When studying chemistry of photodissociation regions (PDRs), time dependence becomes important as visual extinction increases, since certain chemical time-scales are comparable to the cloud lifetime. Dust temperature is also a key factor, since it significantly influences gas temperature and mobility on dust grains, determining the chemistry occurring on grain surfaces. We present a study of the dust temperature impact and time effects on the chemistry of different PDRs, using an updated version of theMeijerink PDR code and combining it with the time-dependent code Nahoon. We find the largest temperature effects in the inner regions of highG0 PDRs,where high dust temperatures favour the formation of simple oxygen-bearing molecules (especially that of O₂), while the formation of complex organic molecules is much more efficient at low dust temperatures. We also find that time-dependent effects strongly depend on the PDR type, since long time-scales promote the destruction of oxygen-bearing molecules in the inner parts of low G0 PDRs, while favouring their formation and that of carbon-bearing molecules in high G0 PDRs. From the chemical evolution, we also conclude that, in dense PDRs, CO₂ is a late-forming ice compared to water ice, and confirm a layered ice structure on dust grains, with H₂O in lower layers than CO₂. Regarding steady state, the PDR edge reaches chemical equilibrium at early times (≲10⁵ yr). This time is even shorter (<10⁴ yr) for high G0 PDRs. By contrast, inner regions reach equilibrium much later, especially low G0 PDRs, where steady state is reached at ∼10⁶-10⁷ yr.

@en

Context. Spectroscopic studies of ices in nearby star-forming regions indicate that ice mantles form on dust grains in two distinct steps, starting with polar ice formation (H2O rich) and switching to apolar ice (CO rich). Aims. We test how well the picture applies to more diffuse and quiescent clouds where the formation of the first layers of ice mantles can be witnessed.
Methods. Medium-resolution near-infrared spectra are obtained toward background field stars behind the Pipe Nebula. Results. The water ice absorption is positively detected at 3.0 µm in seven lines of sight out of 21 sources for which observed spectra are successfully reduced. The peak optical depth of the water ice is significantly lower than those in Taurus with the same AV . The
source with the highest water-ice optical depth shows CO ice absorption at 4.7 µm as well. The fractional abundance of CO ice with respect to water ice is 16+7
−6 %, and about half as much as the values typically seen in low-mass star-forming regions.
Conclusions. A small fractional abundance of CO ice is consistent with some of the existing simulations. Observations of CO2 ice in the early diffuse phase of a cloud play a decisive role in understanding the switching mechanism between polar and apolar ice formation.@en

The temperature of interstellar dust particles is of great importance to astronomers. It plays a crucial role in the thermodynamics of interstellar clouds, because of the gas-dust collisional coupling. It is also a key parameter in astrochemical studies that governs the rate at which molecules form on dust. In 3D (magneto)hydrodynamic simulations often a simple expression for the dust temperature is adopted, because of computational constraints, while astrochemical modelers tend to keep the dust temperature constant over a large range of parameter space. Our aim is to provide an easy-to-use parametric expression for the dust temperature as a function of visual extinction (A_V) and to shed light on the critical dependencies of the dust temperature on the grain composition. We obtain an expression for the dust temperature by semi-analytically solving the dust thermal balance for different types of grains and compare to a collection of recent observational measurements. We also explore the effect of ices on the dust temperature. Our results show that a mixed carbonaceous-silicate type dust with a high carbon volume fraction matches the observations best. We find that ice formation allows the dust to be warmer by up to 15% at high optical depths (A_V> 20 mag) in the interstellar medium. Our parametric expression for the dust temperature is presented as T_d = [11 + 5.7 × tanh(0.61 - log ₁₀(A_V)]χ_uv ^1/5.9, where χ_uv is in units of the Draine (1978, ApJS, 36, 595) UV field.

