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.
ISM: abundances - ISM: molecules - ISM: individual objects: Sgr B2 - radio lines: ISM