with the Morita ALMA Compact Array, have a spatial resolution of ∼290–440 pc and resolve the innerR_gal ≲ 0.6–1 kpc of the targets, as well as the southern bar end of NGC 3627. We complement this data set with publicly available images of lower excitation lines of HCN, HCO⁺, and CO and analyse the behaviour of a representative set of line ratios: HCN(3–2)/HCN(1–0), HCN(3–2)/HCO⁺(3–2), HCN(1–0)/CO(2–1), and HCN(3–2)/CO(2–1). Most of these ratios peak at the galaxy centres and decrease outwards. We compare the HCN and HCO⁺observations with a grid of one-phase, non-local thermodynamic equilibrium (non-LTE) radiative transfer models and find them compatible with models that predict subthermally excited and optically thick lines. We study the systematic variations of the line ratios across the targets as a function of the stellar surface density (Σ_star), the intensity-weighted CO(2–1) (⟨I_CO⟩), and the star formation rate surface density (Σ_SFR). We find no apparent correlation with Σ_SFR, but positive correlations with the other two parameters, which are stronger in the case of ⟨I_CO⟩. The HCN/CO–⟨I_CO⟩ relations show ≲0.3 dex galaxy-to-galaxy offsets, with HCN(3–2)/CO(2–1)–⟨I_CO⟩ being ∼2 times steeper than HCN(1–0)/CO(2–1). In contrast, the HCN(3–2)/HCN(1–0)–⟨I_CO⟩ relation exhibits a tighter alignment between galaxies. We conclude that the overall behaviour of the line ratios cannot be ascribed to variations in a single excitation parameter (e.g., density or temperature). 

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4. A 2–3 mm high-resolution molecular line survey towards the centre of the nearby spiral galaxy NGC 6946

   https://doi.org/10.1051/0004-6361/202142624  

   Eibensteiner, C.; Barnes, A. T.; Bigiel, F.; Schinnerer, E.; Liu, D.; Meier, D. S.; Usero, A.; Leroy, A. K.; Rosolowsky, E.; Puschnig, J.; et al (March 2022, Astronomy & Astrophysics)

   The complex physical, kinematic, and chemical properties of galaxy centres make them interesting environments to examine with molecular line emission. We present new 2 − 4″ (∼75 − 150 pc at 7.7 Mpc) observations at 2 and 3 mm covering the central 50″ (∼1.9 kpc) of the nearby double-barred spiral galaxy NGC 6946 obtained with the IRAM Plateau de Bure Interferometer. We detect spectral lines from ten molecules: CO, HCN, HCO + , HNC, CS, HC 3 N, N 2 H + , C 2 H, CH 3 OH, and H 2 CO. We complemented these with published 1 mm CO observations and 33 GHz continuum observations to explore the star formation rate surface density Σ SFR on 150 pc scales. In this paper, we analyse regions associated with the inner bar of NGC 6946 – the nuclear region (NUC), the northern (NBE), and southern inner bar end (SBE) and we focus on short-spacing corrected bulk (CO) and dense gas tracers (HCN, HCO + , and HNC). We find that HCO + correlates best with Σ SFR , but the dense gas fraction ( f dense ) and star formation efficiency of the dense gas (SFE dense ) fits show different behaviours than expected from large-scale disc observations. The SBE has a higher Σ SFR , f dense , and shocked gas fraction than the NBE. We examine line ratio diagnostics and find a higher CO(2−1)/CO(1−0) ratio towards NBE than for the NUC. Moreover, comparison with existing extragalactic datasets suggests that using the HCN/HNC ratio to probe kinetic temperatures is not suitable on kiloparsec and sub-kiloparsec scales in extragalactic regions. Lastly, our study shows that the HCO + /HCN ratio might not be a unique indicator to diagnose AGN activity in galaxies. 

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5. The Arizona Radio Observatory 1 mm Spectral Survey of the Hypergiant Star NML Cygni (215–285 GHz)

   https://doi.org/10.3847/1538-3881/ac8df0  

   Singh, A. P.; Edwards, J. L.; Ziurys, L. M. (November 2022, The Astronomical Journal)

   Abstract A sensitive (1σrms ≤ 3 mK; 2 MHz resolution) 1 mm spectral survey (214.5–285.5 GHz) of the envelope of the oxygen-rich supergiant star NML Cygni (NML Cyg) has been conducted using the 10 m Submillimeter Telescope of the Arizona Radio Observatory. These data represent the first spectral line survey of NML Cyg and are complementary to a previous 1 mm survey of the envelope of a similar hypergiant, VY Canis Majoris (VY CMa). The complete NML Cyg data set is presented here. In the survey, 104 emission lines were observed, arising from 17 different molecules and 4 unidentified features. Many of the observed features have complex line profiles, arising from asymmetric outflows characteristic of hypergiant stars. While most of the lines in the survey arise from SiO, SO, SO₂, and SiS, CO had the strongest emission. Five other C-bearing species are identified in the survey (HCN, CN, HCO⁺, CS, and HNC), demonstrating an active carbon chemistry despite the O-rich environment. Moreover, NS was observed, but not NO, although favorable transitions of both molecules lie in the surveyed region. Sulfur chemistry appears to be prominent in NML Cyg and plays an important role in the collimated outflows. The refractory species observed, NaCl and AlO, have narrow emission lines, indicating that these molecules do not reach the terminal expansion velocity. NaCl and AlO likely condense into dust grains at r < 50R_*. From NaCl, the chlorine isotope ratio was determined to be³⁵Cl/³⁷Cl = 3.85 ± 0.30. 

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