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The Protoplanetary Discussions conference—held in Edinburgh, UK, from 2016 March 7th–11th—included several open sessions led by participants. This paper reports on the discussions collectively concerned with the multi-physics modelling of protoplanetary discs, including the self-consistent calculation of gas and dust dynamics, radiative transfer, and chemistry. After a short introduction to each of these disciplines in isolation, we identify a series of burning questions and grand challenges associated with their continuing development and integration. We then discuss potential pathways towards solving these challenges, grouped by strategical, technical, and collaborative developments. This paper is not intended to be a review, but rather to motivate and direct future research and collaboration across typically distinct fields based on community-driven input, to encourage further progress in our understanding of circumstellar and protoplanetary discs.
The frequently made assumption that gas and dust temperatures are equal cannot be applied to the low density disks (104 cm−3 < ntot < 108 cm−3) around young A stars (“Vega-type” stars), because the collisional coupling is too weak. It turns out that OI fine structure lines and CO rotational lines are two very important cooling terms in the heating/cooling balance for the gas. Their level population numbers are far from LTE and depend strongly on the background radiation field, given by the thermal dust emission of the disk and the cosmic microwave background.
The chemistry of circumstellar disks around young (a few 10 Myr) solar-type stars is mainly driven by the strong UV radiation field of the central star. As a starting point for a detailed UV radiation field, the rocket and satellite observations of the solar chromosphere are used and scaled according to the time-dependent behaviour of stellar activity. The disk chemistry as well as dust and gas temperatures are then derived self-consistently from the model. The results of these calculations can be used for the identification of the most promising gas tracers as well as for the interpretation of present and future observations.
We started a comparison between different NLTE codes that calculate the statistical equilibrium in solar-type and late type stars (G - A type stars). Discrepancies between different authors analyzing the statistical equilibrium of the same element in the same atmosphere are often surprisingly large. Hence, this study was meant to nail down the origin of these differences. The preliminary results indicate that even if the atmosphere and the atomic input data is fixed, discrepancies of up to 40% in the outer atmospheric layers are still present; the main reason is the different treatment of the background opacity. Following up, we shortly discuss the completeness and accuracy of atomic data used in analyses of the kinetic equilibrium of atoms in the atmospheres of middle and late type stars.
The origin of water and other volatiles in protoplanetary disks can be either interstellar or due to chemical processing during the protoplanetary disk phase. Depending on the strength of the ionization field present during this stage, an active chemical evolution in the protoplanetary disk midplane can lead to formation of complex volatiles on timescales shorter than the disk dissipation timescale. For this reason, we investigate the effects of cosmic rays and the usually neglected cosmic ray induced UV ionization field in time dependent chemical models of protoplanetary disks. These results are benchmarked against our current knowledge of the chemical composition of cometary ices. We conclude that water and other, more complex volatiles can be preserved in the ice mantles of dust grains. This ice mantle growth can also have a significant impact on the dust opacity and hence on the temperature profile of the disk midplane. This effect will be observable in the near future with ALMA.
The small group of λ Bootis stars comprises late B to early F-type stars, with moderate to extreme (up to a factor 100) surface under-abundances of most Fe-peak elements and solar abundances of lighter elements (C, N, O, and S). The main mechanisms responsible for this phenomenon are atmospheric diffusion, meridional mixing, and accretion of material from their surroundings. Especially spectroscopic binary (SB) systems with λ Bootis-type components are very important to investigate the evolutionary status and accretion process in more details. Because also δ Scuti type pulsation was found for several members, it gives the opportunity to use the tools of astroseismology for further investigations. We present the results of our long term efforts of detailed abundance analysis, orbital parameter estimation and photometric time series analysis for five well investigated SB systems.
The structure and chemistry of protoplanetary disks depends strongly on the nature of the central star around which it has formed. The dust temperature is mainly set by the stellar luminosity, while the chemistry of the upper disk layers depends on the amount of intercepted UV and X-ray flux. We discuss here the differences in chemistry, thermal structure and line emission around Herbig Ae/Be, T Tauri stars and low-mass M dwarfs. Predictions will be made for future observations with SOFIA and Herschel.
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