@en

Context. The presence of dust can strongly affect the chemical composition of the interstellar medium. We model the chemistry in photodissociation regions (PDRs) using both gas-phase and dust-phase chemical reactions. Aims. Our aim is to determine the chemical compositions of the interstellar medium (gas/dust/ice) in regions with distinct (molecular) gas densities that are exposed to radiation fields with different intensities. Methods. We have significantly improved the Meijerink PDR code by including 3050 new gas-phase chemical reactions and also by implementing surface chemistry. In particular, we have included 117 chemical reactions occurring on grain surfaces covering different processes, such as adsorption, thermal desorption, chemical desorption, two-body reactions, photo processes, and cosmic-ray processes on dust grains. Results. We obtain abundances for different gas and solid species as a function of visual extinction, depending on the density and radiation field. We also analyse the rates of the formation of CO₂ and H₂O ices in different environments. In addition, we study how chemistry is affected by the presence/absence of ice mantles (bare dust or icy dust) and the impact of considering different desorption probabilities. Conclusions. The type of substrate (bare dust or icy dust) and the probability of desorption can significantly alter the chemistry occurring on grain surfaces, leading to differences of several orders of magnitude in the abundances of gas-phase species, such as CO, H₂CO, and CH₃OH. The type of substrate, together with the density and intensity of the radiation field, also determine the threshold extinction to form ices of CO₂ and H₂O. We also conclude that H₂CO and CH₃OH are mainly released into the gas phase of low, far-ultraviolet illuminated PDRs through chemical desorption upon two-body surface reactions, rather than through photodesorption.

@en

Atoms and molecules, and in particular CO, are important coolants during the evolution of interstellar star-forming gas clouds. The presence of dust grains, which allow many chemical reactions to occur on their surfaces, strongly impacts the chemical composition of a cloud. At low temperatures, dust grains can lock up species from the gas phase which freeze out and form ices. In this sense, dust can deplete important coolants. Our aim is to understand the effects of freeze-out on the thermal balance and the evolution of a gravitationally bound molecular cloud. For this purpose, we perform 3D hydrodynamical simulations with the adaptive mesh code FLASH. We simulate a gravitationally unstable cloud under two different conditions, with and without grain surface chemistry. We let the cloud evolve until one free-fall time is reached and track the thermal evolution and the abundances of species during this time. We see that at a number density of 104 cm^-3 most of the CO molecules are frozen on dust grains in the run with grain surface chemistry, thereby depriving the most important coolant. As a consequence, we find that the temperature of the gas rises up to ̃25 K. The temperature drops once again due to gas-grain collisional cooling when the density reaches a few × 104 cm^-3.We conclude that grain surface chemistry not only affects the chemical abundances in the gas phase, but also leaves a distinct imprint in the thermal evolution that impacts the fragmentation of a star-forming cloud. As a final step, we present the equation of state of a collapsing molecular cloud that has grain surface chemistry included.

@en

Star formation in the centers of galaxies is thought to yield massive stars with a possibly top-heavy stellar mass distribution. It is likely that magnetic fields play a crucial role in the distribution of stellar masses inside star-forming molecular clouds. In this context, we explore the effects of magnetic fields, with a typical field strength of 38 μG, such as in RCW 38, and a field strength of 135 μG, similar to NGC 2024 and the infrared dark cloud G28.34+0.06, on the initial mass function (IMF) near (=10 pc) a 107 solar mass black hole. Using these conditions, we perform a series of numerical simulations with the hydrodynamical code FLASH to elucidate the impact of magnetic fields on the IMF and the star-formation efficiency (SFE) emerging from an 800 solar mass cloud. We find that the collapse of a gravitationally unstable molecular cloud is slowed down with increasing magnetic field strength and that stars form along the field lines. The total number of stars formed during the simulations increases by a factor of 1.5-2 with magnetic fields. The main component of the IMF has a lognormal shape, with its peak shifted to sub-solar (=0.3 M·) masses in the presence of magnetic fields, due to a decrease in the accretion rates from the gas reservoir. In addition, we see a top-heavy, nearly flat IMF above ∼2 solar masses, from regions that were supported by magnetic pressure until high masses are reached. We also consider the effects of X-ray irradiation if the central black hole is active. X-ray feedback inhibits the formation of sub-solar masses and decreases the SFEs even further. Thus, the second contribution is no longer visible. We conclude that magnetic fields potentially change the SFE and the IMF both in active and inactive galaxies, and need to be taken into account in such calculations. The presence of a flat component of the IMF would be a particularly relevant signature for the importance of magnetic fields, as it is usually not found in hydrodynamical simulations.

