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Superhydrogenated polycyclic aromatic hydrocarbons (PAHs) have been suggested to catalyze the formation of H2 in certain regions of space, but it remains unclear under which circumstances this mechanism is viable given the reduced carbon backbone stability of superhydrogenated PAHs. We report a laboratory study on the stability of the smallest pericondensed PAH, pyrene (C16H10+N , with N = 4, 6, and 16 additional H atoms), against photodestruction by single vacuum ultraviolet photons using the photoelectron–photoion coincidence technique. For N = 4, we observe a protective effect of hydrogenation against the loss of native hydrogens, in the form of an increase in the appearance energies of the and C16H8 + daughter ions compared to those reported for pristine pyrene (C16H10). No such effect is seen for N = 6 or 16, where the weakening effect of replacing aromatic bonds with aliphatic ones outweighs the buffering effect of the additional hydrogen atoms. The onset of fragmentation occurs at similar internal energies for N = 4 and 6, but is significantly lower for N = 16. In all three cases, H-loss and C m H n -loss (m ≥ 1, carbon backbone fragmentation) channels open at approximately the same energy. The branching fractions of the primary channels favor H-loss for N = 4, C m H n -loss for N = 16, and are roughly equal for the intermediate N = 6. We conclude that superhydrogenated pyrene is probably too small to support catalytic H2-formation, while trends in the current and previously reported data suggest that larger PAHs may serve as catalysts up to a certain level of hydrogenation.

Molecular hydrogen (H2) is the main constituent of the gas in the planet-forming disks that surround many PMS stars. H2 can be incorporated in the atmosphere of the giant planets. HD has been detected in a few disks and can be considered the most reliable tracer of H2. We wish to form H2 and HD efficiently for the varied conditions encountered in protoplanetary disks: the densities vary from 1E4 to 1E16 cm^-3; the dust temperatures range from 5 to 1500 K, the gas temperatures go from 5 to a few 1000 Kelvin, and the ultraviolet field can be 1E7 stronger than the standard interstellar field. We implemented a comprehensive model of H2 and HD formation on cold and warm grain surfaces and via hydrogenated PAHs in the physico-chemical code ProDiMo. The H2 and HD formation can proceed via the Langmuir-Hinshelwood and Eley-Ridel mechanisms for physisorbed or chemisorbed H (D) atoms. H2 and HD also form by H (D) abstraction from hydrogenated neutral and ionised PAHs and via gas phase reactions. H2 and HD are formed efficiently on dust grain surfaces from 10 to 700 K. All the deuterium is converted into HD in UV shielded regions as soon as H2 is formed by gas-phase D abstraction reactions. The detailed model compares well with standard analytical prescriptions for H2 (HD) formation. At low temperatures, H2 is formed from the encounter of two physisorbed atoms. HD molecules form on the grain surfaces and in the gas-phase. At temperatures greater than 20 K, the meeting between a weakly bound H- (or D-) atom or a gas-phase H (D) atom and a chemisorbed atom is the most efficient H2 formation route. H2 formation through hydrogenated PAHs alone is efficient above 80 K. The contribution of hydrogenated PAHs to the overall H2 and HD formation is relatively low if chemisorption on silicate is taken into account and if a small hydrogen abstraction cross-section is used., Comment: 26 pages, accepted to A&A

Aims: The production of molecular hydrogen and its deuterated forms onto carbonaceous dust grains is investigated in detail. The goal of this study is to estimate the importance of the chemistry occuring on grain surfaces for the deuteration of H2. Furthermore, we aim to find a robust and general surface chemical model which can be used in different astrophysical environments. Methods: Surface processes are described for the cases of graphitic and amorphous--carbon grains, where laboratory work is available. Langmuir--Hinshelwood as well as Eley--Rideal surface chemistries are included in the model and their relative contributions are highlighted. Analytic expressions are derived for H2, HD, and D2 formation efficiencies for both type of grains. Rate equations are tested against stochastic methods. Results: As expected, rate equations and stochastic methods diverge for grain sizes lower than a critical value acrit. For grain sizes below this critical value, D2 formation decreases to favour HD formation. The formation efficiencies of H2 and D2 can be calculated by adding a correction factor to the rate equations methods. We found that because of the presence of chemisorbed sites, which can store atoms to form molecules up to high grain temperatures, the formation efficiency of HD and D2 is very high compared to models where only physisorption sites are taken into account. When considering a realistic distribution of dust grains, we found that the formation rate of H2 and HD is enhanced by an order of magnitude if small grains are taken into account. The processes described in this paper, that allow a strong enhancement of the deuterated forms of molecular hydrogen, could explain the high degree of deuterium fractionation observed in protostellar environments., Comment: 17 pages 25 figures

