D. S. Wiebe

Gas-Phase CO in Protoplanetary Disks: A Challenge for Turbulent Mixing

This is the first paper in a series in which we study the influence of turbulent diffusion and advective transport on the chemical evolution of protoplanetary disks, using a 2D flared-disk model and a 2D-mixing gas-grain chemical code with surface reactions. A first interesting result concerns the abundance of gas-phase CO in the outer regions of protoplanetary disks. In this Letter we argue that the gas-phase CO concentration in the disk regions, where the temperature is lower than ~25 K, can be significantly enhanced due to the combined effect of vertical and radial mixing. This finding has a potential implication for the current observational data on the DM Tau disk chemistry.

IMPACT OF GRAIN EVOLUTION ON THE CHEMICAL STRUCTURE OF PROTOPLANETARY DISKS

We study the impact of dust evolution in a protoplanetary disk (PPD) around a T Tauri star on the disks chemical composition. For the first time, we utilize a comprehensive model of dust evolution, which includes growth, fragmentation, and sedimentation. Specific attention is paid to the influence of grain evolution on the penetration of the UV field in the disk. A chemical model that includes a comprehensive set of gas-phase and grain-surface chemical reactions is used to simulate the chemical structure of the disk. The main effect of grain evolution on the disks chemical composition comes from sedimentation and, to a lesser degree, from reduction of the total grain-surface area. The net effect of grain growth is suppressed by the fragmentation process which maintains a population of small grains, dominating the total grain surface area. We consider three models of dust properties. In model GS, both growth and sedimentation are taken into account. In models A5 and A4, all grains are assumed to be the same size (10?5?cm and 10?4?cm, respectively) with a constant gas-to-dust mass ratio of 100. As in previous studies, the three-layer pattern (cold midplane, warm molecular layer, and hot atmosphere) in the disk-chemical structure is preserved in all models, but shifted closer to the midplane in models with increased grain size (GS and A4). Unlike other similar studies, we find that in models GS and A4, the column densities of most gas-phase species are enhanced by 1-3 orders of magnitude relative to those in a model with pristine dust (A5), while column densities of their surface counterparts are decreased. We show that column densities of certain species, such as C2H, HC2n+1N (n = 0-3), H2O, and some other molecules, as well as the C2H2/HCN abundance ratio, all of which are accessible with Herschel and ALMA, can be used as observational tracers of early stages of the grain evolution process in PPDs.

A NEW MODIFIED-RATE APPROACH FOR GAS-GRAIN CHEMISTRY: COMPARISON WITH A UNIFIED LARGE-SCALE MONTE CARLO SIMULATION

We compare the results of the unified Monte Carlo chemical model with the new modified-rate equation (MRE) method under a wide range of interstellar conditions, using a full gas-grain chemical network. In most of the explored parameter space, the new MRE method reproduces very well the results of the exact approach. Small disagreements between the methods may be remedied by the use of a more complete surface chemistry network, appropriate to the full range of temperatures employed here.

A Coupled Dynamical and Chemical Model of Starless Cores of Magnetized Molecular Clouds: I. Formulation and Initial Results

We develop a detailed chemical model for the starless cores of strongly magnetized molecular clouds, with the ambipolar diffusion-driven dynamic evolution of the clouds coupled to the chemistry through ion abundances. We concentrate on two representative model clouds in this initial study, one with magnetic fields and the other without. The model predictions on the peak values and spatial distributions of the column densities of CO, CCS, N2H+, and HCO+ are compared with those observationally inferred for the well-studied starless core L1544, which is thought to be on the verge of star formation. We find that the magnetic model, in which the cloud is magnetically supported for several million years before collapsing dynamically, provides a reasonable overall fit to the available data on L1544; the fit is significantly worse for the nonmagnetic model, in which the cloud collapses promptly. The observed large-peak column density for N2H+ and clear central depression for CCS favor the magnetically retarded collapse over the free-fall collapse. A relatively high abundance of CCS is found in the magnetic model, resulting most likely from an interplay of depletion and late-time hydrocarbon chemistry enhanced by CO depletion. These initial results lend some support to the standard picture of dense core formation in strongly magnetized clouds through ambipolar diffusion. They are at variance with those of Aikawa et al., who considered a set of models somewhat different from ours and preferred one in which the cloud collapses more or less freely for L1544.

