Absolute evaporation rates of non-rotating neutral PAH clusters

Clusters of polycyclic aromatic hydrocarbons (PAHs) have been proposed as candidates for evaporating very small grains, which are thought to be precursors of free-flying PAHs. Evaporation rates have been calculated so far only for species containing up to a few 100 C atoms, whereas interstellar PAH clusters could contain up to ~1000 C atoms. We present a method that generalises the calculation of the statistical evaporation rate of large PAH clusters and provides rates for species containing up to ~1000 C-atoms. The evaporation of non-rotating neutral homo-molecular PAH clusters containing up to 12 molecules from a family of highly symmetric compact PAHs is studied. Statistical calculations were performed and completed with molecular dynamics simulations at high internal energies to provide absolute values for the evaporation rate and distributions of kinetic energy released. The calculations used explicit atom-atom Lennard-Jones potentials in the rigid molecule approximation. A new method is proposed to take both inter- and intra-molecular vibrations into account. Without any parameter adjustment, the calculated evaporation rates agree well with available experimental data. We find that the non-rotation assumption has a limited impact on the evaporation rates. The photostability of PAH clusters increases dramatically with the size of molecules in the clusters, and to a lesser extent with the number of molecules in the clusters. For values of the UV radiation field that are typical of the regions where evaporating very small grains are observed, the smallest clusters in this study (~50 C-atoms) are found to be quickly photo-evaporated, whereas the largest clusters (~1000 C-atoms) are photostable. Our results support the idea that large PAH clusters are good candidates for evaporating very small grains.Comment: 13 pages, 10 figure

Blind Signal Separation Methods for the Identification of Interstellar Carbonaceous Nanoparticles

The use of Blind Signal Separation methods (ICA and other approaches) for the analysis of astrophysical data remains quite unexplored. In this paper, we present a new approach for analyzing the infrared emission spectra of interstellar dust, obtained with NASA's Spitzer Space Telescope, using FastICA and Non-negative Matrix Factorization (NMF). Using these two methods, we were able to unveil the source spectra of three different types of carbonaceous nanoparticles present in interstellar space. These spectra can then constitute a basis for the interpretation of the mid-infrared emission spectra of interstellar dust in the Milky Way and nearby galaxies. We also show how to use these extracted spectra to derive the spatial distribution of these nanoparticles

The infrared signatures of very small grains in the Universe seen by JWST

The near- and mid-IR spectrum of many astronomical objects is dominated by emission bands due to UV-excited polycyclic aromatic hydrocarbons (PAH) and evaporating very small grains (eVSG). Previous studies with the ISO, Spitzer and AKARI space telescopes have shown that the spectral variations of these features are directly related to the local physical conditions that induce a photo-chemical evolution of the band carriers. Because of the limited sensitivity and spatial resolution, these studies have focused mainly on galactic star-forming regions. We discuss how the advent of JWST will allow to extend these studies to previously unresolved sources such as near-by galaxies, and how the analysis of the infrared signatures of PAHs and eVSGs can be used to determine their physical conditions and chemical composition.Comment: To appear in the Proceedings of the annual meeting of the French society of astronomy and astrophysics (SF2A 2015

Photo-ionization and photo-dissociation of trapped PAH cations

In astrophysical environments, polycyclic aromatic hydrocarbons (PAHs) are submitted to VUV photons of energy up to $\sim$20 eV. In the laboratory, photoelectron-photoion spectroscopy is usually performed using VUV synchrotron radiation, in which the same photon (15-25 eV), is used to ionize and dissociate PAHs\footnote{H.W.~Jochims et al., \textit{Astron. \& Astrophys.} \underline{\textbf{420}} (1994), 307-317P. M.~Mayer et al., \textit{J. Chem. Phys.} \underline{\textbf{134}} (2011), 244312-244312-8}. These experiments explore specific conditions and complementary studies in ion traps are required for a wider investigation of interstellar conditions. We have used the LTQ linear ion trap available on the DESIRS VUV beamline at SOLEIL to study the interaction of PAH cations with photons in the 7-20 eV range. We recorded by action spectroscopy the relative intensities of photo-fragmentation and photo-ionization of eight PAH cations ranging in size from 14 to 24 carbon atoms and with different structures. We found that, at photon energies below $\sim$13.6 eV, fragmentation dominates for the smaller species, while for larger species ionization is competitive immediately above the second ionization potential. At higher photon energies, all species behave similarly, the ionization yield gradually increases, levelling off between 0.8 and 0.9 at $\sim$18 eV \footnote{J.~Zhen et al., \textit{Astron. \& Astrophys.} (2016), in press}. We have also recorded the competition between the different dissociation channels as a function of the VUV photon energy, such as the C$_2$H$_2$ versus H/H$_2$ loss. We will discuss how these data can be compared to results of photoelectron spectroscopy performed on neutral PAHs at the VUV beamline at the Swiss Light Source.\footnote{B.~West et al., \textit{J. Phys. Chem. A } \underline{\textbf{118}} (2014), 7824-7831B.~West et al., \textit{J. Phys. Chem. A } \underline{\textbf{118}} (2014), 9870-9878}. Acknowledgments\footnote{European Research Council grant ERC-2013-SyG, Grant Agreement n.~610256 NANOCOSMOS.

