A SENSITIVE SEARCH FOR ROTATIONAL LINES OF VIBRATIONALLY EXCITED HC5N IN IRC +10-DEGREES-216

We report the results of a search for nu-11 and nu-10 lines of HC5N (both the J = 9-8 and J = 11-10 transitions) in the molecular envelope of the carbon star IRC + 10-degrees-216. The J = 11-10 observations are the first obtained with the new NRAO 30 GHz receiver. No vibrational lines were detected to a level of 2% of the ground state lines. Using new and published intensities for the ground-state lines we find evidence of HC5N at two temperatures. By assuming that the warmer HC5N gas is contained in the inner half of a thick shell similar to HC5N, an excitation analysis reveals that its mean excitation temperature is approximately 25 K. Although it is conceivable that some of the HC3N gas at the inner edge of the shell may have an excitation temperature as high as approximately 30 K, we find this to be too low to produce vibrational lines above our limit of detectability. The cooler gas, which extends to a radius of approximately 25" is characterized by an excitation temperature of approximately 13 K. The density of the warm ground state HC5N gas is found to be N approximately 3.7 x 10(14) cm-2.