Search

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

22 pags., 8 figs., 1 tab., Massive stars disrupt their natal molecular cloud material through radiative and mechanical feedback processes. These processes have profound effects on the evolution of interstellar matter in our Galaxy and throughout the universe, from the era of vigorous star formation at redshifts of 1-3 to the present day. The dominant feedback processes can be probed by observations of the Photo-Dissociation Regions (PDRs) where the far-ultraviolet photons of massive stars create warm regions of gas and dust in the neutral atomic and molecular gas. PDR emission provides a unique tool to study in detail the physical and chemical processes that are relevant for most of the mass in inter-and circumstellar media including diffuse clouds, proto-planetary disks, and molecular cloud surfaces, globules, planetary nebulae, and star-forming regions. PDR emission dominates the infrared (IR) spectra of star-forming galaxies. Most of the Galactic and extragalactic observations obtained with the James Webb Space Telescope (JWST) will therefore arise in PDR emission. In this paper we present an Early Release Science program using the MIRI, NIRSpec, and NIRCam instruments dedicated to the observations of an emblematic and nearby PDR: the Orion Bar. These early JWST observations will provide template data sets designed to identify key PDR characteristics in JWST observations. These data will serve to benchmark PDR models and extend them into the JWST era. We also present the Science-Enabling products that we will provide to the community. These template data sets and Science-Enabling products will guide the preparation of future proposals on star-forming regions in our Galaxy and beyond and will facilitate data analysis and interpretation of forthcoming JWST observations., Support for JWST-ERS program ID 1288 was provided through grants from the STScI under NASA contract NAS5-03127 to STScI (K.G., D.V.D.P., M.R.), Univ. of Maryland (M.W., M.P.), Univ. of Michigan (E.B., F.A.), and Univ. of Toledo (T.S.-Y.L.). O.B. and E.H. are supported by the Programme National “Physique et Chimie du Milieu Interstellaire” (PCMI) of CNRS/INSU with INC/INP co-funded by CEA and CNES, and through APR grants 6315 and 6410 provided by CNES. E. P. and J.C. acknowledge support from the National Science and Engineering Council of Canada (NSERC) Discovery Grant program (RGPIN-2020-06434 and RGPIN-2021-04197 respectively). E.P. acknowledges support from a Western Strategic Support Accelerator Grant (ROLA ID 0000050636). J.R.G. and S.C. thank the Spanish MCINN for funding support under grant PID2019-106110GB-I00. Work by M.R. and Y.O. is carried out within the Collaborative Research Centre 956, subproject C1, funded by the Deutsche Forschungsgemeinschaft (DFG)—project ID 184018867. T.O. acknowledges support from JSPS Bilateral Program, grant No. 120219939. M.P. and M.W. acknowledge support from NASA Astrophysics Data Analysis Program award #80NSSC19K0573. C.B. is grateful for an appointment at NASA Ames Research Center through the San José State University Research Foundation (NNX17AJ88A) and acknowledges support from the Internal Scientist Funding Model (ISFM) Directed Work Package at NASA Ames titled: “Laboratory Astrophysics—The NASA Ames PAH IR Spectroscopic Database.”

Full list of authors: De Marco, Orsola; Akashi, Muhammad; Akras, Stavros; Alcolea, Javier; Aleman, Isabel; Amram, Philippe; Balick, Bruce; De Beck, Elvire; Blackman, Eric G.; Boffin, Henri M. J.; Boumis, Panos; Bublitz, Jesse; Bucciarelli, Beatrice; Bujarrabal, Valentin; Cami, Jan; Chornay, Nicholas; Chu, You-Hua; Corradi, Romano L. M.; Frank, Adam; García-Hernández, D. A.; García-Rojas, Jorge; García-Segura, Guillermo; Gómez-Llanos, Veronica; Gonçalves, Denise R.; Guerrero, Martín A.; Jones, David; Karakas, Amanda I.; Kastner, Joel H.; Kwok, Sun; Lykou, Foteini; Manchado, Arturo; Matsuura, Mikako; McDonald, Iain; Miszalski, Brent; Mohamed, Shazrene S.; Monreal-Ibero, Ana; Monteiro, Hektor; Montez, Rodolfo; Baez, Paula Moraga; Morisset, Christophe; Nordhaus, Jason; Mendes de Oliveira, Claudia; Osborn, Zara; Otsuka, Masaaki; Parker, Quentin A.; Peeters, Els; Quint, Bruno C.; Quintana-Lacaci, Guillermo; Redman, Matt; Ruiter, Ashley J.; Sabin, Laurence; Sahai, Raghvendra; Contreras, Carmen Sánchez; Santander-García, Miguel; Seitenzahl, Ivo; Soker, Noam; Speck, Angela K.; Stanghellini, Letizia; Steffen, Wolfgang; Toalá, Jesús A.; Ueta, Toshiya; Van de Steene, Griet; Van Winckel, Hans; Ventura, Paolo; Villaver, Eva; Vlemmings, Wouter; Walsh, Jeremy R.; Wesson, Roger; Zijlstra, Albert A., Planetary nebulae—the ejected envelopes of red giant stars—provide us with a history of the last, mass-losing phases of 90% of stars initially more massive than the Sun. Here we analyse images of the planetary nebula NGC 3132 from the James Webb Space Telescope (JWST) Early Release Observations. A structured, extended hydrogen halo surrounding an ionized central bubble is imprinted with spiral structures, probably shaped by a low-mass companion orbiting the central star at about 40–60 au. The images also reveal a mid-infrared excess at the central star, interpreted as a dusty disk, which is indicative of an interaction with another closer companion. Including the previously known A-type visual companion, the progenitor of the NGC 3132 planetary nebula must have been at least a stellar quartet. The JWST images allow us to generate a model of the illumination, ionization and hydrodynamics of the molecular halo, demonstrating the power of JWST to investigate complex stellar outflows. Furthermore, new measurements of the A-type visual companion allow us to derive the value for the mass of the progenitor of a central star with excellent precision: 2.86 ± 0.06 M⊙. These results serve as pathfinders for future JWST observations of planetary nebulae, providing unique insight into fundamental astrophysical processes including colliding winds and binary star interactions, with implications for supernovae and gravitational-wave systems. © 2022, The Author(s), under exclusive licence to Springer Nature Limited., We acknowledge the International Astronomical Union that oversees the work of Commission H3 on Planetary Nebulae. It is through the coordinating activity of this committee that this paper came together. S.A. acknowledges support under the grant 5077 financed by IAASARS/NOA. J.A. and V.B. acknowledge support from the EVENTs/Nebulae-Web research programme, Spanish AEI grant PID2019-105203GB-C21. I.A. acknowledges the support of CAPES, Brazil (Finance Code 001). E.D.B. acknowledges financial support from the Swedish National Space Agency. E.G.B. acknowledges NSF grants AST-1813298 and PHY-2020249. J.C. and E.P. acknowledge support from an NSERC Discovery Grant. G.G.-S. thanks M. L. Norman and the Laboratory for Computational Astrophysics for the use of ZEUS-3D. D.A.G.-H. and A.M. acknowledge support from the ACIISI, Gobierno de Canarias and the European Regional Development Fund (ERDF) under grant with reference PROID2020010051 as well as from the State Research Agency (AEI) of the Spanish Ministry of Science and Innovation (MICINN) under grant PID2020-115758GB-I00. J.G.-R. acknowledges support from Spanish AEI under Severo Ochoa Centres of Excellence Programme 2020-2023 (CEX2019-000920-S). J.G.-R. and V.G.-L. acknowledge support from ACIISI and ERDF under grant ProID2021010074. D.R.G. acknowledges the CNPq grant 313016/2020-8. M.A.G. acknowledges support of grant PGC2018-102184-B-I00 of the Ministerio de Educación, Innovación y Universidades cofunded with FEDER funds and from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709). D.J. acknowledges support from the Erasmus+ programme of the European Union under grant number 2020-1-CZ01-KA203-078200. A.I.K. and Z.O. were supported by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013. This research is/was supported by an Australian Government Research Training Program (RTP) Scholarship. M.M. and R.W. acknowledge support from STFC Consolidated grant (2422911). C.M. acknowledges support from UNAM/DGAPA/PAPIIT under grant IN101220. S.S.M. acknowledges funding from UMiami, the South African National Research Foundation and the University of Cape Town VC2030 Future Leaders Award. J.N. acknowledges support from NSF grant AST-2009713. C.M.d.O. acknowledges funding from FAPESP through projects 2017/50277-0, 2019/11910-4 e 2019/26492-3 and CNPq, process number 309209/2019-6. J.H.K. and P.M.B. acknowledge support from NSF grant AST-2206033 and a NRAO Student Observing Support grant to Rochester Institute of Technology. M.O. was supported by JSPS Grants-in-Aid for Scientific Research(C) (JP19K03914 and 22K03675). Q.A.P. acknowledges support from the HKSAR Research grants council. Vera C. Rubin Observatory is a Federal project jointly funded by the National Science Foundation (NSF) and the Department of Energy (DOE) Office of Science, with early construction funding received from private donations through the LSST Corporation. The NSF-funded LSST (now Rubin Observatory) Project Office for construction was established as an operating centre under the management of the Association of Universities for Research in Astronomy (AURA). The DOE-funded effort to build the Rubin Observatory LSST Camera (LSSTCam) is managed by SLAC National Accelerator Laboratory (SLAC). A.J.R. was supported by the Australian Research Council through award number FT170100243. L.S. acknowledges support from PAPIIT UNAM grant IN110122. C.S.C.’s work is part of I+D+i project PID2019-105203GB-C22 funded by the Spanish MCIN/AEI/10.13039/501100011033. M.S.-G. acknowledges support by the Spanish Ministry of Science and Innovation (MICINN) through projects AxIN (grant AYA2016-78994-P) and EVENTs/Nebulae-Web (grant PID2019-105203GB-C21). R.S.’s contribution to the research described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. J.A.T. thanks the Marcos Moshisnky Fundation (Mexico) and UNAM PAPIIT project IA101622. E.V. acknowledges support from the ‘On the rocks II project’ funded by the Spanish Ministerio de Ciencia, Innovación y Universidades under grant PGC2018-101950-B-I00. A.A.Z. acknowledges support from STFC under grant ST/T000414/1. This research made use of Photutils, an Astropy package for detection and photometry of astronomical sources83, of the Spanish Virtual Observatory (https://svo.cab.inta-csic.es) project funded by MCIN/AEI/10.13039/501100011033/ through grant PID2020-112949GB-I00 and of the computing facilities available at the Laboratory of Computational Astrophysics of the Universidade Federal de Itajubá (LAC-UNIFEI, which is maintained with grants from CAPES, CNPq and FAPEMIG).