Electronic spectroscopy of trapped pah photofragments

The PIRENEA set-up combines an ion cyclotron resonance cell mass spectrometer with cryogenic cooling in order to study the physical and chemical properties of polycyclic aromatic hydrocarbons (PAHs) of astrophysical interest. In space, PAHs are submitted to UV photons that lead to their dissociation. It is therefore of interest to study fragmentation pathways and search for species that might be good interstellar candidates because of their stability\footnote{J.~Montillaud, C.~Joblin, D.~Toublanc, \textit{Astron. \& Astrophys.} \underline{\textbf{552}} (2013), id.A15}. Electronic spectroscopy can bring major insights into the structure of species formed by photofragmentation. This is also a way to identify new species in space as recently illustrated in the case of C$_{60}^+$\footnote{E.K.~Campbell, M.~Holz, D.~Gerlich, and J.P.~Maier, \textit{Nature} \underline{\textbf{523}} (2015), 322-323}. In PIRENEA, the trapped ions are not cold enough, and thus we cannot use complexation with rare gas in order to record spectroscopy, as was nicely performed in the work by Campbell et al. on C$_{60}^+$. We are therefore using the dissociation of the trapped ions themselves instead, which requires in general a multiple photon scheme\footnote{F.~Useli-Bacchitta, A.~Bonnamy, G.~Malloci, et al., \textit{Chem.~Phys.} \underline{\textbf{371}} (2010), 16-23J.~Zhen, A. ~Bonnamy, G.~Mulas, C.~Joblin, \textit{Mol. Astrophys.} \underline{\textbf{2}} (2015), 12-17}. This leads to non-linear effects that affect the measured spectrum. We are working on improving this scheme in the specific case of the photofragment obtained by H-loss from 1-methylpyrene cation (CH$_3$-C$_{16}$H$_{9}^+$). A recent theoretical study has shown that a rearrangement can occur from 1-pyrenemethylium cation (CH$_2$-C$_{16}$H$_{9}^+$) to a system containing a seven membered ring (tropylium like pyrene system)\footnote{M.~Rapacioli, A.~Simon, C.C.M.~Marshall, et al., \textit{J. Phys. Chem. A} \underline{\textbf{119}} (2015), 12845-12854}. This study also reports the calculated electronic spectra of both isomers, which are specific enough to distinguish them, and as a function of temperature. We will present experiments that have been performed to study the photophysics of these ions using the PIRENEA set-up and a two-laser scheme for the action spectroscopy. Acknowledgments\footnote{European Research Council grant ERC-2013-SyG, Grant Agreement n.~610256 NANOCOSMOS.

Blind decomposition of Herschel-HIFI spectral maps of the NGC 7023 nebula

Large spatial-spectral surveys are more and more common in astronomy. This calls for the need of new methods to analyze such mega- to giga-pixel data-cubes. In this paper we present a method to decompose such observations into a limited and comprehensive set of components. The original data can then be interpreted in terms of linear combinations of these components. The method uses non-negative matrix factorization (NMF) to extract latent spectral end-members in the data. The number of needed end-members is estimated based on the level of noise in the data. A Monte-Carlo scheme is adopted to estimate the optimal end-members, and their standard deviations. Finally, the maps of linear coefficients are reconstructed using non-negative least squares. We apply this method to a set of hyperspectral data of the NGC 7023 nebula, obtained recently with the HIFI instrument onboard the Herschel space observatory, and provide a first interpretation of the results in terms of 3-dimensional dynamical structure of the region.Comment: Proceedings of the 2012 meeting of the french astronomical society (SF2A) in Nic

Laboratory Photo-chemistry of PAHs: Ionization versus Fragmentation

Interstellar polycyclic aromatic hydrocarbons (PAHs) are expected to be strongly processed by vacuum ultraviolet photons. Here, we report experimental studies on the ionization and fragmentation of coronene (C24H12), ovalene (C32H14) and hexa-peri-hexabenzocoronene (HBC; C42H18) cations by exposure to synchrotron radiation in the range of 8--40 eV. The results show that for small PAH cations such as coronene, fragmentation (H-loss) is more important than ionization. However, as the size increases, ionization becomes more and more important and for the HBC cation, ionization dominates. These results are discussed and it is concluded that, for large PAHs, fragmentation only becomes important when the photon energy has reached the highest ionization potential accessible. This implies that PAHs are even more photo-stable than previously thought. The implications of this experimental study for the photo-chemical evolution of PAHs in the interstellar medium are briefly discussed

Cross-validation of blindly separated interstellar dust spectra

International audienceIn this paper, we investigate the validation of Blind Source Separation (BSS) methods applied to interstellar dust hyperspectral data cubes. Since the original source signals are unknown, we cannot measure the separation accuracy by means of classical objective criteria. As a consequence, we here propose a cross-validation of the extracted spectra by applying to the measured data various BSS techniques based on different criteria. We show that, with all these methods, we obtain quite the same (physically relevant) estimated interstellar dust spectra. Moreover, we then derive a spatial structure of the emission of the chemical species, which is not used in the separation step and which is physically relevant