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 deg 216 are reported. 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 percent of the ground state lines. Using new and published intensities for the ground-state lines, evidence of HC5N at two temperatures is found. 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 about 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 about 30 K, this is found to be too low to produce vibrational lines above the present limit of detectability. The cooler gas, which extends to a radius of about 25 arcsec is characterized by an excitation temperature of about 13 K. The density of the warm ground state HC5N gas is found to be about 3.7 x 10 exp 14/sq cm.
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Bell et al. (1992) studied this question.