New Millimeter Heterodyne Observations of Titan: Vertical Distributions of Nitriles HCN, HC3N, CH3CN, and the Isotopic Ratio N/N in Its Atmosphere

High sensitivity observations were performed at 1.2- and 3-mm wavelengths with the IRAM 30-m telescope (Spain) between April 1996 and December 1999 to investigate the nitrile composition of Titan's stratosphere. A part of our dataset consists of high resolution spectra of HC¹⁴N taken at 88.6 GHz as well as spectra of HC¹⁵N recorded at 258.16 GHz. From a thorough analysis of both lines and with the help of appropriate radiative transfer calculations we show that the isotopic ratio ¹⁵N/¹⁴N is strongly enhanced compared to the terrestrial value. We propose that the range 3.9-4.5 should be considered as a basis for the enrichment factor. Five individual lines of HC₃N were measured at 39-kHz resolution using a frequency-switched technique. Several CH₃CN features were recorded at 78-kHz resolution in two transitions around 147.6 and 220.7 GHz. The high spectral resolution and the good signal-to-noise ratio affecting the spectra permit us to retrieve disk-averaged vertical profiles for HCN up to 450 km and for HC₃N and CH₃CN up to 500 km. Comparison of our inferred vertical profiles with relevant results of presently published photochemical models is presented. We show that the profiles of HCN and HC₃N predicted by various authors below 450-km altitude appear inconsistent with our new observations. We find that the three distributions present very different gradients of abundance below 200-km altitude down to the condensation levels around 80 km. In the upper stratosphere HC₃N and CH₃CN have approximately the same mixing ratio of about 4×10⁻⁸ at 450 km, at least one order of magnitude lower than that of HCN. In the same time, another nitrile HC₅N has been searched for by observing four transitions located between 109 and 221 GHz. As no spectral features could be detected after several hours of integration time, we propose an upper limit for the mixing ratio equal to 4×10⁻¹⁰ assuming a uniform distribution of this compound in the lower stratosphere.