@en

Context. The ultraluminous infrared galaxy Mrk 231, which shows signs of both black hole accretion and star formation, exhibits very strong water rotational lines between λ = 200-670 μm, comparable to the strength of the CO rotational lines. High-redshift quasars also show similar CO and H 2O line properties, while starburst galaxies, such as M 82, lack these very strong H 2O lines in the same wavelength range, but do show strong CO lines. Aims. We explore the possibility of enhancing the gas phase H 2O 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 reforming the ice. Methods. We determined the formation rates of both OH and H 2O on dust grains, with temperatures T dust = 10-60 K, using both Monte Carlo and rate equation method simulations. The acquired formation rates were added to our X-ray chemistry code, which allowed us to calculate the thermal and chemical structure of the interstellar medium near an active galactic nucleus. Results. We derive analytic expressions for the formation of OH and H 2O on bare dust grains as a catalyst. Oxygen atoms arriving on the dust are released into the gas phase in the form of OH and H 2O. The efficiencies of this conversion due to the chemistry occurring on dust are near 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 decline rapidly. When the gas is mostly atomic, molecule formation on dust is dominant over the gas-phase route, which is then quenched by the low H 2 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 explain the observed bright water lines in nearby and high-redshift ULIRGs and quasars.@en

Aims. Because of their catalytic properties, interstellar dust grains 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 formation 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 temperature. 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 coming from the gas phase have to cross a square barrier (usually considered in previous studies) or a barrier obtained by density functional theory (DFT) calculations to become chemisorbed. The product of the 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.@en

Observations of star-forming environments revealed that the abundances of some deuterated interstellar molecules are markedly larger than the cosmic D/H ratio of 10^-5. Possible reasons for this pointed to grain surface chemistry. However, organic molecules and water, which are both ice constituents, do not enjoy the same deuteration. For example, deuterated formaldehyde is very abundant in comets and star-forming regions, while deuterated water rarely is. In this paper, we explain this selective deuteration by following the formation of ices (using the rate equation method) in translucent clouds, as well as their evolution as the cloud collapses to form a star. Ices start with the deposition of gas-phase CO and O onto dust grains. While reaction of oxygen with atoms (H or D) or molecules (H₂) yields H₂O (HDO), CO only reacts with atoms (H and D) to form H ₂CO (HDCO, D₂CO). As a result, the deuteration of formaldehyde is sensitive to the gas D/H ratio as the cloud undergoes gravitational collapse, while the deuteration of water strongly depends on the dust temperature at the time of ice formation. These results reproduce well the deuterium fractionation of formaldehyde observed in comets and star-forming regions and can explain the wide spread of deuterium fractionation of water observed in these environments.

@en

Context. Water and O₂ are important gas phase ingredients for cooling dense gas when forming stars. On dust grains, H₂O is an important constituent of the icy mantle in which a complex chemistry is taking place, as revealed by hot core observations. The formation of water can occur on dust grain surfaces, and can impact gas phase composition. Aims. The formation of molecules such as OH, H₂O, HO₂ and H₂O ₂, as well as their deuterated forms and O₂ and O ₃ is studied to assess how the chemistry varies in different astrophysical environments, and how the gas phase is affected by grain surface chemistry. Methods. We use Monte Carlo simulations to follow the formation of molecules on bare grains as well as the fraction of molecules released into the gas phase. We consider a surface reaction network, based on gas phase reactions, as well as UV photo-dissociation of the chemical species. Results. We show that grain surface chemistry has a strong impact on gas phase chemistry, and that this chemistry is very different for different dust grain temperatures. Low temperatures favor hydrogenation, while higher temperatures favor oxygenation. Also, UV photons dissociate the molecules on the surface, which can subsequently reform. The formation-destruction cycle increases the amount of species released into the gas phase. We also determine the timescales to form ices in diffuse and dense clouds, and show that ices are formed only in shielded environments, as supported by observations.

@en

Context. 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 are investigated to assess the importance of trace amounts of dust, 10-5 - 10-3 Z ⊙ 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 that, for a collapsing gas cloud environment with coupled chemical and thermal balance, 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 an 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 modeling suggest that H2 formation on grains is suppressed while HD formation on grains is not.@en

We study the formation of molecular hydrogen on dust grain surfaces and apply our results to the high-redshift universe. We find that a range of physical parameters - in particular dust temperature and gas temperature, but not so much dust surface composition - influences the formation rate of H 2. The H2 formation rate is found to be suppressed above gas kinetic temperatures of a few hundred K and for dust temperatures above 500 K and below 10 K. We highlight the differences between our treatment of the H2 formation process and other descriptions in the literature. We also study the relative importance of H2 formation on dust grains with respect to molecular hydrogen formation in the gas phase, through the H- route. The ratio of the formation rates of these two routes depends to a large part on the dust abundance, on the electron abundance, and on the relative strength of the far-ultraviolet (extra-)galactic radiation field. We find that for a cosmological evolution of the star formation rate and dust density consistent with the Madau plot, a positive feedback effect on the abundance of H2 due to the presence of dust grains can occur at redshifts z ≥ 3. This effect occurs for a dust-to-gas mass ratio as small as 10-3 of the galactic value.@en