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J.E. Kessler, G.A. Blake (Caltech), C. Qi (Harvard-Smithsonian CfA), Joanna Brown (Caltech)
Models indicate that molecular line emission from CN, HCN and HNC should serve as good tracers of the temperature, and UV and X-ray radiation fields in the surface layers of circumstellar disks. This is consistent with previous interferometric observations, obtained with the Owen's Valley Radio Observatory (OVRO) Milimeter Array, of T Tauri (LkCa 15 and GM Aur) and Herbig Ae disks (HD 163296 and MWC 480) which indicate that the CN/HCN and HCO+/CO ratios increase with increasing UV field, where the UV field can be enhanced either via dust settling or due to a higher luminosity of the central star. Here we will presenton an expanded OVRO study of CN, HCN and HNC in several disks, with a range of X-ray and optical luminosities and dust properties, as described by LFIR/Lbol and stellar AV, in order to examine whether the properties of the disk (dust settling) or the central star (radiation field) exert a stronger influence on the chemistry.
Additionally, observations of the disk around LkCa 15 indicated the existence of large radial gradients in the density of CN and HCN relative to that of CO and HCO+. In order to more fully understand the cause of these radial gradients and to distinguish the phenomenon from instrumental effects, we combine model observations created by non-LTE radiative transfer calculations (Hogerheijde & van der Tak 2001) with the appropriate uv-sampling for our OVRO observations and a variety of density gradients in order to simulate the observations. We find that the emission can be modeled by an anulus of (CN or HCN) gas with inner radius 200AU and outer radius of 400AU. We have also constructed less extreme gradients and will also discuss the results of these models.
This work was supported by the National Science Foundation under grant No.9981546 and the NASA Graduate Student Researchers Program, NGT5-50231.
The author(s) of this abstract have provided an email address for comments about the abstract: kessler@caltech.edu
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.