Submillimeter molecular line observations of IRC +10216: Searches for MgH, SiH2, and HCO(+), and detection of hot HCN
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Abstract
Sensitive searches have been carried out under good observing conditions for submillimeter-wavelength lines of MgH and SiH2 in the circumstellar envelope of IRC +10216. Upper limits for the column densities are N(MgH-24) equal to or less than 6 x 1012/sq cm and N(SiH2) equal to or less than 3.5 x 1015. A deep search has also been made without success for HCO(+). The upper limit for the column density is N(HCO(+)) equal to or less than 1.4 x 1011/sq cm, which corresponds to X(HCO(+)) = 1.8 x 10-11 for a mass-loss rate of 4 x 10-5 solar mass/yr. This is an order of magnitude lower than the predicted theoretical abundance of X(HCO(+)) = 4.5 x 10-10 (Glassgold et al. 1987). The J = 4-3 rotational lines have been observed from four vibrational states of HCN, all with higher excitation energies than have previously been reported. These are the (0330), (111d0), (1200), and (011d1) states, with excitation energies of 3000, 4000, 5100, and 5800 K, respectively. The line profiles are narrower than that of the HC(15)N ground-state line which was observed simultaneously. This suggests the hot HCN emission is not cospatial with the lower excitation lines.
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