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

Flint A.R., Fortenberry R.C.

Westbrook B.R., Fortenberry R.C.

Palmer C.Z., Fortenberry R.C.

Maercker M., Khouri T., Mecina M., De Beck E.

Aims.In this paper, we aim to constrain the dust mass and grain sizes in the interaction regions between the stellar winds and the interstellar medium (ISM) around asymptotic giant branch (AGB) stars. By describing the dust in these regions, we aim to shed light on the role of evolved low-mass stars in the origin of dust in galaxies.Methods.We use images in the far-infrared (FIR) at 70 and 160 µm to derive dust temperatures and dust masses in the wind-ISM interaction regions around a sample of carbon-rich and oxygen-rich AGB stars. The dust temperature and mass are determined in two ways: first, directly from the data using the ratio of the measured fluxes and assuming opacities for dust with a constant grain size of 0.1 µm, and then using three-dimensional dust-radiative transfer models spatially constrained by the observations. Each of the radiative transfer models contains one constant grain size, which is varied between 0.01 and 5.0 µm.Results.We find that the observed dust mass in the wind-ISM interaction regions is consistent with mass accumulated from the stellar winds. For the carbon-rich sources, adding the spatial constraints in the radiative transfer models results in preferentially larger grain sizes (≈2 µm). For the oxygen-rich sources, the spatial constraints result in overly high temperatures in the models, making it impossible to fit the observed FIR ratio irrespective of the grain size used, indicating a more complex interplay of grain properties and the stellar radiation field.Conclusions.Our results have implications for how likely it is for the grains to survive the transition into the ISM, and the properties of dust particles that later act as seeds for grain growth in the ISM. However, our results for the oxygen-rich sources show that the derivation of dust properties is not straight forward, requiring more complex modelling.

Saberi M., Khouri T., Velilla-Prieto L., Fonfría J.P., Vlemmings W.H., Wedemeyer S.

The nucleosynthesis production of fluorine (F) is still a matter of debate. Asymptotic giant branch (AGB) stars are one of the main candidates for F production. However, their contribution to the total F budget is not fully known due to the lack of observations. In this paper, we report the detection of aluminium monofluoride (AlF) line emission, one of the two main carriers of F in the gas-phase in the outflow of evolved stars, towards five nearby oxygen-rich (M-type) AGB stars. We studied the Atacama large millimetre/sub-millimetre array (ALMA) observations of AlF (v = 0, J = 4—3, 9–8, 10–9, and 15–14) and (v = 1, J = 7–6) line emission towards o Ceti, and (v = 0, J = 7–6 and 15–14) lines towards R Leo. We also report a tentative detection of AlF (v = 0, J = 7–6) line in IK Tau, (v = 0, J = 15–14) line towards R Dor, and (v = 0, J = 7–6 and J = 15–14) lines in W Hya. From spatially resolved observations, we estimated the AlF emitting region with a radius ~11R⋆ for o Ceti and ~9R⋆ for R Leo. From population diagram analysis, we report the AlF column densities of ~5.8 × 1015 cm−2 and ~3 × 1015 cm−2 for o Ceti and R Leo, respectively, within these regions. For o Ceti, we used the C18O (v = 0, J = 3–2) observations to estimate the H2 column density of the emitting region. We found a fractional abundance of fAIF/H2 ~ (2.5 ± 1.7) × 10−8. This gives a lower limit on the F budget in o Ceti and is compatible with the solar F budget fF/H2 = (5 ± 2) × 10−8. For R Leo, a fractional abundance fAIF/H2 = (1.2 ± 0.5) × 10−8 is estimated. For other sources, we cannot precisely determine the emitting region based on the available data. Assuming an emitting region with a radius of ~11R⋆ and the rotational temperatures derived for o Ceti and R Leo, we crudely approximated the AlF column density to be ~(1.2−1.5) × 1015 cm−2 in W Hya, ~(2.5−3.0) × 1014 cm−2 in R Dor, and ~(0.6−1.0) × 1016 cm−2 in IK Tau. These result in fractional abundances within a range of fAIF/H2 ~ (0.1 − 4) × 10−8 in W Hya, R Dor, and IK Tau.

