Detection of C₈H^– and Comparison with C₈H toward IRC +10 216

We report the detection of new transitions of octatetraynyl (C₈H) toward the circumstellar envelope IRC +10 216 using data taken with the 100 m Green Bank Telescope (GBT). In addition, we report five features from the Ku, K, and Q bands that have been identified as transitions of the octatetraynyl anion (C₈H^-). From a rotational temperature diagram and an assumed source size of 30'', we find a total C₈H column density of 8(3) × 10¹² cm^-2 and a rotational temperature of ~13 K. From the five detected transitions of C₈H^-, we find a total C₈H^- column density of ~2.1 × 10¹² cm^-2 consistent with a rotational temperature of ~34 K for a total C₈H/C₈H^- column density ratio of ~3.8. This observed C₈H/C₈H^- column density ratio is similar to the theoretical prediction of 3.6, while the observed column densities were lower than that predicted by a factor of ~30. This prompted us to reinvestigate the initial conditions of the circumstellar envelope (CSE) model. The new model results are presented, and they more closely match the C₈H and C₈H^- abundances observed with the GBT. Finally, we use the new CSE model results to predict the abundance of decapentaynyl (C₁₀H), and we compare them with the measured upper limit found from the GBT observations.