Abstract
Models for the interpretation of line observations from protoplanetary disks are summarized. The spectrum ranges from 1D LTE slab models to 2D thermo-chemical radiative transfer models and their use depends largely on the type/nature of observational data that is analyzed. I discuss the various types of observational data and their interpretation in the context of disk physical and chemical properties. The most simple spatially and spectral unresolved data are line fluxes, which can be interpreted using so-called Boltzmann diagrams. The interpretation is often tricky due to optical depth and non-LTE effects and requires care. Line profiles contain kinematic information and thus indirectly the spatial origin of the emission. Using series of line profiles, we can for example deduce radial temperature gradients in disks (CO pure rotational ladder). Spectro-astrometry of e.g. CO ro-vibrational line profiles probes the disk structure in the 1–30 AU region, where planet formation through core accretion should be most efficient. Spatially and spectrally resolved line images from (sub)mm interferometers are the richest datasets we have to date and they enable us to unravel exciting details of the radial and vertical disk structure such as winds and asymmetries.
Highlights
Models for the interpretation of line observations from protoplanetary disks are summarized
2D slab models have been introduced, where the density and temperature are parametrized as radial power laws (Fig. 1, right)
2.3 Disk models with parametrized chemistry Many flavors of this model category exists and we describe in the following only three examples
Summary
There are two main goals in observing line emission: (1) to obtain a chemical inventory in space and (2) to learn about the physical conditions in astrophysical environments. Comparing the line ratio [C i]/CO J=3-2 with 1D spherical PDR models allows to estimate the total mass and density of the region close to the central core Molecular spectra with their wealth of lines tracing different excitation conditions (rotational, vibrational, electronic) are excellent tracers of gas densities, temperatures and kinematics over a wide variety of environments in disks (see chapter by Dionatos 2015). Local Thermodynamical Equilibrium (LTE), for example radiative depopulation, radiative pumping, fluorescence and resonance scattering, can complicate the analysis of line observations from disks. Due to this complexity, we often rely on complex models for the interpretation of line fluxes and/or profiles.
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