Jakobsen P., Ferruit P., Alves de Oliveira C., Arribas S., Bagnasco G., Barho R., Beck T.L., Birkmann S., Böker T., Bunker A.J., Charlot S., de Jong P., de Marchi G., Ehrenwinkler R., Falcolini M., et. al.

We provide an overview of the design and capabilities of the near-infrared spectrograph (NIRSpec) onboard theJames WebbSpace Telescope. NIRSpec is designed to be capable of carrying out low-resolution (R = 30−330) prism spectroscopy over the wavelength range 0.6–5.3 μm and higher resolution (R = 500−1340 orR = 1320−3600) grating spectroscopy over 0.7–5.2 μm, both in single-object mode employing any one of five fixed slits, or a 3.1 × 3.2 arcsec2integral field unit, or in multiobject mode employing a novel programmable micro-shutter device covering a 3.6 × 3.4 arcmin2field of view. The all-reflective optical chain of NIRSpec and the performance of its different components are described, and some of the trade-offs made in designing the instrument are touched upon. The faint-end spectrophotometric sensitivity expected of NIRSpec, as well as its dependency on the energetic particle environment that its two detector arrays are likely to be subjected to in orbit are also discussed.

Grosselin D., Fortenberry R.C.

Gobrecht D., Plane J.M., Bromley S.T., Decin L., Cristallo S., Sekaran S.

Context.Aluminium oxide (alumina; Al2O3) is a promising candidate as a primary dust condensate in the atmospheres of oxygen-rich evolved stars. Therefore, alumina ‘seed’ particles might trigger the onset of stellar dust formation and of stellar mass loss in the wind. However, the formation of alumina dust grains is not well understood.Aims.We aim to shed light on the initial steps of cosmic dust formation (i.e. nucleation) in oxygen-rich environments via a quantum-chemical bottom-up approach.Methods.Starting with an elemental gas-phase composition, we construct a detailed chemical-kinetic network that describes the formation and destruction of aluminium-bearing molecules and dust-forming (Al2O3)nclusters up to the size of dimers (n= 2) coagulating to tetramers (n= 4). Intermediary species include the prevalent gas-phase molecules AlO and AlOH as well as AlxOyclusters withx= 1–5,y= 1–6. The resulting extensive network is applied to two model stars, which represent a semi-regular variable and a Mira type, and to different circumstellar gas trajectories, including a non-pulsating outflow and a pulsating model. The growth of larger-sized (Al2O3)nclusters withn= 4–10 is described by the temperature-dependent Gibbs free energies of the most favourable structures (i.e. the global minima clusters) as derived from global optimisation techniques and calculated via density functional theory. We provide energies, bond characteristics, electrostatic properties, and vibrational spectra of the clusters as a function of size,n, and compare these to corundum, which corresponds to the crystalline bulk limit (n→∞).Results.The circumstellar aluminium gas-phase chemistry in oxygen-rich giants is primarily controlled by AlOH and AlO, which are tightly coupled by the reactions AlO+H2, AlO+H2O, and their reverse. Models of semi-regular variables show comparatively higher AlO abundances, as well as a later onset and a lower efficiency of alumina cluster formation when compared to Mira-like models. The Mira-like models exhibit an efficient cluster production that accounts for more than 90% of the available aluminium content, which is in agreement with the most recent ALMA observations. Chemical equilibrium calculations fail to predict both the alumina cluster formation and the abundance trends of AlO and AlOH in the asymptotic giant branch dust formation zone. Furthermore, we report the discovery of hitherto unreported global minimum candidates and low-energy isomers for cluster sizesn= 7, 9, and 10. A homogeneous nucleation scenario, where Al2O3monomers are successively added, is energetically viable. However, the formation of the Al2O3monomer itself represents an energetic bottleneck. Therefore, we provide a bottom-up interpolation of the cluster characteristics towards the bulk limit by excluding the monomer, approximately following ann−1∕3dependence.

