SUBMILLIMETER MOLECULAR LINE OBSERVATIONS OF IRC- SEARCHES FOR MGH, SIH2, AND HCO+, AND DETECTION OF HOT HCN(10216 )

Sensitive searches have been carried out under good observing conditio ns for submillimeter-wavelength lines of MgH and SiH2 in the circumste llar envelope of IRC + 10216. Upper limits for the column densities ar e N((MgH)-Mg-24) less-than-or-equal-to 6 x 10(12) cm-2 and N(SiH2) les s-than-or-equal-to 3.5 x 10(11) cm-2. A deep search has also been made without success for HCO+. The upper limit for the column density is N (HCO+) less-than-or-equal-to 1.4 x 10(11) cm-2, which corresponds to X [HCO+] = 1.8 x 10(-11) for a mass-loss rate of 4 x 10(-5) M. yr-1. Thi s is an order of magnitude lower than the predicted theoretical abunda nce of X[HCO+] = 4.5 x 10(-10) (Glassgold et al. 1987). The J = 4-3 ro tational lines have been observed from four vibrational states of HCN, all with higher excitation energies than have previously been reporte d. These are the (03(3)0), (11(1d)0), (12(0)10), and (01(1d)1) states, with excitation energies of 3000, 4000, 5100, and 5800 K, respectivel y. The line profiles are narrower than that of the (HCN)-N-15 ground-s tate line which was observed simultaneously. This suggests the hot HCN emission is not cospatial with the lower excitation lines.