Papers
The photodissociation and chemistry of CO isotopologues: applications to interstellar clouds and circumstellar disks
Co-authored with Ewine van Dishoeck and John Black. Published as a highlighted paper in Astronomy & Astrophysics 503, 323-343 (2009)
Aims. Photodissociation by UV light is an important destruction mechanism for carbon monoxide (CO) in many astrophysical environments, ranging from interstellar clouds to protoplanetary disks. The aim of this work is to gain a better understanding of the depth dependence and isotope-selective nature of this process.
Methods. We present a photodissociation model based on recent spectroscopic data from the literature, which allows us to compute depth-dependent and isotope-selective photodissociation rates at higher accuracy than in previous work. The model includes self-shielding, mutual shielding and shielding by atomic and molecular hydrogen, and it is the first such model to include the rare isotopologues C17O and 13C17O. We couple it to a simple chemical network to analyse CO abundances in diffuse and translucent clouds, photon-dominated regions, and circumstellar disks.
Results. The photodissociation rate in the unattenuated interstellar radiation field is 2.6e-10 s^-1, 30% higher than currently adopted values. Increasing the excitation temperature or the Doppler width can reduce the photodissociation rates and the isotopic selectivity by as much as a factor of three for temperatures above 100 K. The model reproduces column densities observed towards diffuse clouds and PDRs, and it offers an explanation for both the enhanced and the reduced N(12CO)/N(13CO) ratios seen in diffuse clouds. The photodissociation of C17O and 13C17O shows almost exactly the same depth dependence as that of C18O and 13C18O, respectively, so 17O and 18O are equally fractionated with respect to 16O. This supports the recent hypothesis that CO photodissociation in the solar nebula is responsible for the anomalous 17O and 18O abundances in meteorites. Grain growth in circumstellar disks can enhance the N(12CO)/N(C17O) and N(12CO)/N(C18O) ratios by a factor of ten relative to the initial isotopic abundances.
Full text is available at http://www.strw.leidenuniv.nl/~ruvisser/files/visser2009b.pdf
Like (1)The chemical history of molecules in circumstellar disks. I. Ices
Co-authored with Ewine van Dishoeck, Steve Doty and Kees Dullemond. Published in Astronomy & Astrophysics 495, 881-897 (2009)
Context. Many chemical changes occur during the collapse of a molecular cloud to form a low-mass star and the surrounding disk. One-dimensional models have been used so far to analyse these chemical processes, but they cannot properly describe the incorporation of material into disks.
Aims. The goal of this work is to understand how material changes chemically as it is transported from the cloud to the star and the disk. Of special interest is the chemical history of the material in the disk at the end of the collapse.
Methods. A two-dimensional, semi-analytical model is presented that, for the first time, follows the chemical evolution from the pre-stellar core to the protostar and circumstellar disk. The model computes infall trajectories from any point in the cloud and tracks the radial and vertical motion of material in the viscously evolving disk. It includes a full time-dependent radiative transfer treatment of the dust temperature, which controls much of the chemistry. A small parameter grid is explored to understand the effects of the sound speed and the mass and rotation of the cloud. The freeze-out and evaporation of carbon monoxide (CO) and water (H2O), as well as the potential for forming complex organic molecules in ices, are considered as important first steps towards illustrating the full chemistry.
Results. Both species freeze out towards the centre before the collapse begins. Pure CO ice evaporates during the infall phase and re-adsorbs in those parts of the disk that cool below the CO desorption temperature of ~18 K. Water remains solid almost everywhere during the infall and disk formation phases and evaporates within ~10 AU of the star. Mixed CO-H2O ices are important in keeping some solid CO above 18 K and in explaining the presence of CO in comets. Material that ends up in the planet- and comet-forming zones of the disk (~5–30 AU from the star) is predicted to spend enough time in a warm zone (several 104 yr at a dust temperature of 20–40 K) during the collapse to form first-generation complex organic species on the grains. The dynamical timescales in the hot inner envelope (hot core or hot corino) are too short for abundant formation of second-generation molecules by high-temperature gas-phase chemistry.
