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Project Progress

We have worked on two of our sub-goals this year: (i) constraining the abundances of Polycyclic Aromatic Hydrocarbons (PAHs) in disks, and (ii) implementing gas-grain chemistry in disk models.

PAH abundances in disks:

We are modeling PAH emission from disks around intermediate mass Herbig stars and from disks around the low-mass T Tauri stars (which are young solar-type objects). Our models attempt to discriminate between two proposed mechanisms that explain the high rates of detection of PAH features in Herbig stars and the non-detection in T Tauri stars: (i) a depletion of PAHs, or, (ii) a lower excitation rate of PAHs due to low UV fields in low mass stars.

We make use of existing thermochemical disk models [1,2] that can compute the abundance of PAH cations, anions, and neutrals in a chemical framework, and are self-consistently determined from a dust collisional coagulation/fragmentation equilibrium model. The latter approach depends on the gas density and temperature that are iteratively determined; here PAHs are assumed to an extension of the small dust population down to molecular sizes (e.g., [3]). The output results (example shown in Figure 1) are fed into the Ames PAH Database [4], using a template-fitting procedure. The disk parameters (density, local UV field, extinction, and PAH species abundances at every spatial gridpoint) are scaled according to chosen templates that were constructed using an extensive array of Spitzer observations drawn from many astrophysical environments.

A composite disk spectrum is then produced for comparison with observations. The Monte Carlo Radiative transfer tool RADMC-3D (Dullemond and others, Heidelberg) is used to co-add the infrared emission from the dust with the PAH emission features from the database for each model run.

We find from our modeling that the non-detection of PAHs in T Tauri disks is consistent with depletion factors of 100 or more from the interstellar medium, in agreement with previous results. We also find that the strength of the UV field has a strong influence on the detection of PAH features – a drop in UV luminosity by 10 is found to cause a significant decrease in the excitation of PAH molecules, which then do not decay in the infrared and emit.

We have thus established a methodology for a detailed investigation of PAH emission from disks. We are now working on implementing X-ray chemistry and photo-destruction of PAHs into the chemical network, and on better integrating the RADMC-3D interface with the PAH emission from the database. We have repeated this exercise for the Herbig Ae disks as well, and are extending our parameter study to include different initial disk conditions (Gorti & Boersma, in preparation).

Dust rings and holes in disks and their chemical signatures:

Recent ALMA observations show the presence of many ring-like structures in emission from dust at the midplane of protoplanetary disks. These dust structures are often accompanied by intriguing chemical rings and holes/depletions observed in high resolution observations from cold-gas tracers like CS and rarer isotopes of CO.

We are investigating changes in disk chemistry caused by changes in dust properties in disks. The latter are likely caused by planet formation processes and if the changes in dust spatial distribution affects the chemistry as well, then these results could hold implications for the chemical composition of the forming planetesimals in the disk.

Using thermo-chemical disk models that can handle spatially distinct distributions of gas and dust, as well as spatially varying dust grain size distributions, we are modeling ALMA observations of some evolved disks believed to be undergoing planet formation. Gas-grain surface chemistry, hitherto absent in our models, is now being added to the chemical network using results from the KIDA database [5]. In the coming year, gas-grain chemistry using the Heidelberg code ALCHEMIC (developed by long-term collaborator, Dmitry Semenov) will be tested and the results compared with ALMA observations of disks to calibrate the chemical models and make inferences on the physical and chemical structure of disks that are believed to be undergoing planet formation.

References:
[1] Gorti, U., Hollenbach, D., Najita, J., & Pascucci, I. (2011) Astrophys. J., 735, 90.
[2] Gorti, U., Hollenbach, D., & Dullemond, C. P. (2015) Astrophys. J., 804, 29.
[3] Tielens, A. G. G. M. (2010) The Physics and Chemistry of the Interstellar Medium, Cambridge, UK: Cambridge University Press.
[4] Boersma, C., Bregman, J., & Allamandola, L. J. (2015) Astrophys. J., 806, 121.
[5] Wakelam. V., Loison, J. C., Herbst, E., et al. (2015) Astrophys. J. Suppl. Series, 217, p20.