Modeling and interpretation of line observations

Abstract

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. The power of line observationsThere 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. An example of the first is the HEXOS program, a Herschel/HIFI key program lead by E.A. Bergin, which besides other goals aims at a chemical inventory of the Orion and Sagittarius B2 star forming regions. They identified complex organic species such as methanol (CH 3 OH), methyl formate (CH 3 OCHO) and dimethyl-ether (CH 3 OCH 3 ) as well as hydrogen cyanide (HCN) and methyl cyanide (CH 3 CN), which are thought to be pre-cursors of amino acids (Crockett et al. 2014). An example for learning about physical conditions are the fine structure and molecular lines observed towards the dark cloud core Barnard 5 (Bensch 2006). 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). However, disks are complicated three-dimensional structures, in which different molecular lines originate from different regions (radial and vertical) Article available at