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

Context. Nearby photo-dissociation regions (PDRs), where the gas and dust are heated by the far-ultraviolet (FUV) irradiation emitted from stars, are ideal templates with which to study the main stellar feedback processes. Aims. With this study, we aim to probe the detailed structures at the interfaces between ionized, atomic, and molecular gas in the Orion Bar. This nearby prototypical strongly irradiated PDR are among the first targets of the James Webb Space Telescope (JWST) within the framework of the PDRs4All Early Release Science program. Methods. We employed the subarcsecond resolution accessible with Keck-II NIRC2 and its adaptive optics system to obtain images of the vibrationally excited line H2 1−0 S(1) at 2.12 µm that are more detailed and complete than ever before. H2 1−0 S(1) traces the dissociation front (DF), and the [FeII] and Brγ lines, at 1.64 and 2.16 µm, respectively, trace the ionization front (IF). The former is a powerful tracer of the FUV radiation field strength and gas density distribution at the PDR edge, while the last two trace the temperature and density distribution from the ionized gas to the PDR. We obtained narrow-band filter images in these key gas line diagnostics over ~40″ at spatial scales of ~0.1″ (~0.0002 pc or ~40 AU at 414 pc). Results. The Keck/Near Infrared Camera 2 (NIRC2) observations spatially resolve a plethora of irradiated substructures such as ridges, filaments, globules, and proplyds. This portends what JWST should accomplish and how it will complement the highest resolution Atacama Large Millimeter/submillimeter Array (ALMA) maps of the molecular cloud. We observe a remarkable spatial coincidence between the H2 1−0 S(1) vibrational and HCO+ J = 4−3 rotational emission previously obtained with ALMA. This likely indicates the intimate link between these two molecular species and highlights that in high-pressure PDRs, the H/H2 and C+/C/CO transitions zones come closer than in a typical layered structure of a constant density PDR. The H/H2 dissociation front appears as a highly structured region containing substructures with a typical thickness of a few ~10−3 pc.

We carried out a principal component analysis (PCA) of the fluxes of five polycyclic aromatic hydrocarbon (PAH) bands at 6.2, 7.7, 8.6, 11.0, and 11.2 $\mu$m in the reflection nebula NGC 7023 comprising of the photodissociation region (PDR) and a cavity. We find that only two principal components (PCs) are required to explain the majority of the observed variance in PAH fluxes (98 %). The first PC ($PC_{1}$), which is the primary driver of the variance, represents the total PAH emission. The second PC ($PC_{2}$) is related to the ionization state of PAHs across the nebula. This is consistent with the results of a similar analysis of the PAH emission in NGC 2023. The biplots and the correlations of PCs with the various PAH ratios show that there are two subsets of ionic bands with the 6.2 and 7.7 $\mu$m bands forming one subset and the 8.6 and 11.0 $\mu$m bands the other. However, the distinction between these subsets is only present in the PDR. We have also carried out a separate PCA analysis of the PAH fluxes, this time only considering variations in the cavity. This shows that in the cavity, $PC_{2}$ is not related to the charge state of PAHs, but possibly to structural molecular changes., Comment: 15 pages, 12 figures, accepted for publication in MNRAS

We present a charge distribution based model that computes the infrared spectrum of polycyclic aromatic hydrocarbon (PAH) molecules using recent measurements or quantum chemical calculations of specific PAHs. The model is applied to a sample of well-studied photodissociation regions (PDRs) with well-determined physical conditions (the radiation field strength, $G_{0}$, electron density $n_{e}$, and the gas temperature, $T_{\rm gas}$). Specifically, we modelled the emission of five PAHs ranging in size from 18 to 96 carbon atoms, over a range of physical conditions characterized by the ionization parameter $\gamma = G_{0}\times T_{\rm gas}^{1/2}/n_{e}$. The anions emerge as the dominant charge carriers in low $\gamma $ ($< 2\times 10^{2}$) environments, neutrals in the intermediate $\gamma$ ($10^{3} - 10^{4}$) environments, and cations in the high $\gamma$ ($ > 10^{5}$) environments. Furthermore, the PAH anions and cations exhibit similar spectral characteristics. The similarity in the cationic and anionic spectra translates into the interpretation of the 6.2/(11.0+11.2) band ratio, with high values of this ratio associated with large contributions from either cations or anions. The model's predicted values of 6.2/(11.0+11.2) and 3.3/6.2 compared well to the observations in the PDRs NGC 7023, NGC 2023, the horsehead nebula, the Orion bar, and the diffuse ISM, demonstrating that changes in the charge state can account for the variations in the observed PAH emission. We also reassess the diagnostic potential of the 6.2/(11.0+11.2) vs 3.3/(11.0+11.2) ratios and show that without any prior knowledge about $\gamma$, the 3.3/(11.0+11.2) can predict the PAH size, but the 6.2/(11.0+11.2) cannot predict the $\gamma$ of the astrophysical environment., Comment: 27 pages, 24 figures, Accepted for publication 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

