1999A&A...341L..59N

We analyse the excitation of 114 rotational transitions of vinyl cyanide (acrylonitrile), C₂H₃CN, in its ν₁₁ and ν₁₅ vibrational states, together with 98 transitions in the ground state. The data were taken from the SEST line survey of Sgr B2(N) in the 1.3mm band. For the lines with energies above 200K a rotational temperature of 440_–90⁺¹⁹⁰K, a C₂H₃CN column density of 5.1_–2.5^+7.2x10¹⁸cm^–2, and a source size in the range 0.025-0.037pc are most consistent with observations. The resulting abundance of C₂H₃CN is about 6x10^–8. We argue that the high rotation temperature of the high-energy lines is most likely due to direct radiative excitation through far-infrared emission from hot dust. A Monte-Carlo analysis was applied to the radiative transfer of the lines with upper-state energies below 200K. A good fit to the intensities of the a-type lines is achieved by a model cloud with a fractional C₂H₃CN abundance of 4x10^–10. By increasing the abundance to 6x10^–9 in the outer portion of the cloud also the intensities of the b-type lines could be reasonably well fitted. This model is, however, inconsistent with the low intensities of low-energy a-type lines observed at longer wavelengths. Hence, we favour a constant C₂H₃CN abundance of 4x10^–10 in the part of the cloud where the kinetic temperature is below 180K, and regard the excitation of the intrinsically weak b-type lines as anomalous.