Influence of uncertainties in the rate constants of chemical reactions on astrochemical modeling results

Abstract. With the chemical reaction rate database UMIST95 (Millar etal. 1997) we analyze how uncertainties in rate constants of gas-phase chemical reactions influence the modelling of the molecular abundances in the interstellar medium. Random variations are introduced into the rate constants to estimate the scatter in theoretical abundances. Calculations were performed for the dark and translucent molecular clouds where gas phase chemistry is adequate (Terzieva & Herbst 1998). Similar approach was used by Pineau des Forets & Roueff (2000) for the study of chemical bistability. All the species are divided into 6 sensitivity groups according to the value of the scatter in their model abundances computed with varied rate constants. It is shown that the distribution of species within these groups depends on the number of atoms in them and on the adopted physical conditions. The simple method is suggested which allows to single out reactions that are most important for the evolution of a given species.We analyze the influence of errors in the rate constants of gas-phase chemical reactions on the model abundances of molecules in the interstellar medium using the UMIST 95 chemical database. By randomly varying the rate constants within the limits of the errors given in UMIST 95, we have estimated the scatters in theoretical abundances for dark and diffuse molecular clouds. All of the species were divided into six groups by the scatter in their model equilibrium abundances when varying the rate constants of chemical reactions. The distribution of the species in groups depends on the physical conditions. The scatters in the abundances of simple species lie within 0.5–1 order of magnitude, but increase significantly as the number of atoms in the molecule increases. We suggest a simple method for identifying the reactions whose rate constants have the strongest effect on the abundance of a selected species. This method is based on an analysis of the correlations between the abundance of species and the reaction rate constants and allows the extent to which an improvement in the rate constant of a specific reaction reduces the uncertainty in the abundance of the species concerned to be directly estimated.

A Coupled Dynamical and Chemical Model of Starless Cores of Magnetized Molecular Clouds. II. Chemical Differentiation

Dense cores of molecular clouds are the basic units of isolated low-mass star formation. They have been observed extensively in various molecule lines and dust continuum with the aim of revealing their chemical and dynamical state. In a previous paper, we formulated a coupled dynamical and chemical model for data interpretation and carried out an initial investigation focusing on the effects of a magnetic field on the core dynamics and chemistry. Here, we update our chemical network and the treatment of magnetic field–matter coupling and explore the effects of changing various parameters, including the initial gas-phase metal abundances, adsorption energies, cosmic-ray ionization rate, sticking probability onto dust grains, cloud mass, as well as magnetic field strength. The model results are compared with the velocity field and column density distributions of CO, CS, CCS, NH3 ,N 2H + , and HCO + inferred observationally for the well-studied starless core L1544. We find that, in agreement with previous work, models with the so-called high metal abundances produce excessive CS and CCS by more than 2 orders of magnitude. Models of magnetized clouds with ‘‘ low metal ’’ and ‘‘ mixed metal ’’ (with a strong initial depletion of sulphur) abundances can fit the available data on L1544 reasonably well, with the low-metal model fitting somewhat better the chemical data (except for CS) and the mixed-metal model the velocity field. Taking into account of a newly recalculated rate for the neutral-neutral reaction S + CCH ! CCS + H increases the abundance of CCS substantially, leading to a better agreement with observation for the mixed-metal model. We considered two sets of adsorption energies, compiled respectively by Aikawa et al. and Hasegawa & Herbst. Our results favor the former over the latter. For our standard models, we adopted a cosmic ionization rate of 1:3 � 10 � 17 s � 1 and a sticking probability of 0.3. Increasing their values does not improve the model fits. Somewhat surprisingly, removing the magnetic support of the cloud leads to relatively modest changes in the peak column densities of the species except for CS. However, the spatial distributions of CS and CCS become more centrally concentrated than observed in L1544, and the infall speed is too large to be acceptable. This illustrates the need for both chemical and dynamical data to provide the tightest possible model constraints. A generic feature of our coupled dynamical and chemical model is that NH3 and, to a lesser extent, N2H + are concentrated in the slowly contracting, central plateau region of the growing core, whereas CS and CCS are most abundant in the lower density envelope surrounding the plateau, which has a faster infall motion. The chemical differentiation offers an exciting possibility of directly probing the velocity field of core evolution leading to star formation. Subject headings: ISM: clouds — ISM: individual (L1544) — ISM: magnetic fields — ISM: molecules — MHD — stars: formation