Infrared emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 $\mu$m, attributed to polycyclic aromatic hydrocarbons, show variations in relative intensity, shape, and peak position. These variations depend on the physical conditions of the photodissociation region (PDR) in which strong PAH emission arises, but their relationship has yet to be fully quantified. We aim to better calibrate the response of PAH species to their environment using observations with matching apertures and spatial resolution. We present observations from the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS) on board the Stratospheric Observatory for Infrared Astronomy (SOFIA) of the gas cooling lines [OI] 63, 146 $\mu$m and [CII] 158 $\mu$m in the reflection nebula NGC 1333 and use archival dust continuum observations from the Photodetector Array Camera and Spectrometer (PACS) on board Herschel. We employ PDR modelling to derive the physical conditions and compare these with the characteristics of the PAH emission as observed with the Infrared Spectrometer (IRS) on board Spitzer. We find distinct spatial characteristics for the various PAH spectral components. We conclude that the ionic bands (6.2, 7.7, 8.6, and 11.0) and the 7-9 $\mu$m emission are due to multiple PAH sub-populations and that the plateaus are distinct from the features perched on top. The 6-9 $\mu$m PAH emission exhibit a significant change in behaviour between the irradiated PDR and diffuse outskirts, confirming these bands arise from multiple PAH sub-populations with different underlying molecular properties. We find multiple promising relationships between PAH ratios and the FUV radiation field strength but no clear correlations with the PAH ionization parameter., Comment: 22 pages, 21 figures, to be published in MNRAS

We present observations from the First Light Infrared TEst CAMera (FLITECAM) on board the Stratospheric Observatory for Infrared Astronomy (SOFIA), the Spitzer Infrared Array Camera (IRAC) and the Spitzer Infrared Spectrograph (IRS) SH mode in three well-known Photodissocation Regions (PDRs), the reflection nebulae (RNe) NGC 7023 and NGC 2023 and to the southeast of the Orion Bar, which are well suited to probe emission from Polycyclic Aromatic Hydrocarbon molecules (PAHs). We investigate the spatial behaviour of the FLITECAM 3.3 um filter as a proxy for the 3.3 um PAH band, the integrated 11.2 um PAH band, and the IRAC 8.0 um filter as a proxy for the sum of the 7.7 and 8.6 um PAH bands. The resulting ratios of 11.2/3.3 and IRAC 8.0/11.2 provide an approximate measure of the average PAH size and PAH ionization respectively. In both RNe, we find that the relative PAH ionization and the average PAH size increases with decreasing distance to the illuminating source. The average PAH sizes derived for NGC 2023 are greater than those found for NGC 7023 at all points. Both results indicate that PAH size is dependent on the radiation field intensity. These results provide additional evidence of a rich carbon-based chemistry driven by the photo-chemical evolution of the omnipresent PAH molecules within the interstellar medium. In contrast, we did not detect a significant variation in the average PAH size found in the region southeast of the Orion Bar and report a peculiar PAH ionization radial profile., 20 pages, 11 figures, accepted for publication in ApJ

We present a new method to accurately describe the ionization fraction and the size distribution of polycyclic aromatic hydrocarbons (PAHs) within astrophysical sources. To this purpose, we have computed the mid-infrared emission spectra of 308 PAH molecules of varying sizes, symmetries, and compactness, generated in a range of radiation fields. We show that the intensity ratio of the solo CH out-of-plane bending mode in PAH cations and anions (referred to as the 11.0 $\mu$m band, falling in the 11.0-11.3 $\mu$m region for cations and anions) to their 3.3 $\mu$m emission, scales with PAH size, similarly to the scaling of the 11.2/3.3 ratio with the number of carbon atoms (N$_{\mathrm{C}}$) for neutral molecules. Among the different PAH emission bands, it is the 3.3 $\mu$m band intensity which has the strongest correlation with N$_{\mathrm{C}}$, and drives the reported PAH intensity ratio correlations with N$_{\mathrm{C}}$ for both neutral and ionized PAHs. The 6.2/7.7 intensity ratio, previously adopted to track PAH size, shows no evident scaling with N$_{\mathrm{C}}$ in our large sample. We define a new diagnostic grid space to probe PAH charge and size, using the (11.2+11.0)/7.7 and (11.2+11.0)/3.3 PAH intensity ratios respectively. We demonstrate the application of the (11.2+11.0)/7.7 - (11.2+11.0)/3.3 diagnostic grid for galaxies M82 and NGC 253, for the planetary nebula NGC 7027, and the reflection nebulae NGC 2023 and NGC 7023. Finally, we provide quantitative relations for PAH size determination depending on the ionization fraction of the PAHs and the radiation field they are exposed to., Comment: 25 pages, 16 figures, 16 tables. Accepted for publication in 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 mid-infrared (MIR) spectra of HII regions within star-forming galaxies M83 and M33. Their emission features are compared with Galactic and extragalactic HII regions, HII-type galaxies, starburst galaxies, and Seyfert/LINER type galaxies. Our main results are as follows: (i) the M33 and M83 HII regions lie in between Seyfert/LINER galaxies and HII-type galaxies in the 7.7/11.3 - 6.2/11.3 plane, while the different sub-samples exhibiting different 7.7/6.2 ratios; (ii) Using the NASA Ames PAH IR Spectroscopic database, we demonstrate that the 6.2/7.7 ratio does not effectively track PAH size, but the 11.3/3.3 PAH ratio does; (iii) variations on the 17 $\mu$m PAH band depends on object type; however, there is no dependence on metallicity for both extragalactic HII regions and galaxies; (iv) the PAH/VSG intensity ratio decreases with the hardness of the radiation field and galactocentric radius (Rg), yet the ionization alone cannot account for the variation seen in all of our sources; (v) the relative strength of PAH features does not change significantly with increasing radiation hardness, as measured through the [NeIII]/[NeII] ratio and the ionization index; (vi) We present PAH SFR calibrations based on the tight correlation between the 6.2, 7.7, and 11.3 $\mu$m PAH luminosities with the 24 $\mu$m luminosity and the combination of the 24 $\mu$m and H$\alpha$ luminosity; (vii) Based on the total luminosity from PAH and FIR emission, we argue that extragalactic HII regions are more suitable templates in modeling and interpreting the large scale properties of galaxies compared to Galactic HII regions., Comment: 26 pages, 24 figures, 6 tables. Accepted for publication in MNRAS

We examine polycyclic aromatic hydrocarbon (PAH), dust and atomic/molecular emission toward the Galactic bulge using Spitzer Space Telescope observations of four fields: C32, C35, OGLE and NGC 6522. These fields are approximately centered on (l, b) = (0.0{\deg}, 1.0{\deg}), (0.0{\deg}, -1.0{\deg}), (0.4{\deg}, -2.1{\deg}) and (1.0{\deg}, -3.8{\deg}), respectively. Far-infrared photometric observations complement the Spitzer/IRS spectroscopic data and are used to construct spectral energy distributions. We find that the dust and PAH emission are exceptionally similar between C32 and C35 overall, in part explained due to their locations---they reside on or near boundaries of a 7 Myr-old Galactic outflow event and are partly shock-heated. Within the C32 and C35 fields, we identify a region of elevated H{\alpha} emission that is coincident with elevated fine-structure and [O IV] line emission and weak PAH feature strengths. We are likely tracing a transition zone of the outflow into the nascent environment. PAH abundances in these fields are slightly depressed relative to typical ISM values. In the OGLE and NGC 6522 fields, we observe weak features on a continuum dominated by zodiacal dust. SED fitting indicates that thermal dust grains in C32 and C35 have comparable temperatures to those of diffuse, high-latitude cirrus clouds. Little variability is detected in the PAH properties between C32 and C35, indicating that a stable population of PAHs dominates the overall spectral appearance. In fact, their PAH features are exceptionally similar to that of the M82 superwind, emphasizing that we are probing a local Galactic wind environment., Comment: Accepted for publication in the Astrophysical Journal