We present a far-ultraviolet (PDR) and an X-ray dominated region (XDR) code. We include and discuss thermal and chemical processes that pertain to irradiated gas. An elaborate chemical network is used and a careful treatment of PAHs and H(2) formation, destruction and excitation is included. For both codes we calculate four depth-dependent models for different densities and radiation fields, relevant to conditions in starburst galaxies and active galactic nuclei. A detailed comparison between PDR and XDR physics is made for total gas column densities between similar to 10(20) and similar to 10(25) cm(-2). We show cumulative line intensities for a number of fine-structure lines (e.g., [CII], [OI], [CI], [SiII], [FeII]), as well as cumulative column densities and column density ratios for a number of species ( e. g., CO/H(2), CO/C, HCO(+)/HCN, HNC/HCN). The comparison between the results for the PDRs and XDRs shows that column density ratios are almost constant up to N(H) = 1022 cm(-2) for XDRs, unlike those in PDRs. For example, CO/C in PDRs changes over four orders of magnitude from the edge to NH = 10(22) cm(-2). The CO/C and CO/H(2) ratios are lower in XDRs at low column densities and rise at NH > 1023 cm-2. At most column densities NH > 10(21.5) cm-2, HNC/HCN ratios are lower in XDRs too, but they show a more moderate increase at higher N(H).

We present a far-ultraviolet (PDR) and an X-ray dominated region (XDR) code. We include and discuss thermal and chemical processes that pertain to irradiated gas. An elaborate chemical network is used and a careful treatment of PAHs and H(2) formation, destruction and excitation is included. For both codes we calculate four depth-dependent models for different densities and radiation fields, relevant to conditions in starburst galaxies and active galactic nuclei. A detailed comparison between PDR and XDR physics is made for total gas column densities between similar to 10(20) and similar to 10(25) cm(-2). We show cumulative line intensities for a number of fine-structure lines (e.g., [CII], [OI], [CI], [SiII], [FeII]), as well as cumulative column densities and column density ratios for a number of species ( e. g., CO/H(2), CO/C, HCO(+)/HCN, HNC/HCN). The comparison between the results for the PDRs and XDRs shows that column density ratios are almost constant up to N(H) = 1022 cm(-2) for XDRs, unlike those in PDRs. For example, CO/C in PDRs changes over four orders of magnitude from the edge to NH = 10(22) cm(-2). The CO/C and CO/H(2) ratios are lower in XDRs at low column densities and rise at NH > 1023 cm-2. At most column densities NH > 10(21.5) cm-2, HNC/HCN ratios are lower in XDRs too, but they show a more moderate increase at higher N(H).