Molecular Emission Line Formation in Prestellar Cores

We investigate general aspects of molecular line formation under conditions typical of prestellar cores. Focusing on simple linear molecules, we study the formation of their rotational lines with radiative transfer simulations. We present a thermalization diagram to show the effects of collisions and radiation on the level excitation. We construct a detailed scheme (contribution chart) to illustrate the formation of emission-line profiles. This chart can be used as an efficient tool to identify which parts of the cloud contribute to a specific line profile. We show how molecular line characteristics for uniform model clouds depend on hydrogen density, molecular column density, and kinetic temperature. The results are presented in a two-dimensional plane to illustrate mutual effects of the physical factors. We also use a core model with a nonuniform density distribution and chemical stratification to study the effects of cloud contraction and rotation on spectral line maps. We discuss the main issues that should be taken into account when dealing with interpretation and simulation of observed molecular lines.

CB 17: Inferring the Dynamical History of a Prestellar Core with Chemodynamical Models

We present a detailed theoretical study of the isolated Bok globule CB 17 (L1389) based on spectral maps of CS, HCO+, C18O, C34S, and H13CO+ lines. The intensity of the external UV field, the probability for molecules to stick onto dust grains, the core age, the infall, and rotation velocity all significantly affect the molecular line spectra. We demonstrate that these parameters are well constrained when results of the modeling are compared to observations in multiple lines of sight through the core. We use a detailed chemical model to compute the time-dependent abundances in a number of locations within the core. Both static and dynamically evolving cloud configurations are considered. These abundances are then used to simulate the spectral maps. We developed a general criterion that allows us to quantify the difference between observed and simulated spectral maps. By minimizing this difference, we isolate the model that represents a good approximation to the core chemical and kinematic structure. The chemical age of the core is about 2 Myr, while the most probable effective sticking probability value is 0.3-0.5. The spatial distribution of intensities and self-absorption features of optically thick lines is indicative of attenuated UV radiation of the core. The line asymmetry pattern in CB 17 is reproduced by a combination of infall, rotation, and turbulent motions with velocities of ~0.05, ~0.1, and ~0.1 km s-1, respectively. These parameters correspond to energy ratios Erot/Egrav ≈ 0.03, Etherm/Egrav ≈ 0.8, and Eturb/Egrav ≈ 0.05 (the rotation parameters are determined for i = 90°). Based on the angular momentum value, we argue that the core is going to fragment, i.e., to form a binary (multiple) system.

Dust dynamics and evolution in expanding H ii regions. I. Radiative drift of neutral and charged grains

We consider dust drift under the influence of stellar radiation pressure during the pressure-driven expansion of an HII region using the chemo-dynamical model MARION. Dust size distribution is represented by four dust types: conventional polycyclic aromatic hydrocarbons (PAHs), very small grains (VSGs), big grains (BGs) and also intermediate-sized grains (ISGs), which are larger than VSGs and smaller than BGs. The dust is assumed to move at terminal velocity determined locally from the balance between the radiation pressure and gas drag. As Coulomb drag is an important contribution to the overall gas drag, we evaluate a grain charge evolution within the HII region for each dust type. BGs are effectively swept out of the HII region. The spatial distribution of ISGs within the HII region has a double peak structure, with a smaller inner peak and a higher outer peak. PAHs and VSGs are mostly coupled to the gas. The mean charge of PAHs is close to zero, so they can become neutral from time to time because of charge fluctuations. These periods of neutrality occur often enough to cause the removal of PAHs from the very interior of the HII region. For VSGs, the effect of charge fluctuations is less pronounced but still significant. We conclude that accounting for charge dispersion is necessary to describe the dynamics of small grains.

A mechanism for the formation of oxygen and iron bimodal radial distribution in the disc of our Galaxy

Recently, it has been proposed that there are two type Ia supernova progenitors: short-lived and long-lived. On the basis of this idea, we develop a theory of a unified mechanism for the formation of the bimodal radial distribution of iron and oxygen in the Galactic disc. The underlying cause for the formation of the fine structure of the radial abundance pattern is the influence of the spiral arms, specifically the combined effect of the corotation resonance and turbulent diffusion. From our modelling, we conclude that in order to explain the bimodal radial distributions simultaneously for oxygen and iron and to obtain approximately equal total iron output from different types of supernovae, the mean ejected iron mass per supernova event should be the same as quoted in the literature if the maximum mass of stars, which eject heavy elements, is 50 M� . For the upper mass limit of 70 M� , the production of iron by a type II supernova explosion should increase by about 1.5 times.