We present 5-20 micron spectral maps of the reflection nebula NGC2023 obtained with the Infrared Spectrograph SL and SH modes on board the Spitzer Space Telescope which reveal emission from polycyclic aromatic hydrocarbons (PAHs), C60, and H2 superposed on a dust continuum. We show that several PAH emission bands correlate with each other and exhibit distinct spatial distributions revealing a spatial sequence with distance from the illuminating star. We explore the distinct morphology of the 6.2, 7.7 and 8.6 micron PAH bands and find that at least two spatially distinct components contribute to the 7--9 micron PAH emission in NGC2023. We report that the PAH features behave independently of the underlying plateaus. We present spectra of compact oval PAHs ranging in size from C_66 to C_210, determined computationally using density functional theory, and investigate trends in the band positions and relative intensities as a function of PAH size, charge and geometry. Based on the NASA Ames PAH database, we discuss the 7--9 micron components in terms of band assignments and relative intensities. We assign the plateau emission to very small grains with possible contributions from PAH clusters and identify components in the 7--9 micron emission that likely originates in these structures. Based on the assignments and the observed spatial sequence, we discuss the photochemical evolution of the interstellar PAH family as they are more and more exposed to the radiation field of the central star in the evaporative flows associated with the PDRs in NGC2023., Comment: 42 pages, 25 figures, 2 animations, accepted for ApJ

We present a sample of resolved galactic HII regions and photodissociation regions (PDRs) observed with the Spitzer infrared spectrograph (IRS) in spectral mapping mode between the wavelengths of 5--15 $\mu$m. For each object we have spectral maps at a spatial resolution of $\sim$4" in which we have measured all of the mid-infrared emission and absorption features. These include the PAH emission bands, primarily at 6.2, 7.7, 8.6, 11.2 and 12.7 $\mu$m, as well as the spectral emission lines of neon and sulfur and the absorption band caused by silicate dust at around 9.8 $\mu$m. In this work we describe the data in detail, including the data reduction and measurement strategies, and subsequently present the PAH emission band intensity correlations for each of the objects and the sample as a whole. We find that there are distinct differences between the sources in the sample, with two main groups, the first comprising the HII regions and the second the reflection nebulae (RNe). Three sources, the reflection nebula NGC~7023, the Horsehead nebula PDR (an interface between the HII region IC~434 and the Orion B molecular cloud) and M 17, resist this categorization, with the Horsehead PDR points mimicking the RNe and the NGC~7023 fluxes displaying unique bifurcated appearance in our correlation plots. These discrepancies seem to be due to the very low radiation field experienced by the Horsehead PDR and the very clean separation between the PDR environment and a diffuse environment in the NGC~7023 observations., Comment: 17 pages, 9 figures. ApJ accepted

We performed a detailed spectroscopic analysis of the fullerene C60-containing planetary nebula (PN) Lin49 in the Small Magellanic Cloud using XSHOOTER at the ESO VLT and the Spitzer/IRS instruments. We derived nebular abundances for nine elements. We used TLUSTY to derive photospheric parameters for the central star. Lin49 is C-rich and metal-deficient PN (Z~0.0006). The nebular abundances are in good agreement with Asymptotic Giant Branch nucleosynthesis models for stars with initial mass 1.25 Msun and metallicity Z = 0.001. Using the TLUSTY synthetic spectrum of the central star to define the heating and ionising source, we constructed the photoionisation model with CLOUDY that matches the observed spectral energy distribution (SED) and the line fluxes in the UV to far-IR wavelength ranges simultaneously. We could not fit the ~1-5 um SED using a model with 0.005-0.1 um-sized graphite grains and a constant hydrogen density shell owing to the prominent near-IR excess, while at other wavelengths the model fits the observed values reasonably well. We argue that the near-IR excess might indicate either (1) the presence of very small particles in the form of small carbon clusters, small graphite sheets, or fullerene precursors, or (2) the presence of a high-density structure surrounding the central star. We found that SMC C60 PNe show a near-IR excess component to lesser or greater degree. This suggests that these C60 PNe might maintain a structure nearby their central star., 24 pages, 17 figures, 17 tables, Accepted for publication by MNRAS

We recently identified several emission bands in the Spitzer-IRS spectrum of the unusual planetary nebula Tc 1 with the infrared active vibrational modes of the neutral fullerene species C60 and C70. Since then, the fullerene bands have been detected in a variety of sources representing circumstellar and interstellar environments. Abundance estimates suggest that C60 represents ~0.1%-1.5% of the available carbon in those sources. The observed relative band intensities in various sources are not fully compatible with single-photon heating and fluorescent cooling, and are better reproduced by a thermal distribution at least in some sources. The observational data suggests that fullerenes form in the circumstellar environments of evolved stars, and survive in the interstellar medium. Precisely how they form is still a matter of debate., 12 pages, 5 figures. To appear in the proceedings of IAU symposium 280 "The Molecular Universe"

We present new mid-infrared spectra for a sample of 15 targets (1 FU Orionis object, 4 Herbig Ae stars, 5 T Tauri stars and 5 Vega type stars), obtained with the TIMMI2 camera at La Silla Observatory (ESO). Three targets are members of the beta Pic moving group (HD 155555, HD 181296 and HD 319139). PAH bands are observed towards the T Tauri star HD 34700 and the Herbig Ae star PDS 144 N. For HD 34700, the band profiles indicate processed PAHs. The spectrum of the Vega-type object eta Corvi (HD 109085), for which a resolved disk at sub-mm wavelengths is known, is entirely stellar between 8--13 micron. Similarly, no indication for circumstellar matter at mid-infrared wavelengths is found towards the Vega-like stars HD 3003, HD 80951, HD 181296 and, surprisingly, the T Tauri system HD 155555. The silicate emission features of the remaining eight sources are modelled with a mixture of silicates of different grain sizes and composition. Unprocessed dust dominates FU Ori, HD 143006 and CD-43 344. Large amorphous grains are the main dust component around HD 190073, HD 319139, KK Oph and PDS 144 S. Both small grains and crystalline dust is found for the Vega-type HD 123356, with a dominance of small amorphous grains. We show that the infrared emission of the binary HD 123356 is dominated by its late-type secondary, but optical spectroscopy is still required to confirm the age of the system and the spectral class of the companion. For most targets this is their first mid-infrared spectroscopic observation. We investigate trends between stellar, disk and silicate properties and confirm correlations of previous studies. Several objects present an exciting potential for follow-up high-resolution disk studies., Comment: Accepted for publication in A&A

