Detection of vibrationally excited HC₇N and HC₉N in IRC+10216 ^⋆

Observations of IRC +10216 with the Yebes 40m telescope between 31 and 50 GHz have revealed more than 150 unidentified lines. Some of them can be grouped into a new series of 26 doublets, harmonically related with integer quantum numbers ranging from J _up=54 to 80. The separation of the doublets increases systematically with J, i.e., as expected for a linear species in one of its bending modes. The rotational parameters resulting from the fit to these data are B = 290.8844 ± 0.0004 MHz, D = 0.88 ± 0.04 Hz, q = 0.1463 ± 0.0001 MHz. The rotational constant is very close to that of the ground state of HC₉N. Ab initio calculations show an excellent agreement between these parameters and those predicted for the lowest energy vibrationally excited state, ν ₁₉=1, of HC₉N. This is the first detection, and complete characterization in space, of vibrationally excited HC₉N. An energy of 41.5 cm^-1 is estimated for the ν ₁₉ state. In addition, 17 doublets of HC₇N in the ν ₁₅=1 state, for which laboratory spectroscopy is available, have been detected for the first time in IRC+10216. Several doublets of HC₅N in its ν ₁₁=1 state have been also observed. The column density ratio between the ground and the lowest excited vibrational states are ≈127, 9.5, and 1.5 for HC₅N, HC₇N, and HC₉N, respectively. We find that these lowest-lying vibrational states are most probably populated via infrared pumping to vibrationally excited states lying at ≈600 cm^-1. The lowest vibrationally excited states thus need to be taken into account to precisely determine absolute abundances and abundanceratios for long carbon chains. The abundance ratios N(HC₅N)/N(HC₇N) and N(HC₇N)/N(HC₉N) are 2.4 and 7.7 respectively.