Observation of C$_8$H$^-$toward IRC +10216

Five rotational lines of the C$_8$H$^-$ion were observed in the circumstellar envelope of IRC $+$10216 with the Nobeyama 45 m telescope. An improved value of the column density of the C$_8$H radical yielded a [C$_8$H$^-$]$/$[C$_8$H] ratio of 37%—nearly 4 times larger than the [C$_6$H$^-$]$/$[C$_6$H] ratio (8.6%) and nearly 1500 times the [C$_4$H$^-$]$/$[C$_4$H] ratio (0.024%), which may indicate more efficient formation of longer carbon chain anions. The excitation temperature of C$_8$H$^-$(16$\;\pm\;$2 K) derived here is somewhat lower than that of the two smaller anions C$_6$H$^-$(32 K) and C$_4$H$^-$(23 K) in IRC $+$10216.