The mid-IR spectra of six large, irregular PAHs with formulae (C84H24 - C120H36) have been computed using Density Functional Theory (DFT). Trends in the dominant band positions and intensities are compared to those of large, compact PAHs as a function of geometry, size and charge. Irregular edge moieties that are common in terrestrial PAHs, such as bay regions and rings with quartet hydrogens, are shown to be uncommon in astronomical PAHs. As for all PAHs comprised solely of C and H reported to date, mid-IR emission from irregular PAHs fails to produce a strong CCstr band at 6.2 um, the position characteristic of the important, class A astronomical PAH spectra. Earlier studies showed inclusion of nitrogen within a PAH shifts this to 6.2 um for PAH cations. Here we show this band shifts to 6.3 um in nitrogenated PAH anions, close to the position of the CC stretch in class B astronomical PAH spectra. Thus nitrogenated PAHs may be important in all sources and the peak position of the CC stretch near 6.2 um appears to directly reflect the PAH cation to anion ratio. Large irregular PAHs exhibit features at 7.8 um but lack them near 8.6 um. Hence, the 7.7 um astronomical feature is produced by a mixture of small and large PAHs while the 8.6 um band can only be produced by large compact PAHs. As with the CCstr, the position and profile of these bands reflect the PAH cation to anion ratio., Comment: accepted by ApJ

Photodissociation regions (PDRs) contain a large fraction of all of the interstellar matter in galaxies. Classical examples include the boundaries between ionized regions and molecular clouds in regions of massive star formation, marking the point where all of the photons energetic enough to ionize hydrogen have been absorbed. In this paper we determine the physical properties of the PDRs associated with the star forming regions IRAS 23133+6050 and S 106 and present them in the context of other Galactic PDRs associated with massive star forming regions. We employ Herschel PACS and SPIRE spectroscopic observations to construct a full 55-650 ��m spectrum of each object from which we measure the PDR cooling lines, other fine- structure lines, CO lines and the total far-infrared flux. These measurements are then compared to standard PDR models. Subsequently detailed numerical PDR models are compared to these predictions, yielding additional insights into the dominant thermal processes in the PDRs and their structures. We find that the PDRs of each object are very similar, and can be characterized by a two-phase PDR model with a very dense, highly UV irradiated phase (n $\sim$ 10^6 cm^(-3), G$_0$ $\sim$ 10^5) interspersed within a lower density, weaker radiation field phase (n $\sim$ 10^4 cm^(-3), G$_0$ $\sim$ 10^4). We employed two different numerical models to investigate the data, firstly we used RADEX models to fit the peak of the $^{12}$CO ladder, which in conjunction with the properties derived yielded a temperature of around 300 K. Subsequent numerical modeling with a full PDR model revealed that the dense phase has a filling factor of around 0.6 in both objects. The shape of the $^{12}$CO ladder was consistent with these components with heating dominated by grain photoelectric heating. An extra excitation component for the highest J lines (J > 20) is required for S 106., 20 pages, 10 figures, A&A Accepted

We present Spitzer/Infrared Spectrograph (IRS) 5-21 micron spectroscopic maps towards 12 regions in the Andromeda galaxy (M31). These regions include the nucleus, bulge, an active region in the star-forming ring, and 9 other regions chosen to cover a range of mid-to-far-infrared colours. In line with previous results, PAH feature ratios (6.2 micron and 7.7 micron features compared to the 11.2 micron feature) measured from our extracted M31 spectra, except the nucleus, strongly correlate. The equivalent widths of the main PAH features, as a function of metallicity and radiation hardness, are consistent with those observed for other nearby spiral and starburst galaxies. Reprocessed data from the ISOCAM instrument on the Infrared Space Observatory agree with the IRS data; early reports of suppressed 6-8 micron features and enhanced 11.3 micron feature intensity and FWHM apparently resulted from background-subtraction problems. The nucleus does not show any PAH emission but does show strong silicate emission at 9.7 micron. Furthermore, different spectral features (11.3 micron PAH emission, silicate emission and [NeIII] 15.5 micron line emission) have distinct spatial distributions in the nuclear region: the silicate emission is strongest towards the stellar nucleus, while the PAH emission peaks 15 arcsec north of the nucleus. The PAH feature ratios at this position are atypical with strong emission at 11.2 microns and 15-20 microns but weak emission at 6--8 microns. The nucleus itself is dominated by stellar light giving rise to a strong blue continuum and silicate emission., Comment: 14 pages, 13 figures; MNRAS in press

We present the first mid-infrared spectra of SMP LMC 11 in the Large Magellanic Cloud. While this object resembles a planetary nebula in the optical, its infrared properties are more similar to an object in transition from the asymptotic giant branch to the planetary nebula phase. A warm dust continuum dominates the infrared spectrum. The peak emission corresponds to a mean dust temperature of 330 K. The spectrum shows overlapping molecular absorption bands from 12 to 17 um corresponding to acetylene and polyacetylenic chains and benzene. This is the first detection of C4H2, C6H2, C6H6 and other molecules in an extragalactic object. The infrared spectrum of SMP LMC 11 is similar in many ways to that of the pre-planetary nebula AFGL 618. The IRS spectrum shows little evidence of nitrogen-based molecules which are commonly seen in Galactic AGB stars. Polycyclic aromatic hydrocarbons are also absent from the spectrum. The detection of the [NeII] 12.8 um line in the infrared and other forbidden emission lines in the optical indicates that an ionized region is present., 5 pages (in emulateapj), 1 table, 2 figures. Accepted for publication in ApJ Letters

We present a detailed photoionization model of G29.96-0.02 (hereafter G29.96), one of the brightest Galactic Ultra Compact HII (UCHII) regions in the Galaxy. This source has been observed extensively at radio and infrared wavelengths. The most recent data include a complete ISO (SWS and LWS) spectrum, which displays a remarkable richness in atomic fine-structure lines. The number of observables is twice as much as the number available in previous studies. In addition, most atomic species are now observed in two ionization stages. The radio and infrared data on G29.96 are best reproduced using a nebular model with two density components: a diffuse (n_e~680cm-3) extended (~1 pc) component surrounding a compact (~0.1 pc) dense (n_e~57000cm-3) core. The properties of the ionizing star were derived using state-of-the-art stellar atmosphere models. CoStar models yield an effective temperature of \~30^{+2}_{-1} kK whereas more recent non-LTE line blanketed atmospheres with stellar winds indicate somewhat higher values, Teff~32--38 kK. This range in Teff is compatible with all observational constraints, including near-infrared photometry and bolometric luminosity. The range 33-36 kK is also compatible with the spectral type O5-O8 determined by Watson and Hanson (97) when recent downward revisions of the effective temperature scale of O stars are taken into account. The age of the ionizing star of G29.96 is found to be a few 10^6 yr, much older than the expected lifetime of UCHII regions. Accurate gas phase abundances are derived with the most robust results being Ne/S=7.5 and N/O=0.43 (1.3 and 3.5 times the solar values, respectively)., Accepted in Astronomy and Astrophysics

The massive star forming region W49A represents one of the largest complexes of massive star formation present in the Milky Way and contains at least fifty young massive stars still enshrouded in their natal molecular cloud. We employ Spitzer/IRS spectral mapping observations of the northern part of W49A to investigate the mid-infrared (MIR) spatial appearance of the polycyclic aromatic hydrocarbon (PAH) bands, PAH plateau features, atomic lines and continuum emission. We examine the spatial variations of the MIR emission components in slices through two of the ultra compact-HII (UC-HII) regions. We find that the PAH bands reproduce known trends, with the caveat that the 6.2 $\mu$m PAH band seems to decouple from the other ionized PAH bands in some of the UC-HII regions -- an effect previously observed only in one other object: the giant star forming region N66 in the LMC. Furthermore, we compare the nature of the emission surrounding W49A to that of `diffuse' sightlines. It is found that the surrounding emission can be explained by line of sight emission, and does not represent true `diffuse' material. Additionally, we examine the MIR appearance of star formation on various scales from UC-HII regions to starburst galaxies, including a discussion of the fraction of PAH emission in the 8 $\mu$m IRAC filter. We find that the MIR appearance of W49A is that of a starburst on large scales yet its individual components are consistent with other galactic HII regions., Comment: 23 pages, 14 figures, ApJ accepted

