SIMBAD references

We present sensitive, high resolution maps of HC₃N emission in the circumstellar envelope of IRC+10216. We have mapped the J=16-15, J=18-17 and J=24-23 trasitions at 1mm and 2mm wavelength. Contrary to previous maps of lower transitions, they show centrally peaked intensity distributions. We resolve this discrepancy by modelling the vibrational and rotational excitation of HC₃N, showing that it largely results from radiative and collisional excitation in the envelope inner layers. The HC₃N abundance is changed by this Result in the sense that HC₃N is more abundant than previously thought. Our new results are compared with chemical models.