Submillimeter Molecular Line Observations of IRC +10216: Searches for MgH, SiH 2, and HCO +, and Detection of Hot HCN

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), (11^1d0), (12⁰0), and (01^1d1) 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.