Discovery of HC₉N ν₁9 overtones in IRC+10216

The sub-milliKelvin sensitivity achieved in the Q-band (31-50 GHz) line survey of IRC, +10216 obtained with the Yebes 40m telescope reveals the presence of several new vibrational HC₉N overtones 19 beyond the already reported ν₁9=1. The newly found series of rotational lines appears in seven groups with rotational constants (B) between 291. 252 and 293. 465 MHz, with integer quantum numbers from J_ up=54 to 81 and upper-level excitation energies up to ∼600 K, well above what is typical of lines found in the outer part of this circumstellar envelope. All evidence, especially the spectral splitting of the ell-doubling components and the spacing between B values, unambiguously points toward rotational lines within ν₁9=2 to 8 that appear surprisingly strong, possibly due to very efficient infrared pumping and slow decay via the ν₁9 mode. The detection of ν₁9=5, 6, 7, and 8 has been possible thanks to collapsed ell-doubling components to a single or only two spectral features per rotational transition. To obtain a correct determination of the abundance of HC₉N in IRC+10216 one has to take into account that about ∼70% is estimated to be in the vibrationally excited ν₁9 states.