Detection of vibrationally excited HC<sub>7</sub>N and HC<sub>9</sub>N in IRC +10216
J. R. Pardo, Celina Bermúdez, C. Cabezas, M. Agúndez, J. D. Gallego, J. P. Fonfría, L. Velilla-Prieto, G. Quintana-Lacaci, B. Tercero, M. Guélin, J. Cernicharo
Abstract
Observations of IRC +10216 with the Yebes 40 m 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 , that is to say, 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, and q = 0.1463 ± 0.0001 MHz. The rotational constant is very close to that of the ground state of HC 9 N. Our 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, were detected for the first time in IRC +10216. Several doublets of HC 5 N in its ν 11 = 1 state were 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 abundance ratios 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.