Chemistry in evolving protoplanetary disks

Planets form in disks of gas and dust around young stars. Since the gas makes up 99 % of the disk mass, it is critical for our understanding of planet formation to gain direct information from the gas, independently of what can be learned from dust emission. In this thesis, calculations are presented of the chemistry and gas temperature in disks, and the resulting atomic and molecular emission lines are investigated. The main focus of the thesis is on the effects of dust settling on gas-phase emission lines of disks around T-Tauri and Herbig Ae stars. It is found that dust settling has little effect on the overall chemistry and molecular lines; the main effect is a decrease in the gas temperature, which is reflected in atomic fine-structure lines and especially in the [O I] lines. The chemistry, and especially the CO abundance and HCN/CN ratio, is affected more by the total gas mass than by the dust gas ratio in a disk. The models were also applied to the disk around HD 141569A, which is in a transitional stage between a gas-rich Herbig Ae disk and a debris disk. Using chemical models to fit the observed CO rotational lines it is concluded that gas and small dust particles have an approximately interstellar mass ratio, and that gas is still present in the inner hole in the dust distributionUBL - phd migration 201

Photoprocesses in protoplanetary disks

Circumstellar disks are exposed to intense ultraviolet radiation from the young star. In the inner disks, the UV radiation can be enhanced by more than seven orders of magnitude compared with the average interstellar field, resulting in a physical and chemical structure that resembles that of a dense photon-dominated region (PDR). This intense UV field affects the chemistry, the vertical structure of the disk, and the gas temperature, especially in the surface layers of the disk. The parameters which make disks different from traditional PDRs are discussed, including the shape of the UV radiation field, grain growth, the absence of PAHs, the gas/dust ratio and the presence of inner holes. New photorates for selected species, including simple ions, are presented. Also, a summary of available cross sections at Lyman alpha 1216 A is made. Rates are computed for radiation fields with color temperatures ranging from 4000 to 30,000 K, and can be applied to a wide variety of astrophysical regions including exo-planetary atmospheres. The importance of photoprocesses is illustrated for a number of representative disk models, including disk models with grain growth and settling.Comment: A website with the final published version and all photodissociation cross sections and rates can be found at http://www.strw.leidenuniv.nl/~ewine/phot

A photon dominated region code comparison study

International audienceAims:We present a comparison between independent computer codes, modeling the physics and chemistry of interstellar photon dominated regions (PDRs). Our goal was to understand the mutual differences in the PDR codes and their effects on the physical and chemical structure of the model clouds, and to converge the output of different codes to a common solution. Methods: A number of benchmark models have been created, covering low and high gas densities n = 10^3,105.5 cm-3 and far ultraviolet intensities chi = 10, 105 in units of the Draine field (FUV: 6 Results: We investigated the impact of PDR geometry and agreed on the comparison of results from spherical and plane-parallel PDR models. We identified a number of key processes governing the chemical network which have been treated differently in the various codes such as the effect of PAHs on the electron density or the temperature dependence of the dissociation of CO by cosmic ray induced secondary photons, and defined a proper common treatment. We established a comprehensive set of reference models for ongoing and future PDR model bench-marking and were able to increase the agreement in model predictions for all benchmark models significantly. Nevertheless, the remaining spread in the computed observables such as the atomic fine-structure line intensities serves as a warning that there is still a considerable uncertainty when interpreting astronomical data with our models