Westbrook B.R., Patel D.J., Dallas J.D., Swartzfager G.C., Lee T.J., Fortenberry R.C.

The recent detection of ethynyl-functionalized cyclopropenylidene (c-C3HC2H) has initiated the search for other functional forms of cyclopropenylidene (c-C3H2) in space. There is existing gas-phase rotational spectroscopic data for cyano-cyclopropenylidene (c-C3HCN), but the present work provides the first anharmonic vibrational spectral data for that molecule, as well as the first full set of both rotational and vibrational spectroscopic data for fluoro- and chloro-cyclopropenylidenes (c-C3HF and c-C3HCl). All three molecules have fundamental vibrational frequencies with substantial infrared intensities. Namely, c-C3HCN has a moderately intense fundamental frequency at 1244.4 cm-1, while c-C3HF has two large intensity modes at 1765.4 and 1125.3 cm-1 and c-C3HCl again has two large intensity modes at 1692.0 and 1062.5 cm-1. All of these frequencies are well within the spectral range covered by the high-resolution EXES instrument on NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA). Further, all three molecules have dipole moments of around 3.0 D in line with c-C3H2, enabling them to be observed by pure rotational spectroscopy, as well. Thus, the rovibrational spectral data presented herein should assist with future laboratory studies of functionalized cyclopropenylidenes and may lead to their interstellar or circumstellar detection.

Danilovich T., Van de Sande M., Plane J.M., Millar T.J., Royer P., Amor M.A., Hammami K., Decock L., Gottlieb C.A., Decin L., Richards A.M., De Beck E., Baudry A., Bolte J., Cannon E., et. al.

Context. S-type asymptotic giant branch (AGB) stars are thought to be intermediates in the evolution of oxygen- to carbon-rich AGB stars. The chemical compositions of their circumstellar envelopes are also intermediate but have not been studied in as much detail as their carbon- and oxygen-rich counterparts. W Aql is a nearby S-type star, with well-known circumstellar parameters, making it an ideal object for in-depth study of less common molecules. Aims. We aim to determine the abundances of AlCl and AlF from rotational lines, which have been observed for the first time towards an S-type AGB star. In combination with models based on PACS observations, we aim to update our chemical kinetics network based on these results. Methods. We analyse ALMA observations towards W Aql of AlCl in the ground and first two vibrationally excited states and AlF in the ground vibrational state. Using radiative transfer models, we determine the abundances and spatial abundance distributions of Al35Cl, Al37Cl, and AlF. We also model HCl and HF emission and compare these models to PACS spectra to constrain the abundances of these species. Results. AlCl is found in clumps very close to the star, with emission confined within 0′′.1 of the star. AlF emission is more extended, with faint emission extending 0′′.2 to 0′′.6 from the continuum peak. We find peak abundances, relative to H2, of 1.7 × 10−7 for Al35Cl, 7 × 10−8 for Al37Cl, and 1 × 10−7 for AlF. From the PACS spectra, we find abundances of 9.7 × 10−8 and ≤10−8, relative to H2, for HCl and HF, respectively. Conclusions. The AlF abundance exceeds the solar F abundance, indicating that fluorine synthesised in the AGB star has already been dredged up to the surface of the star and ejected into the circumstellar envelope. From our analysis of chemical reactions in the wind, we conclude that AlF may participate in the dust formation process, but we cannot fully explain the rapid depletion of AlCl seen inthe wind.

Labiano A., Argyriou I., Álvarez-Márquez J., Glasse A., Glauser A., Patapis P., Law D., Brandl B.R., Justtanont K., Lahuis F., Martínez-Galarza J.R., Mueller M., Noriega-Crespo A., Royer P., Shaughnessy B., et. al.

