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We present a charge distribution-based emission model that calculates the infrared spectrum of fullerenes (C60). Analysis of the modelled spectrum of C60 in various charge states shows that the relative intensity of the features in the 5–10 μm versus 15–20 μm can be used to probe the C60 charge state in interstellar spectra. We further used our model to simulate emission from polycyclic aromatic hydrocarbons (PAHs) and C60 at five positions in the cavity of reflection nebula NGC 7023. Specifically, we modelled the 6.2/11.2 band ratio for circumcoronene and circumcircumcoronene and the 7.0/19.0 band ratio for C60 as a function of the ionization parameter γ. A comparison of the model results with the observed band ratios shows that the γ values in the cavity do not vary significantly, suggesting that the emission in the cavity does not originate from locations at the projected distances. Furthermore, we find that the C60-derived γ values are lower than the PAH-derived values by an order of magnitude. We discuss likely scenarios for this discrepancy. In one scenario, we attribute the differences in the derived γ values to the uncertainties in the electron recombination rates of PAHs and C60. In the other scenario, we suggest that PAHs and C60 are not co-spatial resulting in different γ values from their respective models. We highlight that experiments to determine necessary rates will be required in validating either one of the scenarios.

We present a detailed study on the spatially resolved polycyclic aromatic hydrocarbon (PAH) emission properties in the (circum)nuclear region (NR) and extranuclear regions (ENRs) of M51a using Spitzer-IRS observations. Correlations among PAH intensity ratios are examined with respect to each other, local physical parameters, galactocentric distance ($R_{g}$), and very small grain (VSG) emission. Additional comparison is performed with the mid-infrared emission features in the H$_{\mathrm{II}}$ regions of M33 and M83. The NR exhibits the strongest correlation among the PAH intensity ratios, whereas ENRs are showing increased scatter attributed to ISM emission. Overall, the radiation field hardness has a higher impact on PAH emission than metallicity, with the latter regulating PAH variance as a function of $R_{g}$. Specifically, the variance of PAH emission with respect to the different physical parameters suggests a higher rate of small/medium PAH processing compared to large PAHs and a higher ratio of small-to-large PAHs formed with increasing galactocentric distance. We find similarities between the 7.7 $\mu$m carriers in M51a's NR and M83's H$_{\mathrm{II}}$ regions, the 8.6 $\mu$m carriers in M51a's NR and M33 H$_{\mathrm{II}}$ regions, and both types of carriers between M51a's ENRs, M33's, and M83's H$_{\mathrm{II}}$ regions. We have identified a positive correlation between PAH/VSG and the PAH intensity ratios. We conclude that the relative abundance of PAHs and VSG is not solely driven by the hardness of the radiation field., Comment: 17 pages, 12 figures, 7 tables. Accepted for publication in MNRAS

We present 5--14~$\mu$m spectra at two different positions across the Orion Bar photodissociation region (PDR) obtained with the Infrared Spectrograph onboard the Spitzer Space Telescope and 3.3~$\mu$m PAH observations obtained with the Stratospheric Observatory for Infrared Astronomy (SOFIA). We aim to characterize emission from Polycyclic Aromatic Hydrocarbon (PAH), dust, atomic and molecular hydrogen, argon, sulfur, and neon as a function of distance from the primary illuminating source. We find that all the major PAH bands peak between the ionization front and the PDR front, as traced by H$_{2}$, while variations between these bands become more pronounced moving away from this peak into the face-on PDRs behind the PDR front and at the backside of the \HII\, region. While the relative PAH intensities are consistent with established PAH characteristics, we report unusual behaviours and attribute these to the PDR viewing angle and the strength of the FUV radiation field impinging on the PDRs. We determine the average PAH size which varies across the Orion Bar. We discuss subtle differences seen between the cationic PAH bands and highlight the photo-chemical evolution of carbonaceous species in this PDR environment. We find that PAHs are a good tracer of environmental properties such as the strength of the FUV radiation field and the PAH ionization parameter., Comment: 25 pages, 19 Figures. To be published in MNRAS

We report a computational study of the stability and infrared (IR) vibrational spectra of neutral and singly ionised fullerene cages containing between 44 and 70 carbon atoms. The stability is characterised in terms of the standard enthalpy of formation per CC bond, the HOMO-LUMO gap, and the energy required to eliminate a C$_2$ fragment. We compare the simulated IR spectra of these fullerene species to the observed emission spectra of several planetary nebulae (Tc 1, SMP SMC 16, and SMP LMC 56) where strong C$_{60}$ emission has been detected. Although we could not conclusively identify fullerenes other than C$_{60}$ and C$_{70}$, our results point to the possible presence of smaller (44, 50, and 56-atom) cages in those astronomical objects. Observational confirmation of our prediction should become possible when the James Webb Space Telescope comes online., 11 pages, 13 figures, 1 table. Accepted for publication on MNRAS

We present a detailed analysis of deep VLT/X-Shooter observations of the planetary nebula Tc 1. We calculate gas temperature, density, extinction, and abundances for several species from the empirical analysis of the total line fluxes. In addition, a spatially resolved analysis of the most intense lines provides the distribution of such quantities across the nebula. The new data reveal that several lines exhibit a double peak spectral profile consistent with the blue- and red-shifted components of an expanding spherical shell. The study of such components allowed us to construct for the first time a three-dimensional morphological model, which reveals that Tc 1 is a slightly elongated spheroid with an equatorial density enhancement seen almost pole on. A few bright lines present extended wings (with velocities up to a few hundred km/s), but the mechanism producing them is not clear. We constructed photoionization models for the main shell of Tc 1. The models predict the central star temperature and luminosity, as well as the nebular density and abundances similar to previous studies. Our models indicate that Tc 1 is located at a distance of approximately 2 kpc. We report the first detection of the [Kr III] 6825 A emission line, from which we determine the Krypton abundance. Our model indicates that the main shell of Tc 1 is matter bounded; leaking H ionizing photons may explain the ionization of its faint AGB-remnant halo., Comment: Accepted for publication in MNRAS. 27 pages, 20 figures

We present infrared (IR) and submillimeter observations of the Crab-like supernova remnant (SNR) G54.1+0.3 including 350 micron (SHARC-II), 870 micron (LABOCA), 70, 100, 160, 250, 350, 500 micron (Herschel) and 3-40 micron (Spitzer). We detect dust features at 9, 11 and 21 micron and a long wavelength continuum dust component. The 21 micron dust coincides with [Ar II] ejecta emission, and the feature is remarkably similar to that in Cas A. The IRAC 8 micron image including Ar ejecta is distributed in a shell-like morphology which is coincident with dust features, suggesting that dust has formed in the ejecta. We create a cold dust map that shows excess emission in the northwestern shell. We fit the spectral energy distribution of the SNR using the continuous distributions of ellipsoidal (CDE) grain model of pre-solar grain SiO2 that reproduces the 21 and 9 micron dust features and discuss grains of SiC and PAH that may be responsible for the 10-13 micron dust features. To reproduce the long-wavelength continuum, we explore models consisting of different grains including Mg2SiO4, MgSiO3, Al2O3, FeS, carbon, and Fe3O4. We tested a model with a temperature-dependent silicate absorption coefficient. We detect cold dust (27-44 K) in the remnant, making this the fourth such SNR with freshly-formed dust. The total dust mass in the SNR ranges from 0.08-0.9 Msun depending on the grain composition, which is comparable to predicted masses from theoretical models. Our estimated dust masses are consistent with the idea that SNe are a significant source of dust in the early Universe., MNRAS: accepted on June 28, 2018 and published on July 4, 2018