THE LOCATION OF WARM HC3N AND WARM HC5N GAS IN THE CIRCUMSTELLAR ENVELOPE OF IRC +10216

Duplicate observations of warm HCN (J = 31 to J = 37) and cool HC3N (J = 8 to J = 12) in IRC +10216, made with quite different beam sizes, are used to determine the diameters of the warm HC5N and cool HC3N gas emitting regions. The results show that the HC5N and HC3N gas excited to these levels is located inside regions with diameters of 30'' and 51'', respectively. An estimate of the accuracy of the technique is obtained by comparison with recent maps which shows that the paired antenna technique may be more accurate than generally assumed and that it may be a valuable alternative for measuring the diameter of the emission region when interferometer measurements are not available. Duplicate observations of HC3N (J = 24-23) obtained using the Kitt Peak and JCMT telescopes are used to estimate the diameter of warm HC3N. This gas is located inside a region of diameter 18.5''. A second high-J transition of HC3N (J = 28-27), observed with the JCMT, is used to determine T(ex) = 48 K for this warm gas compared to T(ex) = 12.7 for the cool HC3N gas. The excitation temperature for warm HC5N inside a diameter of 30'' is found to be T(ex) is similar to 35 K. The excitation temperatures and locations of the warm and cool gas in the stellar envelope indicate that for 7'' < r < 25'', the excitation temperature curves for both HC3N and HC5N fall off with a slope of approximately -1.8, which is much steeper than predicted for the kinetic temperature by Kwan and Linke and closer to an inverse square law decrease. This may indicate that for these species the excitation is almost entirely due to IR radiation. A search for very warm HC5N in IRC +10216 (J = 83 and J = 86 transitions) using the 22'' beam of the JCMT has resulted in an upper limit of T(A)* < 10 mK. It is now clear from the map of Lucas that there is no HC5N gas close enough to the star to be excited to these levels.