Context. The Mid-Infrared Instrument (MIRI) onboard the James Webb Space Telescope (JWST) will provide imaging, coronagraphy, low-resolution spectroscopy, and medium-resolution spectroscopy at unprecedented sensitivity levels in the mid-infrared wavelength range. The Medium Resolution Spectrometer (MRS) of MIRI is an integral field spectrograph that provides diffraction-limited spectroscopy between 4.9 and 28.3 μm, within a field of view (FOV) varying from ∼13 to ∼56 arcsec square. The design for MIRI MRS conforms with the goals of the JWST mission to observe high redshift galaxies and to study cosmology as well as observations of galactic objects, and stellar and planetary systems. Aims. From ground testing, we calculate the physical parameters essential for general observers and calibrating the wavelength solution and resolving power of the MRS which is critical for maximizing the scientific performance of the instrument. Methods. We have used ground-based observations of discrete spectral features in combination with Fabry-Perot etalon spectra to characterize the wavelength solution and spectral resolving power of the MRS. We present the methodology used to derive the MRS spectral characterization, which includes the precise wavelength coverage of each MRS sub-band, computation of the resolving power as a function of wavelength, and measuring slice-dependent spectral distortions. Results. The ground calibration of the MRS shows that it will cover the wavelength ranges from 4.9 to 28.3 μm, divided in 12 overlapping spectral sub-bands. The resolving power is R ≳ 3500 in channel 1, R ≳ 3000 in channel 2, R ≳ 2500 in channel 3, and R ≳ 1500 in channel 4. The MRS spectral resolution optimizes the sensitivity for detection of spectral features with a velocity width of ∼100 km s−1 which is characteristic of most astronomical phenomena JWST aims to study in the mid-infrared. Based on the ground test data, the wavelength calibration accuracy is estimated to be below one-tenth of a pixel (0.1 nm at 5 μm and 0.4 at 28 μm), with small systematic shifts due to the target position within a slice for unresolved sources that have a maximum amplitude of about 0.25 spectral resolution elements. The absolute wavelength calibration is presently uncertain at the level of 0.35 nm at 5 μm and 46 nm at 28 μm, and it will be refined using in-flight commissioning observations. Conclusions. Based on ground test data, the MRS complies with the spectral requirements for both the R and wavelength accuracy for which it was designed. We also present the commissioning strategies and targets that will be followed to update the spectral characterization of the MRS.

Ramal-Olmedo J.C., Menor-Salván C.A., Fortenberry R.C.

Context. Formaldehyde is a potential biogenic precursor involved in prebiotic chemical evolution. The cold conditions of the interstellar medium (ISM) allow H2CO to be reactive, playing a significant role as a chemical intermediate in formation pathways leading to interstellar complex organic molecules. However, gas-phase molecular formation mechanisms in cold regions of the ISM are poorly understood. Aims. We computationally determine the most favored gas-phase molecular formation mechanisms at local thermodynamic equilibrium conditions that can produce the detected amounts of H2CO in diffuse molecular clouds (DMCs), in dark, cold, and dense molecular clouds (DCDMCs), and in three regions of circumstellar envelopes of low-mass protostars (CELMPs). Methods. The potential energy surfaces, thermodynamic functions, and single-point energies for transition states were calculated at the CCSD(T)-F12/cc-pVTZ-F12 and MP2/aug-cc-pVDZ levels of theory and basis sets. Molecular thermodynamics and related partition functions were obtained by applying the Maxwell-Boltzmann quantum statistics theory from energies computed at CCSD(T)-F12/cc-pVTZ-F12 with corrections for zero-point energy. A literature review on detected abundances of reactants helped us to propose the most favorable formation routes. Results. The most probable reactions that produce H2CO in cold astrophysical regions are: 1CH2 + ⋅3O2 →1H2CO + O⋅(3P) in DMCs, ⋅3CH2 + ⋅3O2 →1H2CO + ⋅O(3P) in DCDMCs, and ⋅CH3 + ⋅O(3P) →1H2CO + ⋅H in region III, ⋅CH3 +⋅O(1D) →1H2CO + ⋅H in region II, and 1CH2 + ⋅3O2 →1H2CO + ⋅O(3P) in region I belonging to CELMPs. Conclusions. Quantum chemical calculations suggest that the principal carbonaceous precursors of H2CO in cold regions for the gas-phase are CH2(a1A1), and ⋅CH2(X3B1) combined with ⋅O2(3Σg) and ⋅CH3(2A”) + ⋅O(3P) / O(1D). Reactions based on more complex reagents yield less effective thermodynamics in the gas-phase H2CO molecular formation.