Full text is available at http://www.strw.leidenuniv.nl/~ruvisser/files/visser2009a.pdf
PAH chemistry and IR emission from circumstellar disks
Co-authored with Vincent Geers, Kees Dullemond, Jean-Charles Augereau, Klaus Pontoppidan and Ewine van Dishoeck. Published in Astronomy & Astrophysics 466, 229-241 (2007)
Aims. The chemistry of, and infrared (IR) emission from, polycyclic aromatic hydrocarbons (PAHs) in disks around Herbig Ae/Be and T Tauri stars are investigated. PAHs can exist in different charge states and they can bear different numbers of hydrogen atoms. The equilibrium (steady-state) distribution over all possible charge/hydrogenation states depends on the size and shape of the PAHs and on the physical properties of the star and surrounding disk.
Methods. A chemistry model is created to calculate the equilibrium charge/hydrogenation distribution. Destruction of PAHs by ultraviolet (UV) photons, possibly in multi-photon absorption events, is taken into account. The chemistry model is coupled to a radiative transfer code to provide the physical parameters and to combine the PAH emission with the spectral energy distribution (SED) from the star+disk system.
Results. Normally hydrogenated PAHs in Herbig Ae/Be disks account for most of the observed PAH emission, with neutral and positively ionized species contributing in roughly equal amounts. Close to the midplane, the PAHs are more strongly hydrogenated and negatively ionized, but these species do not contribute to the overall emission because of the low UV/optical flux deep inside the disk. PAHs of 50 carbon atoms are destroyed out to 100 AU in the disk’s surface layer, and the resulting spatial extent of the emission does not agree well with observations. Rather, PAHs of about 100 carbon atoms or more are predicted to cause most of the observed emission. The emission is extended on a scale similar to that of the size of the disk, with the short-wavelength features less extended than the long-wavelength features. For similar wavelengths, the continuum emission is less extended than the PAH emission. Furthermore, the emission from T Tauri disks is much weaker and concentrated more towards the central star than that from Herbig Ae/Be disks. Positively ionized PAHs are predicted to be largely absent in T Tauri disks because of the weaker radiation field.
Full text is available at http://www.strw.leidenuniv.nl/~ruvisser/files/visser2007a.pdf
Sub-Keplerian accretion onto circumstellar disks
Co-authored with Kees Dullemond. Submitted to Astronomy & Astrophysics.
Context. Models of the formation, evolution and photoevaporation of circumstellar disks are an essential ingredient in many theories of the formation of planetary systems. The ratio of disk mass over stellar mass in the circumstellar phase of a disk is for a large part determined by the angular momentum of the original cloud core from which the system was formed. While full 3D or 2D axisymmetric hydrodynamical models of accretion onto the disk will automatically treat all aspects of angular momentum, this is not so trivial for 1D and semi-2D viscous disk models.
Aims. Since 1D and semi-2D disk models are still very useful for long-term evolutionary modelling of disks with relatively little numerical effort, we wish to investigate how the 2D nature of accretion affects the formation and evolution of the disk in such models. A proper treatment of this problem also requires a correction for the sub-Keplerian velocity at which accretion takes place.
Methods. We develop an update of our semi-2D time-dependent disk formation and evolution model to properly treat the effects of sub-Keplerian accretion. The model also accounts for the effects of the vertical extent of the disk on the infall trajectories.
Results. The disks produced with the new method are smaller than those obtained previously, but their mass is mostly unchanged. The new disks are a few degrees warmer in the outer parts, so they contain less solid CO. Otherwise, the results for ices are unaffected. The 2D treatment of the accretion results in material accreting at larger radii, so a smaller fraction comes close enough to the star for amorphous silicates to be thermally annealed into crystalline form. The lower crystalline abundances thus predicted correspond more closely to the abundances found observationally. We argue that thermal annealing followed by radial mixing must be responsible for at least part of the observed crystalline material.
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