Mid-infrared (MIR) quasar spectra exhibit a suite of emission features including high ionization coronal lines from the narrow line region (NLR) illuminated by the ionizing continuum, and hot dust features from grains, as well as polycyclic aromatic hydrocarbons (PAH) features from star formation in the host galaxy. Few features are detected in most spectra because of typically low signal-to-noise ratio (S/N) data. By generating spectral composites in three different luminosity bins from over 180 Spitzer Infrared Spectrograph (IRS) observations, we boost the S/N and reveal important features in the complex spectra. We detect high-ionization, forbidden emission lines in all templates, PAH features in all but the most luminous objects, and broad silicate and graphite features in emission whose strength increases relative to the continuum with luminosity. We find that the intrinsic quasar spectrum for all luminosity templates is consistent, and the differences in the spectra can be explained by host galaxy contamination in the lower luminosity templates. We also posit that star formation may be active in most quasar host galaxies, but the spectral features of star formation are only detectable if the quasar is faint., 4 pages, 2 figures, IAU Symposium #304 "Multiwavelength AGN Surveys and Studies"

Fullerenes are a particularly stable class of carbon molecules in the shape of a hollow sphere or ellipsoid that might be formed in the outflows of carbon stars. Once injected into the interstellar medium (ISM), these stable species survive and are thus likely to be widespread in the Galaxy where they contribute to interstellar extinction, heating processes, and complex chemical reactions. In recent years, the fullerene species C60 (and to a lesser extent C70) have been detected in a wide variety of circumstellar and interstellar environments showing that when conditions are favourable, fullerenes are formed efficiently. Fullerenes are the first and only large aromatics firmly identified in space. The detection of fullerenes is thus crucial to provide clues as to the key chemical pathways leading to the formation of large complex organic molecules in space, and offers a great diagnostic tool to describe the environment in which they reside. Since fullerenes share many physical properties with PAHs, understanding how fullerenes form, evolve and respond to their physical environment will yield important insights into one of the largest reservoirs of organic material in space. In spite of all these detections, many questions remain about precisely which members of the fullerene family are present in space, how they form and evolve, and what their excitation mechanism is. We present here an overview of what we know from astronomical observations of fullerenes in these different environments, and discuss current thinking about the excitation process. We highlight the various formation mechanisms that have been proposed, discuss the physical conditions conducive to the formation and/or detection of fullerenes in carbon stars, and their possible connection to PAHs, HACs and other dust features., Comment: 6 pages, 2 figures. Proceedings of "The Life Cycle of Dust in the Universe: Observations, Theory, and laboratory Experiments". Taipei, Taiwan 18-22 November 2013. Editors: A. Andersen, M. Baes, H. Gomez, C. Kemper, D. Watson

Mid-infrared (MIR) quasar spectra exhibit a suite of emission features including high ionization coronal lines from the narrow line region illuminated by the ionizing continuum, broad dust bumps from silicates and graphites, and polycyclic aromatic hydrocarbon (PAH) features from star formation in the host galaxy. However, in Spitzer Infared Spectrograph (IRS) data, few features are detected in most individual spectra because of typically low signal-to-noise ratios (S/N). By generating spectral composites from over 180 IRS observations of Sloan Digital Sky Survey broad-line quasars, we boost the S/N and reveal features in the complex spectra that are otherwise lost in the noise. In addition to an overall composite, we generate composites in three different luminosity bins that span the range of 5.6 micron luminosities of 10$^{40}$--10$^{46}$ (erg~s$^{-1}$). We detect the high-ionization, forbidden emission lines of [SIV], [OIV], and [NeV] 14 micron in all templates and PAH features in all but the most luminous template. Ratios of lines with a range of ionization potentials show no evidence for a strong difference in the shape of the 41--97 eV ionizing continuum over this range of luminosities. The scaling of the emission-line luminosities as a function of continuum luminosity is consistent with what is expected, and shows no indication of a ``disappearing narrow-line region.'' The broad 10 and 18 micron silicate features in emission increase in strength with increasing luminosity, and a broad 3--5 micron black body consistent with graphite emission at 750 K is evident in the highest luminosity template. We find that the intrinsic quasar continua for all luminosity templates are consistent; apparent differences arise primarily from host galaxy contamination most evident at low luminosity., 12 pages, 15 figures, Accepted to MNRAS

(abridged)The focus of this work is to study mid-infrared point sources and the diffuse interstellar medium (ISM) in the low-metallicity (~0.2 solar) giant HII region N66 using the Spitzer Space Telescope's Infrared Spectrograph. We study 14 targeted infrared point sources as well as spectra of the diffuse ISM that is representative of both the photodissociation regions (PDRs) and the HII regions. Among the point source spectra, we spectroscopically confirm that the brightest mid-infrared point source is a massive embedded young stellar object, we detect silicates in emission associated with two young stellar clusters, and we observe spectral features of a known B[e] supergiant that are more commonly associated with Herbig Be stars. In the diffuse ISM, we provide additional evidence that the very small grain population is being photodestroyed in the hard radiation field. The 11.3 um PAH complex emission exhibits an unexplained centroid shift in both the point source and ISM spectra that should be investigated at higher signal-to-noise and resolution. Unlike studies of other regions, the 6.2 um and 7.7 um band fluxes are decoupled; the data points cover a large range of I7.7/I11.3 PAH ratio values within a narrow band of I6.2/I11.3 ratio values. Furthermore, there is a spread in PAH ionization, being more neutral in the dense PDR where the radiation field is relatively soft, but ionized in the diffuse ISM/PDR. By contrast, the PAH size distribution appears to be independent of local ionization state. Important to unresolved studies of extragalactic low-metallicity star-forming regions, we find that emission from the infrared-bright point sources accounts for only 20-35% of the PAH emission from the entire region. These results make a comparative dataset to other star-forming regions with similarly hard and strong radiation fields., ApJ, in press. ~60 pages, 18 figures

We compare and analyze the Spitzer mid-infrared spectrum of three fullerene-rich planetary nebulae in the Milky Way and the Magellanic Clouds; Tc1, SMP SMC16, and SMP LMC56. The three planetary nebulae share many spectroscopic similarities. The strongest circumstellar emission bands correspond to the infrared active vibrational modes of the fullerene species C60 and little or no emission is present from Polycyclic Aromatic Hydrocarbons (PAHs). The strength of the fullerene bands in the three planetary nebulae is very similar, while the ratio of the [NeIII]15.5um/[NeII]12.8um fine structure lines, an indicator of the strength of the radiation field, is markedly different. This raises questions about their excitation mechanism and we compare the fullerene emission to fluorescent and thermal models. In addition, the spectra show other interesting and common features, most notably in the 6-9um region, where a broad plateau with substructure dominates the emission. These features have previously been associated with mixtures of aromatic/aliphatic hydrocarbon solids. We hypothesize on the origin of this band, which is likely related to the fullerene formation mechanism, and compare it with modeled Hydrogenated Amorphous Carbon that present emission in this region., Comment: 13 pages, 2 tables, 7 figures, Accepted for publication in ApJ

Fullerenes have recently been identified in space and they may play a significant role in the gas and dust budget of various astrophysical objects including planetary nebulae (PNe), reflection nebulae (RNe) and H II regions. The tenuous nature of the gas in these environments precludes the formation of fullerene materials following known vaporization or combustion synthesis routes even on astronomical timescales. We have studied the processing of hydrogenated amorphous carbon (a-C:H or HAC) nano-particles and their specific derivative structures, which we name "arophatics", in the circumstellar environments of young, carbon-rich PNe. We find that UV-irradiation of such particles can result in the formation of fullerenes, consistent with the known physical conditions in PNe and with available timescales., Accepted for publication in ApJ. Revised version after comments from the referee. Few references added. Calculation in Sec. 3 moved to the Appendix. Results unchanged. 9 pages, 4 figures, 1 table, 1 Appendix

