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Register a new userEffects of turbulent dust grain motion to interstellar chemistry
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Theoretical studies have revealed that dust grains are usually moving fast through the turbulent interstellar gas, which could have significant effects upon interstellar chemistry by modifying grain accretion. This effect is investigated in this work on the basis of numerical gas-grain chemical modeling. Major features of the grain motion effect in the typical environment of dark clouds (DC) can be summarised as follows: 1) decrease of gas-phase (both neutral and ionic) abundances and increase of surface abundances by up to 2-3 orders of magnitude; 2) shifts of the existing chemical jumps to earlier evolution ages for gas-phase species and to later ages for surface species by factors of about ten; 3) a few exceptional cases in which some species turn out to be insensitive to this effect and some other species can show opposite behaviors too. These effects usually begin to emerge from a typical DC model age of about 10^5 yr. The grain motion in a typical cold neutral medium (CNM) can help overcome the Coulomb repulsive barrier to enable effective accretion of cations onto positively charged grains. As a result, the grain motion greatly enhances the abundances of some gas-phase and surface species by factors up to 2-6 or more orders of magnitude in the CNM model. The grain motion effect in a typical molecular cloud (MC) is intermediate between that of the DC and CNM models, but with weaker strength. The grain motion is found to be important to consider in chemical simulations of typical interstellar medium.
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It has recently been shown that turbulence in the interstellar medium (ISM) can significantly accelerate the growth of dust grains by accretion of molecules, but the turbulent gas-density distribution also plays a crucial role in shaping the grain-size distribution. The growth velocity, i.e., the rate of change of the mean grain radius, is proportional to the local gas density if the growth species (molecules) are well-mixed in the gas. As a consequence, grain growth happens at vastly different rates in different locations, since the gas-density distribution of the ISM shows a considerable variance. Here, it is shown that grain-size distribution (GSD) rapidly becomes a reflection of the gas-density distribution, irrespective of the shape of the initial GSD. This result is obtained by modelling ISM turbulence as a Markov process, which in the special case of an Ornstein-Uhlenbeck process leads to a lognormal gas-density distribution, consistent with numerical simulations of isothermal compressible turbulence. This yields an approximately lognormal GSD; the sizes of dust grains in cold ISM clouds may thus not follow the commonly adopted power-law GSD with index -3.5, but corroborates the use of a log-nomral GSD for large grains, suggested by several studies. It is also concluded that the very wide range of gas densities obtained in the high Mach-number turbulence of molecular clouds must allow formation of a tail of very large grains reaching radii of several microns.
The ULIRG Mrk 231 exhibits very strong water rotational lines between lambda = 200-670mu 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.
Dust grains are aligned with the interstellar magnetic field and drift through the interstellar medium (ISM). Evolution of interstellar dust is driven by grain motion. In this paper, we study the effect of grain alignment with magnetic fields and grain motion on grain growth in molecular clouds. We first discuss characteristic timescales of internal alignment (i.e., alignment of the grain axis with its angular momentum, ${bf J}$) and external alignment (i.e., alignment of ${bf J}$ with the magnetic field) and find the range of grain sizes that have efficient alignment. Then, we study grain growth for such aligned grains drifting though the gas. Due to the motion of aligned grains along the magnetic field, gas accretion would increase the grain elongation rather than decrease, as in the case of random orientation. Grain coagulation also gradually increases grain elongation, leading to the increase of elongation with the grain size. The coagulation of aligned grains can form dust aggregates that contain the elongated binaries comprising a pair of grains with parallel short axes. The presence of superparamagnetic iron clusters within dust grains enhances internal alignment and thus increases the maximum size of aligned grains from $sim 2$ to $sim 10mu m$ for dense clouds of $n_{rm H}sim 10^{5}rm cm^{-3}$. Determining the size of such aligned grains with parallel axes within a dust aggregate would be important to constrain the location of grain growth and the level of iron inclusions. We find that grains within dust aggregates in 67P/Churyumov-Gerasimenko obtained by {it Rosetta} have the grain elongation increasing with the grain radius, which is not expected from coagulation by Brownian motion but consistent with the grain growth from aligned grains.
Networks of reactions on dust grain surfaces play a crucial role in the chemistry of interstellar clouds, leading to the formation of molecular hydrogen in diffuse clouds as well as various organic molecules in dense molecular clouds. Due to the sub-micron size of the grains and the low flux, the population of reactive species per grain may be very small and strongly fluctuating. Under these conditions rate equations fail and the simulation of surface-reaction networks requires stochastic methods such as the master equation. However, the master equation becomes infeasible for complex networks because the number of equations proliferates exponentially. Here we introduce a method based on moment equations for the simulation of reaction networks on small grains. The number of equations is reduced to just one equation per reactive specie and one equation per reaction. Nevertheless, the method provides accurate results, which are in excellent agreement with the master equation. The method is demonstrated for the methanol network which has been recently shown to be of crucial importance.
Advanced telescopes, such as ALMA and JWST, are likely to show that the chemical universe may be even more complex than currently observed, requiring astrochemical modelers to improve their models to account for the impact of new data. However, essential input information for gas-grain models, such as binding energies of molecules to the surface, have been derived experimentally only for a handful of species, leaving hundreds of species with highly uncertain estimates. We present in this paper a systematic study of the effect of uncertainties in the binding energies on an astrochemical two-phase model of a dark molecular cloud, using the rate equations approach. A list of recommended binding energy values based on a literature search of published data is presented. Thousands of simulations of dark cloud models were run, and in each simulation a value for the binding energy of hundreds of species was randomly chosen from a normal distribution. Our results show that the binding energy of H$_{2}$ is critical for the surface chemistry. For high binding energy, H$_{2}$ freezes out on the grain forming an H$_{2}$ ice. This is not physically realistic and we suggest a change in the rate equations. The abundance ranges found are in reasonable agreement with astronomical ice observations. Pearson correlation coefficients revealed that the binding energy of HCO, HNO, CH$_{2}$, and C correlate most strongly with the abundance of dominant ice species. Finally, the formation route of complex organic molecules was found to be sensitive to the branching ratios of H$_{2}$CO hydrogenation.
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