Journal
ASTROPHYSICAL JOURNAL
Volume 714, Issue 2, Pages 1511-1520Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/714/2/1511
Keywords
astrochemistry; molecular processes; planetary systems; protoplanetary disks; stars: formation; stars: individual (DM Tau, LkCa 15, MWC 480)
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Funding
- DeutscheForschungsGemeinschaft (DFG) [SPP 1385, SE 1962/1-1]
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We studied several representative circumstellar disks surrounding the Herbig Ae star MWC 480 and the T Tauri stars LkCa 15 and DM Tau at (sub-) millimeter wavelengths in lines of CCH. Our aim is to characterize photochemistry in the heavily UV-irradiated MWC 480 disk and compare the results to the disks around cooler T Tauri stars. We detected and mapped CCH in these disks with the IRAM Plateau de Bure Interferometer in the C and D configurations in the (1-0) and (2-1) transitions. Using an iterative minimization technique, the CCH column densities and excitation conditions are constrained. Very low excitation temperatures are derived for the T Tauri stars. These values are compared with the results of advanced chemical modeling, which is based on a steady-state flared disk structure with a vertical temperature gradient, and a gas-grain chemical network with surface reactions. Both model and observations suggest that CCH is a sensitive tracer of the X-ray and UV irradiation. The predicted radial dependency and source-to-source variations of CCH column densities qualitatively agree with the observed values, but the predicted column densities are too low by a factor of several. The chemical model fails to reproduce high concentrations of CCH in very cold disk midplane as derived from the observed low excitation condition for both the (1-0) and (2-1) transitions.
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