Infrared pumping for HC5N in the circumstellar envelope of IRC + 10216

Infrared excitation is investigated for the rotational transitions of HC5N with special emphasis on the circumstellar envelope of IRC + 10216. Infrared radiation from the dust shell surrounding the carbon star elevates the molecule to the higher rotational levels through the excitation of transitions associated with the bending modes (Delta J = 2 selection rule) as well as the collision with hydrogen molecules. The vibrational frequencies and infrared absorption intensities of HC5N are estimated from ab initio molecular orbital calculations. The modeling of the circumstellar envelope shows that the rate for infrared pumping through the nu(7) and nu(8) bands exceeds the collisional excitation rate by a factor of 3. From the calculated emission-line intensities of high-J and low-J transitions, the abundance of HC5N is found to be 2.5 x 10 to the -7th per hydrogen in the circumstellar envelope of IRC + 10216.