The most abundant interstellar molecule, H-2, is generally thought to form by recombination of H atoms on grain surfaces. On surfaces, hydrogen atoms can be physisorbed and chemisorbed and their mobility can be governed by quantum mechanical tunneling or thermal hopping. We have developed a model for molecular hydrogen formation on surfaces. This model solves the time-dependent kinetic rate equation for atomic and molecular hydrogen and their isotopes, taking the presence of physisorbed and chemisorbed sites, as well as quantum mechanical diffusion and thermal hopping, into account. The results show that the time evolution of this system is mainly governed by the binding energies and barriers against migration of the adsorbed species. We have compared the results of our model with experiments on the formation of HD on silicate and carbonaceous surfaces under irradiation by atomic H and D beams at low and at high temperatures. This comparison shows that including both isotopes, both physisorbed and chemisorbed wells, and both quantum mechanical tunneling and thermal hopping is essential for a correct interpretation of the experiments. This comparison allows us to derive the characteristics of these surfaces. For the two surfaces we consider, we determine the binding energy of H atoms and H-2 molecules, as well as the barrier against diffusion for the H atoms to move from one site to another. We conclude that molecular hydrogen formation is efficient until quite high (similar to 500 K) temperatures. At low temperatures, recombination between mobile physisorbed atoms and trapped chemisorbed atoms dominates. At higher temperatures, chemisorbed atoms become mobile, and this then drives molecular hydrogen formation. We have extended our model to astrophysically relevant conditions. The results show that molecular hydrogen formation proceeds with near unity efficiency at low temperatures ( T less than or equal to 20 K). While the efficiency drops, molecular hydrogen formation in the ISM can be very efficient even at high temperatures, depending on the physical characteristics of the surface.

The most abundant interstellar molecule, H-2, is generally thought to form by recombination of H atoms on grain surfaces. On surfaces, hydrogen atoms can be physisorbed and chemisorbed and their mobility can be governed by quantum mechanical tunneling or thermal hopping. We have developed a model for molecular hydrogen formation on surfaces. This model solves the time-dependent kinetic rate equation for atomic and molecular hydrogen and their isotopes, taking the presence of physisorbed and chemisorbed sites, as well as quantum mechanical diffusion and thermal hopping, into account. The results show that the time evolution of this system is mainly governed by the binding energies and barriers against migration of the adsorbed species. We have compared the results of our model with experiments on the formation of HD on silicate and carbonaceous surfaces under irradiation by atomic H and D beams at low and at high temperatures. This comparison shows that including both isotopes, both physisorbed and chemisorbed wells, and both quantum mechanical tunneling and thermal hopping is essential for a correct interpretation of the experiments. This comparison allows us to derive the characteristics of these surfaces. For the two surfaces we consider, we determine the binding energy of H atoms and H-2 molecules, as well as the barrier against diffusion for the H atoms to move from one site to another. We conclude that molecular hydrogen formation is efficient until quite high (similar to 500 K) temperatures. At low temperatures, recombination between mobile physisorbed atoms and trapped chemisorbed atoms dominates. At higher temperatures, chemisorbed atoms become mobile, and this then drives molecular hydrogen formation. We have extended our model to astrophysically relevant conditions. The results show that molecular hydrogen formation proceeds with near unity efficiency at low temperatures ( T less than or equal to 20 K). While the efficiency drops, molecular hydrogen formation in the ISM can be very efficient even at high temperatures, depending on the physical characteristics of the surface.

The ULIRG Mrk 231 exhibits very strong water rotational lines between \lambda = 200-670\mu m, comparable to the strength of the CO rotational lines. High redshift quasars also show similar CO and H2O line properties, while starburst galaxies, such as M82, lack these very strong H2O lines in the same wavelength range, but do show strong CO lines. We explore the possibility of enhancing the gas phase H2O abundance in X-ray exposed environments, using bare interstellar carbonaceous dust grains as a catalyst. Cloud-cloud collisions cause C and J shocks, and strip the grains of their ice layers. The internal UV field created by X-rays from the accreting black hole does not allow to reform the ice. We determine formation rates of both OH and H2O on dust grains, having temperature T_dust=10-60 K, using both Monte Carlo as well as rate equation method simulations. The acquired formation rates are added to our X-ray chemistry code, that allows us to calculate the thermal and chemical structure of the interstellar medium near an active galactic nucleus. We derive analytic expressions for the formation of OH and H2O on bare dust grains as a catalyst. Oxygen atoms arriving on the dust are released into the gas phase under the form of OH and H2O. The efficiencies of this conversion due to the chemistry occurring on dust are of order 30 percent for oxygen converted into OH and 60 percent for oxygen converted into H_2O between T_dust=15-40 K. At higher temperatures, the efficiencies rapidly decline. When the gas is mostly atomic, molecule formation on dust is dominant over the gas-phase route, which is then quenched by the low H2 abundance. Here, it is possible to enhance the warm (T> 200 K) water abundance by an order of magnitude in X-ray exposed environments. This helps to explain the observed bright water lines in nearby and high-redshift ULIRGs and Quasars., Comment: 13 pages, 8 figures, accepted by A&A

