DPS 35th Meeting, 1-6 September 2003
Session 21. Titan III
Poster, Highlighted on, Wednesday, September 3, 2003, 3:00-5:30pm, Sierra Ballroom I-II

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[21.03] Nitrile Compounds Observed on Titan

A. Marten (LESIA, Obs. Paris), R. Moreno (IRAM)

Heterodyne millimeter observations were performed on Titan with the IRAM Plateau-de-Bure Interferometer array in February-March 2003 near greatest eastern elongations. The most extended configuration of the array was used. The Titan's angular diameter, corresponding to the solid body value, was 0.8 arc sec. However, a larger diameter of about 1 arc sec needs to be considered in the analysis of emitted flux measurements.

Two dual frequency receivers were utilized at 3- and 1.2-mm wavelengths, giving access to the 82-116 and 210-245 GHz spectral ranges. Therefore, to optimize our mapping program, observations were carried out in the HCN(1-0), HC3N(12-11), CH3CN(12-11), HC3N(25-24) and CO(2-1) transitions, near 88.6, 109.2, 220.7, 227.4 and 230.5 GHz, respectively.

An angular resolution of 0.6 arc sec was obtained at shorter wavelengths, yielding disk-resolved spectra of Titan. Most of the HCN(1-0) and HC3N(12-11) data correspond to full-disk measurements since the equivalent synthesized beam of the array was larger than 1.3 arc sec at longer wavelengths.

Narrow isolated lines of HC3N and CH3CN as well as the three components of HCN(1-0) were analyzed at a very high spectral resolution of 40 kHz. Lower values of 160 kHz and 2.5 MHz were chosen for recording broad-band spectra of HCN, CH3CN and CO.

Disk-averaged spectra taken at the same frequencies with the IRAM single-dish 30-m telescope (Marten et al., 2002, Icarus, 158, 532) have been used for comparison. The vertical distributions of nitrile abundances inferred from those data served as a preliminary basis for radiative transfer computations considering a spherical geometry for Titan's atmosphere and an elliptical gaussian synthesized beam. Numerical calculations of HCN and CO spectra are found in remarkable agreement with the interferometric data. Significant differences exist for HC3N in the northern latitudes and CH3CN in midlatitude regions.

Measured maps are presented at all observing frequencies along with representative cases for well-fit synthetic computations, with an emphasis on the nitrile composition variations across Titan's disk.

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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.