The infrared spectra of many galactic and extragalactic objects are dominated by emission features at 3.3, 6.2, 7.7, 8.6 and 11.2 \mu m. The carriers of these features remained a mystery for almost a decade, hence the bands were dubbed the unidentified infrared (UIR) bands. Since the mid-80's, the UIR bands are generally attributed to the IR fluorescence of Polycyclic Aromatic Hydrocarbon molecules (PAHs) upon absorption of UV photons -- the PAH hypothesis. Here we review the progress made over the past 25 years in understanding the UIR bands and their carriers., Comment: 13 pages, 7 figures. To appear in the proceedings of IAU symposium 280 "The Molecular Universe"

Context. The circumstellar environment of evolved stars is generally rich in molecular gas and dust. Typically, the entire environment is either oxygen-rich or carbon-rich, depending on the evolution of the central star. Aims. In this paper we discuss three evolved disc sources with evidence of atypical emission lines in their infrared spectra. The stars were taken from a larger sample of post-AGB binaries for which we have Spitzer infrared spectra, characterised by the presence of a stable oxygen-rich circumbinary disc. Our previous studies have shown that the infrared spectra of post-AGB disc sources are dominated by silicate dust emission, often with an extremely high crystallinity fraction. However, the three sources described here are selected because they show a peculiar molecular chemistry. Methods. Using Spitzer infrared spectroscopy, we study in detail the peculiar mineralogy of the three sample stars. Using the observed emission features, we identify the different observed dust, molecular and gas species. Results. The infrared spectra show emission features due to various oxygen-rich dust components, as well as CO2 gas. All three sources show the strong infrared bands generally ascribed to polycyclic aromatic hydrocarbons. Furthermore, two sample sources show C60 fullerene bands. Conclusions. Even though the majority of post-AGB disc sources are dominated by silicate dust in their circumstellar environment, we do find evidence that, for some sources at least, additional processing must occur to explain the presence of large carbonaceous molecules. There is evidence that some of these sources are still oxygen-rich, which makes the detection of these molecules even more surprising., Comment: Accepted for publication in Astronomy and Astrophysics, 10 pages, 7 figures

We present a Spitzer Space Telescope spectroscopic study of a sample of 25 planetary nebulae in the Magellanic Clouds. The low-resolution modules are used to analyze the dust features present in the infrared spectra. This study complements a previous work by the same authors where the same sample was analyzed in terms of neon and sulfur abundances. Over half of the objects (14) show emission of polycyclic aromatic hydrocarbons, typical of carbon-rich dust environments. We compare the hydrocarbon emission in our objects to those of Galactic HII regions and planetary nebulae, and LMC/SMC HII regions. Amorphous silicates are seen in just two objects, enforcing the now well-known-fact that oxygen-rich dust is less common at low metallicities. Besides these common features, some planetary nebulae show very unusual dust. Nine objects show a strong silicon carbide feature at 11um and twelve of them show magnesium sulfide emission starting at 25um. The high percentage of spectra with silicon carbide in the Magellanic Clouds is not common. Two objects show a broad band which may be attributed to hydrogenated amorphous carbon and weak low-excitation atomic lines. It is likely that these nebulae are very young. The spectra of the remaining eight nebulae are dominated by the emission of fine-structure lines with a weak continuum due to thermal emission of dust, although in a few cases the S/N in the spectra is low, and weak dust features may not have been detected., 13 pages, 2 tables, 7 figures, Accepted for publication in ApJ

To show the importance of high-spatial resolution observations of HII regions when compared with observations obtained with larger apertures such as ISO, we present mid-infrared spectra of two Magellanic Cloud HII regions, N88A and N160A. We obtained mid-infrared (8-13 um), long-slit spectra with TIMMI2 on the ESO 3.6m telescope. These are combined with archival spectra obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope, and are compared with the low-spatial resolution ISO-SWS data. An inventory of the spectra in terms of atomic fine-structure lines and molecular bands is presented. Concerning N88A, an isolated HII region with no adjacent infrared sources, the observations indicate that the line fluxes observed by ISO-SWS and Spitzer-IRS come exclusively from the compact HII region of about 3 arcsec in diameter. This is not the case for N160A, which has a more complex morphology. We have spectroscopically isolated for the first time the individual contributions of the three components of N160A, two high-excitation blobs, A1 and A2, and the young stellar object N160A-IR. In addition, extended [SIV] emission is observed with TIMMI2 and is most likely associated with the central star cluster located between A1 and A2. We show the value of these high-spatial resolution data in determining source characteristics, such as the degree of ionization of each high-excitation blob or the bolometric luminosity of the YSO. This luminosity is about one order of magnitude lower than previously estimated. For each high-excitation blob, we also determine the electron density and the elemental abundances of Ne, S, and Ar., 13 pages, 8 figures, accepted for publication in A&A

We present the results of a systematic study of mid-IR spectra of Galactic regions, Magellanic HII regions, and galaxies of various types (dwarf, spiral, starburst), observed by the satellites ISO and Spitzer. We study the relative variations of the 6.2, 7.7, 8.6 and 11.3 micron features inside spatially resolved objects (such as M82, M51, 30 Doradus, M17 and the Orion Bar), as well as among 90 integrated spectra of 50 objects. Our main results are that the 6.2, 7.7 and 8.6 micron bands are essentially tied together, while the ratios between these bands and the 11.3 micron band varies by one order of magnitude. This implies that the properties of the PAHs are remarkably universal throughout our sample, and that the relative variations of the band ratios are mainly controled by the fraction of ionized PAHs. In particular, we show that we can rule out both the modification of the PAH size distribution, and the mid-infrared extinction, as an explanation of these variations. Using a few well-studied Galactic regions (including the spectral image of the Orion Bar), we give an empirical relation between the I(6.2)/I(11.3) ratio and the ionization/recombination ratio G0/ne.Tgas^0.5, therefore providing a useful quantitative diagnostic tool of the physical conditions in the regions where the PAH emission originates. Finally, we discuss the physical interpretation of the I(6.2)/I(11.3) ratio, on galactic size scales., Comment: Accepted by the ApJ, 67 pages, 70 figures

In order to show the importance of high spatial resolution observations of extra-galactic sources when compared to observations obtained with larger apertures such as ISO, we present N-band spectra (8-13 um) of some locations in three starburst galaxies. In particular, the two galactic nuclei of the spiral galaxy NGC3256, the compact IR supernebula in the dwarf galaxy IIZw40 and the two brightest IR knots in the central starburst of the WR galaxy He2-10. The spectra have been obtained with TIMMI2 on the ESO 3.6m telescope. An inventory of the spectra in terms of atomic fine-structure lines and molecular bands is presented. We show the great value of these high spatial resolution data at constraining properties such as the extinction in the mid-IR, metallicity or stellar content (age, IMF, etc.). Regarding this, we have constrained the stellar content of the IR compact knot in IIZw40 by using the mid-IR fine-structure lines and setting restrictions on the nebular geometry. Considering the PAH bands, we have constructed a new mid-/far-IR diagnostic diagram based on the 11.2 um PAH and continuum, accessible to ground-based observations. Finally, we find a dependence between the presence or non-presence of PAHs and the hardness of the radiation field as measured by the [SIV]/[NeII] ratio. In particular, sources with PAH emission have in general a [SIV]/[NeII] ratio < 0.35. We investigate possible origins for this relation and conclude that it does not necessarily imply PAH destruction, but could also be explained by the PAH-dust competition for FUV photons., 22 pages, 10 figures, accepted for publication in A&A

