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Discovery of the C8H radical

J. Cernicharo , M. Guélin
Observatorio Astronomico Nacional, apartado 1143, E--28800 Alcalá de Henares, Spain
IRAM, 300 Rue de la piscine, F--38406 S Martin d'Hères, France
Abstract: Searching for CH and CH in the carbon star envelope IRC+10216, we have identified 10 lines with harmonically related frequencies and with regularly increasing intensities (3 lines in the IRAM 3 mm spectral survey and 7 lines in the 7 mm Nobeyama survey of Kawaguchi et al., 1995). The rest frequencies can be closely fitted with the standard expression for the energies of a linear molecule with a rotational constant MHz, a distortion constant Hz, and half-integer quantum numbers J. The value of B is within 0.1% of that predicted by Pauzat et al. (1991) for the ground state of CH and the value of D typical of a linear acetylenic chain of CH's weight. Since the half-integer J numbers are also consistent with a state, we concluded that we had very probably discovered CH. The non detection of -doubling, as well as of a second series of lines corresponding to the other fine structure state, suggested that the CH ground state is inverted and that we were observing the state.

Our identification is now confirmed in the laboratory: M. Carthy et al. (1996) succeeded in detecting 30 millimeter lines arising from the new radical in an acetylene discharge and confirmed that their carrier is indeed CH.

The rotation temperature diagram of the CH lines observed in IRC+10216 is shown in Fig. 1. The fitted straight line yields a rotation temperature = 52 K and a beam-averaged column density cm. This latter is a factor of 30 lower than the column density of CH; it is similar to the column density predicted by Millar & Herbst (1991) in the case of a medium-large acetylene abundance ([HC]/[H]= 2.5 10)

We found no trace of CH in either the 30-m telescope, or the 45-m telescope data.

Figure 3: The rotational temperature diagram of the 10 lines assigned to CH. Typical errors on the line integrated intensities W are 20%.


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