THE RADIAL-DISTRIBUTION OF HC3N MOLECULES IN IRC+10216

We present sensitive, high resolution maps of HC3N emission in the cir cumstellar envelope of IRC+10216. We have mapped the J = 16 - 15, J = 18 - 17 and J = 24 - 23 transitions at 1mm and 2mm wavelength. Contrar y to previous maps of lower transitions, they show centrally peaked in tensity distributions. We resolve this discrepancy by modelling the vi brational and rotational excitation of HC3N, showing that it largely r esults from radiative and collisional excitation in the envelope inner layers. The HC3N abundance is changed by this result in the sense tha t HC3N is more abundant than previously thought. Our new results are c ompared with chemical models.