The initial mass function (IMF) of the first (Population III) stars and Population II (Pop II) stars is poorly known due to a lack of observations of the period between recombination and reionization. In simulations of the formation of the first stars, it has been shown that, due to the limited ability of metal-free primordial gas to cool, the IMF of the first stars is a few orders of magnitude more massive than the current IMF. The transition from a high-mass IMF of the first stars to a lower-mass current IMF is thus important to understand. To study the underlying physics of this transition, we performed several simulations using the cosmological hydrodynamic adaptive mesh refinement code Enzo for metallicities of 10^{-4}, 10^{-3}, 10^{-2}, and 10^{-1} Z_{\odot}. In our simulations we include a star formation prescription that is derived from a metallicity dependent multi-phase ISM structure, an external UV radiation field, and a mechanical feedback algorithm. We also implement cosmic ray heating, photoelectric heating and gas-dust heating/cooling, and follow the metal enrichment of the ISM. It is found that the interplay between metallicity and UV radiation leads to the co-existence of Pop III and Pop II star formation in non-zero metallicity (Z/Z_{\odot} \geq10^{-2}) gas. A cold (T10^{-22} g cm^{-3}) gas phase is fragile to ambient UV radiation. In a metal-poor (Z/Z_{\odot} \leq10^{-3}) gas, the cold and dense gas phase does not form in the presence of a radiation field of F_{0}\sim10^{-5}-10^{-4} erg cm^{-2} s^{-1}. Therefore, metallicity by itself is not a good indicator of the Pop III-Pop II transition. Metal-rich (Z/Z_{\odot}\geq10^{-2}) gas dynamically evolves two to three orders of magnitude faster than metal poor gas (Z/Z_{\odot}\leq10^{-3}). The simulations including SNe show that pre-enrichment of the halo does not affect the mixing of metals., Published in ApJ

Context: The HD and H2 molecules play important roles in the cooling of primordial and very metal-poor gas at high redshift. Aims: Grain surface and gas phase formation of HD and H2 is investigated to assess the importance of trace amounts of dust, 10^{-5}-10^{-3} Zo, in the production of HD and H2. Methods: We consider carbonaceous and silicate grains and include both physisorption and chemisorption, tunneling, and realistic grain surface barriers. We find, for a collapsing gas cloud environment with coupled chemical and thermal balance, that dust abundances as small as 10^{-5} solar lead to a strong boost in the H2 formation rate due to surface reactions. As a result of this enhancement in H2, HD is formed more efficiently in the gas phase through the D+ + H2 reaction. Direct formation of HD on dust grains cannot compete well with this gas phase process for dust temperatures below 150 K. We also derive up-to-date analytic fitting formulae for the grain surface formation of H2 and HD, including the different binding energies of H and D. Results: Grain surface reactions are crucial to the availability of H2 and HD in very metal-poor environments. Above metallicities of 10^{-5} solar, the grain surface route dominates the formation of H2, which in turn, drives the formation of HD in the gas phase. At dust temperatures above 150 K, laboratory experiments and theoretical modelling suggest that H2 formation on grains is suppressed while HD formation on grains is not., typos corrected, accepted for publication in Astronomy and Astrophysics