Watrous A.G., Davis M.C., Fortenberry R.C.

Small, inorganic hydrides are likely hiding in plain sight, waiting to be detected toward various astronomical objects. AlH2OH can form in the gas phase via a downhill pathway, and the present, high-level quantum chemical study shows that this molecule exhibits bright infrared features for anharmonic fundamentals in regions above and below that associated with polycyclic aromatic hydrocarbons. AlH2OH along with HMgOH, HMgNH2, and AlH2NH2 are also polar with AlH2OH having a 1.22 D dipole moment. AlH2OH and likely HMgOH have nearly unhindered motion of the hydroxyl group but are still strongly bonded. This could assist in gas phase synthesis, where aluminum oxide and magnesium oxide minerals likely begin their formation stages with AlH2OH and HMgOH. This work provides the spectral data necessary to classify these molecules such that observations as to the buildup of nanoclusters from small molecules can possibly be confirmed.

Gardner M.B., Westbrook B.R., Fortenberry R.C., Lee T.J.

• Quartic force fields of the highest level provide rotational constants to within, on average, 34 MHz of experiment. • Tetraatomic species with low anharmonicities can be within 20 MHz. • These same methods provide vibrational frequencies to within, on average, 5.8 cm −1 . The CcCR quartic force field (QFF) methodology is capable of computing B 0 and C 0 rotational constants to within 35 MHz (0.14%) of experiment for triatomic and larger molecules with at least two heavy atoms. Additionally, the same constants for molecules with four or more atoms agree to within 20 MHz (0.12%) of experiment for the current test set. This work also supports previous claims that the same QFF methodology can produce fundamental vibrational frequencies with a deviation less than 5.7 cm −1 from experiment. Consequently, this approach of augmenting complete basis set extrapolated energies with treatments of core electron correlation and scalar relativity produces some of the most accurate rovibrational spectroscopic data available.

Franke P.R., Stanton J.F., Douberly G.E.

This article primarily discusses the utility of vibrational perturbation theory for the prediction of X-H stretching vibrations with particular focus on the specific variant, second-order vibrational perturbation theory with resonances (VPT2+K). It is written as a tutorial, reprinting most important formulas and providing numerous simple examples. It discusses the philosophy and practical considerations behind vibrational simulations with VPT2+K, including but not limited to computational method selection, cost-saving approximations, approaches to evaluating intensity, resonance identification, and effective Hamiltonian structure. Particular attention is given to resonance treatments, beginning with simple Fermi dyads and gradually progressing to arbitrarily large polyads that describe both Fermi and Darling-Dennison resonances. VPT2+K combined with large effective Hamiltonians is shown to be a reliable framework for modeling the complicated CH stretching spectra of alkenes. An error is also corrected in the published analytic formula for the VPT2 transition moment between the vibrational ground state and triply excited states.

Dotson J.R., Palmer C.Z., Fortenberry R.C.

Fortenberry R.C., McGuire B.A.

Abstract The formation of silicon monosulfide (SiS) in space appears to be a difficult process, but the present work shows that a previously excluded pathway may contribute to its astronomical abundance. Reaction of the radicals SH + SiH produces SiS with a submerged transition state and generates a stabilizing H2 molecule as a product to dissipate the kinetic energy. Such is a textbook chemical reaction for favorable gas-phase chemistry. While previously proposed mechanisms reacting atomic sulfur and silicon with SiH, SH, and H2S will still be major contributors to the production of SiS, an abundance of SiS in certain regions could be a marker for the presence of SiH where it has previously been unobserved. These quantum chemically computed reaction profiles imply that the silicon-chalcogen chemistry of molecular clouds, shocked regions, or protoplanetary disks may be richer than previously thought. Quantum chemical spectral data for the intermediate cis- and trans-HSiSH are also provided to aid in their potential spectroscopic characterization.

Fortenberry R.C.