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Register a new userThe Photolysis of Aromatic Hydrocarbons Adsorbed on the Surfaces of Cosmic Dust Grains
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The work is devoted to the adaptation of the results of laboratory studies of the laser-induced dissociation of molecules of benzene adsorbed on a quartz substrate to the conditions of the interstellar medium. Adsorption was performed under conditions of low temperature and deep vacuum. The difference between the photolysis of adsorbed molecules and molecules in the gas phase is identified. Significance of process of photolytic desorption in the interstellar conditions is analyzed, in particular, in the conditions of photodissociation regions. It is shown that the efficiency and dissociation channels of photolysis of adsorbed and gas phase benzene differ substantially. It is concluded that the photolysis of aromatic hydrocarbons adsorbed on the interstellar dust grains contributes a negligible fraction to the abundance of small hydrocarbons in the interstellar medium.
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The cosmic-ray ionization rate ($zeta$, s$^{-1}$) plays an important role in the interstellar medium. It controls ion-molecular chemistry and provides a source of heating. Here we perform a grid of calculations using the spectral synthesis code CLOUDY along nine sightlines towards, HD 169454, HD 110432, HD 204827, $lambda$ Cep, X Per, HD 73882, HD 154368, Cyg OB2 5, Cyg OB2 12. The value of $zeta$ is determined by matching the observed column densities of H$_3^+$ and H$_2$. The presence of polycyclic aromatic hydrocarbons (PAHs) affects the free electron density, which changes the H$_3^+$ density and the derived ionization rate. PAHs are ubiquitous in the Galaxy, but there are also regions where PAHs do not exist. Hence, we consider clouds with a range of PAH abundances and show their effects on the H$_3^+$ abundance. We predict an average cosmic-ray ionization rate for H$_2$ ($zeta$(H$_2$))= (7.88 $pm$ 2.89) $times$ 10$^{-16}$ s$^{-1}$ for models with average Galactic PAHs abundances, (PAH/H =10$^{-6.52}$), except Cyg OB2 5 and Cyg OB2 12. The value of $zeta$ is nearly 1 dex smaller for sightlines toward Cyg OB2 12. We estimate the average value of $zeta$(H$_2$)= (95.69 $pm$ 46.56) $times$ 10$^{-16}$ s$^{-1}$ for models without PAHs.
Context. The formation of water on the dust grains in the interstellar medium may proceed with hydrogen peroxide (H2O2) as an intermediate. Recently gas-phase H2O2 has been detected in {rho} Oph A with an abundance of ~1E-10 relative to H2. Aims. We aim to reproduce the observed abundance of H2O2 and other species detected in {rho} Oph A quantitatively. Methods. We make use of a chemical network which includes gas phase reactions as well as processes on the grains; desorption from the grain surface through chemical reaction is also included. We run the model for a range of physical parameters. Results. The abundance of H2O2 can be best reproduced at ~6E5 yr, which is close to the dynamical age of {rho} Oph A. The abundances of other species such as H2CO, CH3OH, and O2 can be reasonably reproduced also at this time. In the early time the gas-phase abundance of H2O2 can be much higher than the current detected value. We predict a gas phase abundance of O2H at the same order of magnitude as H2O2, and an abundance of the order 1E-8 for gas phase water in {rho} Oph A. A few other species of interest are also discussed. Conclusions. We demonstrate that H2O2 can be produced on the dust grains and released into the gas phase through non-thermal desorption via surface exothermic reactions. The H2O2 molecule on the grain is an important intermediate in the formation of water. The fact that H2O2 is over-produced in the gas phase for a range of physical conditions suggests that its destruction channel in the current gas phase network may be incomplete.
We report on a common fragment ion formed during the electron-ionization-induced fragmentation of three different three-ring polycyclic aromatic hydrocarbons (PAHs), fluorene (C$_{13}$H$_{10}$), 9,10-dihydrophenanthrene (C$_{14}$H$_{12}$), and 9,10-dihydroanthracene (C$_{14}$H$_{12}$). The infrared spectra of the mass-isolated product ions with $m/z=165$ were obtained in a Fourier transform ion cyclotron resonance mass spectrometer whose cell was placed inside the optical cavity of an infrared free-electron laser, thus providing the high photon fluence required for efficient infrared multiple-photon dissociation. The infrared spectra of the $m/z=165$ species generated from the three different precursors were found to be similar, suggesting the formation of a single C$_{13}$H$_{9}^+$ isomer. Theoretical calculations using density functional theory (DFT) revealed the fragments identity as the closed-shell fluorenyl cation. Decomposition pathways from each parent precursor to the fluorenyl ion are proposed on the basis of DFT calculations. The identification of a single fragmentation product from three different PAHs supports the notion of the existence of common decomposition pathways of PAHs in general and can aid in understanding the fragmentation chemistry of astronomical PAH species.
The electronic and optical properties of polycyclic aromatic hydrocarbons (PAHs) present a strong dependence on their size and geometry. We tackle this issue by analyzing the spectral features of two prototypical classes of PAHs, belonging to D6h and D2h symmetry point groups and related to coronene as multifunctional seed. While the size variation induces an overall red shift of the spectra and a redistribution of the oscillator strength between the main peaks, a lower molecular symmetry is responsible for the appearance of new optical features. Along with broken molecular orbital degeneracies, optical peaks split and dark states are activated in the low-energy part of the spectrum. Supported by a systematic analysis of the composition and the character of the optical transitions, our results contribute in shedding light to the mechanisms responsible for spectral modifications in the visible and near UV absorption bands of medium-size PAHs.
With the high-resolution spectroscopy now available in the optical and satellite UV, it is possible to determine the neutral/ionized column density ratios for several different elements in a single cloud. Assuming ionization equilibrium for each element, one can make several independent determinations of the electron density. For the clouds for which such an analysis has been carried out, these different estimates disagree by large factors, suggesting that some process (or processes) besides photoionization and radiative recombination might play an important role in the ionization balance. One candidate process is collisions of ions with dust grains. Making use of recent work quantifying the abundances of polycyclic aromatic hydrocarbon molecules and other grains in the interstellar medium, as well as recent models for grain charging, we estimate the grain-assisted ion recombination rates for several astrophysically important elements. We find that these rates are comparable to the rates for radiative recombination for conditions typical of the cold neutral medium. Including grain-assisted ion recombination in the ionization equilibrium analysis leads to increased consistency in the various electron density estimates for the gas along the line of sight to 23 Orionis. However, not all of the discrepancies can be eliminated in this way; we speculate on some other processes that might play a role. We also note that grain-assisted recombination of H+ and He+ leads to significantly lower electron fractions than usually assumed for the cold neutral medium.
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