We present a far-ultraviolet (PDR) and an X-ray dominated region (XDR) code. We include and discuss thermal and chemical processes that pertain to irradiated gas. An elaborate chemical network is used and a careful treatment of PAHs and H(2) formation, destruction and excitation is included. For both codes we calculate four depth-dependent models for different densities and radiation fields, relevant to conditions in starburst galaxies and active galactic nuclei. A detailed comparison between PDR and XDR physics is made for total gas column densities between similar to 10(20) and similar to 10(25) cm(-2). We show cumulative line intensities for a number of fine-structure lines (e.g., [CII], [OI], [CI], [SiII], [FeII]), as well as cumulative column densities and column density ratios for a number of species ( e. g., CO/H(2), CO/C, HCO(+)/HCN, HNC/HCN). The comparison between the results for the PDRs and XDRs shows that column density ratios are almost constant up to N(H) = 1022 cm(-2) for XDRs, unlike those in PDRs. For example, CO/C in PDRs changes over four orders of magnitude from the edge to NH = 10(22) cm(-2). The CO/C and CO/H(2) ratios are lower in XDRs at low column densities and rise at NH > 1023 cm-2. At most column densities NH > 10(21.5) cm-2, HNC/HCN ratios are lower in XDRs too, but they show a more moderate increase at higher N(H).

The most abundant interstellar molecule, H-2, is generally thought to form by recombination of H atoms on grain surfaces. On surfaces, hydrogen atoms can be physisorbed and chemisorbed and their mobility can be governed by quantum mechanical tunneling or thermal hopping. We have developed a model for molecular hydrogen formation on surfaces. This model solves the time-dependent kinetic rate equation for atomic and molecular hydrogen and their isotopes, taking the presence of physisorbed and chemisorbed sites, as well as quantum mechanical diffusion and thermal hopping, into account. The results show that the time evolution of this system is mainly governed by the binding energies and barriers against migration of the adsorbed species. We have compared the results of our model with experiments on the formation of HD on silicate and carbonaceous surfaces under irradiation by atomic H and D beams at low and at high temperatures. This comparison shows that including both isotopes, both physisorbed and chemisorbed wells, and both quantum mechanical tunneling and thermal hopping is essential for a correct interpretation of the experiments. This comparison allows us to derive the characteristics of these surfaces. For the two surfaces we consider, we determine the binding energy of H atoms and H-2 molecules, as well as the barrier against diffusion for the H atoms to move from one site to another. We conclude that molecular hydrogen formation is efficient until quite high (similar to 500 K) temperatures. At low temperatures, recombination between mobile physisorbed atoms and trapped chemisorbed atoms dominates. At higher temperatures, chemisorbed atoms become mobile, and this then drives molecular hydrogen formation. We have extended our model to astrophysically relevant conditions. The results show that molecular hydrogen formation proceeds with near unity efficiency at low temperatures ( T less than or equal to 20 K). While the efficiency drops, molecular hydrogen formation in the ISM can be very efficient even at high temperatures, depending on the physical characteristics of the surface.

Aims. Interstellar dust grains, because of their catalytic properties, are crucial to the formation of H2, the most abundant molecule in the Universe. The formation of molecular hydrogen strongly depends on the ability of H atoms to stick on dust grains. In this study we determine the sticking coefficient of H atoms chemisorbed on graphitic surfaces, and estimate its impact on the forma- tion of H2. Methods. The sticking probability of H atoms chemisorbed onto graphitic surfaces is obtained using a mixed classical-quantum dynamics method. In this, the H atom is treated quantum- mechanically and the vibrational modes of the surface are treated classically. The implications of sticking for the formation of H2 are addressed by using Kinetic Monte Carlo simulations that follow how atoms stick, move and associate with each other on dust surfaces of different temper- ature. Results. In our model, molecular hydrogen forms very efficiently for dust temperatures lower than 15 K through the involvement of physisorbed H atoms. At dust temperatures higher than 15 K and gas temperatures lower than 2000 K, H2 formation differs strongly if the H atoms com- ing from the gas phase have to cross a square barrier (usually considered in previous studies) or a barrier obtained by DFT calculations to become chemisorbed. The product of sticking times efficiency can be increased by many orders of magnitude when realistic barriers are considered. If graphite phonons are taken into account in the dynamics calculations, then H atoms stick better on the surface at high energies, but the overall H2 formation efficiency is only slightly affected. Our results suggest that H2 formation can proceed efficiently in photon dominated regions, X-ray dominated regions, hot cores and in the early Universe when the first dust is available., Comment: 6 figures, 14 pages