We present Spitzer high spectral resolution IRS spectroscopy of three positions in the carbon-rich outflow of post-AGB star HD 44179, better known as the Red Rectangle. Surprisingly, the spectra show some strong unknown mid-infrared resonances, in the 13-20 micron range. The shape and position of these resonances varies with position in the nebula, and are not correlated with the PAH features. We conclude these features are due to oxygen-rich minerals, located in a region which is believed to be predominantly carbon-rich. We provide possible explanations for the presence of oxygen-rich dust in the carbon-rich outflows. Simple Mg-Fe-oxides are suggested as carriers of these unidentified features., Comment: accepted by ApJL; 5 pages; 4 figures

An overview is given of ISO results on regions of high excitation ISM and gas, i.e. HII regions, the Galactic Centre and Supernovae Remnants. IR emission due to fine-structure lines, molecular hydrogen, silicates, polycyclic aromatic hydrocarbons and dust are summarised, their diagnostic capabilities illustrated and their implications highlighted., Comment: To be publised in the review book : "ISO science legacy - a compact review of ISO major achievements"

Observations of infrared fine-structure lines provide direct information on the metallicity and ionization structure of HII regions and indirectly on the hardness of the radiation field ionizing these nebulae. We have analyzed a sample of Galactic and Magellanic Cloud HII regions observed by the Infrared Space Observatory (ISO) to examine the interplay between stellar content, metallicity and the ionization structure of HII regions. The observed [SIV]10.5/[SIII]18.7 mum and [NeIII]15.5/[NeII]12.8 mum line ratios are shown to be highly correlated over more than two orders of magnitude. We have compared the observed line ratios to the results of photoionization models using different stellar energy distributions. The derived characteristics of the ionizing star depend critically on the adopted stellar model as well as the (stellar) metallicity. We have compared the stellar effective temperatures derived from these model studies for a few well-studied HII regions with published direct spectroscopic determinations of the spectral type of the ionizing stars. This comparison supports our interpretation that stellar and nebular metallicity influences the observed infrared ionic line ratios. We can explain the observed increase in degree of ionization, as traced by the [SIV]\[SIII] and [NeIII]\[NeII] line ratios, by the hardening of the radiation field due to the decrease of metallicity. The implications of our results for the determination of the ages of starbursts in starburst galaxies are assessed., 9 pages; accepted for publication in A&A; figure 3 modified

Infrared spectra of carbon-rich objects which have evolved off the asymptotic giant branch reveal a range of dust properties, including fullerenes, polycyclic aromatic hydrocarbons (PAHs), aliphatic hydrocarbons, and several unidentified features, including the 21 um emission feature. To test for the presence of fullerenes, we used the position and width of the feature at 18.7-18.9 um and examined other features at 17.4 and 6-9 um. This method adds three new fullerene sources to the known sample, but it also calls into question three previous identifications. We confirm that the strong 11 um features seen in some sources arise primarily from SiC, which may exist as a coating around carbonaceous cores and result from photo-processing. Spectra showing the 21 um feature usually show the newly defined Class D PAH profile at 7-9 um. These spectra exhibit unusual PAH profiles at 11-14 um, with weak contributions at 12.7 um, which we define as Class D1, or show features shifted to ~11.4, 12.4, and 13.2 um, which we define as Class D2. Alkyne hydrocarbons match the 15.8 um feature associated with 21 um emission. Sources showing fullerene emission but no PAHs have blue colors in the optical, suggesting a clear line of sight to the central source. Spectra with 21 um features and Class D2 PAH emission also show photometric evidence for a relatively clear line of sight to the central source. The multiple associations of the 21 um feature to aliphatic hydrocarbons suggest that the carrier is related to this material in some way., 30 pages, 30 figures, ApJ, in press

Based on the ISO spectral catalogue of compact HII regions by Peeters et al. (2001), we present a first analysis of the hydrogen recombination and atomic fine-structure lines originated in the ionized gas. The sample consists of 34 HII regions located at galactocentric distances between Rgal = 0 and 15 kpc. The SWS HI recombination lines between 2 and 8 mum are used to estimate the extinction law at these wavelengths for 14 HII regions. An extinction in the K band between 0 and $\sim$ 3 mag. has been derived. The fine-structure lines of N, O, Ne, S and Ar are detected in most of the sources. Most of these elements are observed in two different ionization stages probing a range in ionization potential up to 41 eV. The ISO data, by itself or combined with radio data taken from the literature, is used to derive the elemental abundances relative to hydrogen. The present data thus allow us to describe for each source its elemental abundance, its state of ionization and to constrain the properties of the ionizing star(s)., Comment: Accepted in Astronomy and Astrophysics, 22 pages, 20 figures, 9 tables

We present 10-15 micron spectra of a sample of H II regions, YSOs and evolved stars that show strong unidentified infrared emission features, obtained with the ISO/SWS spectrograph on-board ISO. These spectra reveal a plethora of emission features with bands at 11.0, 11.2, 12.0, 12.7, 13.5 and 14.2 micron. These features are observed to vary considerably in relative strength to each-other from source to source. In particular, the 10-15 micron spectra of the evolved stars are dominated by the 11.2 micron band while for H II regions the 12.7 is typically as strong as the 11.2 micron band. Analysing the ISO data we find a good correlation between the 11.2 micron band and the 3.3 micron band, and between the 12.7 micron and the 6.2 micron band. There is also a correlation between the ratio of the UIR bands to the total dust emission and the 12.7 over 11.2 micron ratio. Bands in the 10-15 micron spectral region are due to CH out-of-plane (OOP) bending modes of polycyclic aromatic hydrocarbons (PAHs). We summarise existing laboratory data and theoretical quantum chemical calculations of these modes for neutral and cationic PAHs. Due to mode coupling, the exact peak position of these bands depends on the number of adjacent CH groups and hence the observed interstellar 10-15 micron spectra can be used to determine the molecular structure of the interstellar PAHs emitting in the different regions. We conclude that evolved stars predominantly inject compact ~100-200 C-atom PAHs into the ISM where they are subsequently processed, resulting in more open and uneven PAH structures., 14 pages, 9 figures, accepted for A&A

We present 15-20 um spectral maps towards the reflection nebula NGC2023 obtained with the Infrared Spectrograph in short-wavelength, high-resolution mode on board the Spitzer Space Telescope. These spectra reveal emission from PAHs, C60, and H2 superposed on a dust continuum. These emission components exhibit distinct spatial distributions: with increasing distance from the illuminating star, we observe the PAH emission followed by the dust continuum emission and the H2 emission. The C60 emission is located closest to the illuminating star in the south while in the north, it seems to be associated with the H/H2 transition. Emission from PAHs and PAH-related species produce features at 15.8, 16.4, 17.4, and 17.8 um and the 15-18 um plateau. These different PAH features show distinct spatial distributions. The 15.8 um band and 15-18 um plateau correlate with the 11.2 um PAH band and with each other, and are attributed to large, neutral PAHs. Conversely, the 16.4 um feature correlates with the 12.7 um PAH band, suggesting that both arise from species that are favored by the same conditions that favor PAH cations. The PAH contribution to the 17.4 um band is displaced towards the illuminating star with respect to the 11.2 and 12.7 um emission and is assigned to doubly ionized PAHs and/or a subset of cationic PAHs. The spatial distribution of the 17.8 um band suggests it arises from both neutral and cationic PAHs. In contrast to their intensities, the profiles of the PAH bands and the 15-18 um plateau do not vary spatially. Consequently, we conclude that the carrier of the 15-18 um plateau is distinct from that of the PAH bands., 34 pages, 8 fig. To appear in ApJ (accepted)

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