Detection of vibrationally excited HC 7 N and HC 9 N 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 HC ₉ N. Ab initio calculations show an excellent agreement between these parameters and those predicted for the lowest energy vibrationally excited state, ν ₁₉ =1, of HC ₉ N. This is the first detection, and complete characterization in space, of vibrationally excited HC ₉ N. An energy of 41.5 cm ^-1 is estimated for the ν ₁₉ state. In addition, 17 doublets of HC ₇ N 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 HC ₅ N 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 HC ₅ N, HC ₇ N, and HC ₉ N, 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(HC ₅ N)/N(HC ₇ N) and N(HC ₇ N)/N(HC ₉ N) are 2.4 and 7.7 respectively.