MOLECULAR-EMISSION FROM THE DIFFUSE GAS AROUND ZETA-OPHIUCHI

We have mapped lambda 18cm OH and lambda 9cm CH emission from the diff use gas around zeta Oph, and find no trace of emission from the +4.5 k m s(-1) gas whose OH line was previously believed to represent pre-sho ck gas. The pattern of OH emission is strikingly dissimilar to those o f CO, HCO+ and CH in that the strong +1 km s(-1) component seen to the South is largely absent in OH. CH more closely resembles CO, peaking on either side of the star, and comparison of CO and CH shows that the CO line 'turns on' over a very narrow range of CH line strength. Alte rnatively, OW emission actually weakens as CO brightens in the +0.4, k m s(-1) line, but this may be only an excitation effect, and not the r esult of a declining OH abundance. We mapped lambda 3mm HCO+ emission over the inner portion of the gas distribution occulting zeta Oph, fin ding that its behaviour roughly parallels that of CO. We searched for emission from HCN, CS (J=2-1), CN, and C2H at the CO and HCO+ emission peak 30' South of the star. Of these, only HCN was found and it is qu ite weak compared to HCO+ (T-r approximate to 0.03 K vs. 0.09 K). Emi ssion from the other mm-wave species is absent in spectra with rms noi se Delta T-r approximate to 0.01 K; CS J=2-1 emission was sought also unsuccessfully toward the star. The column densities inferred from th ese intensities are uncertain owing to unknown physical conditions alo ng the line of sight. chiefly the kinetic temperature, but they are no t necessarily much larger than would be found toward the star. Relativ e abundances of the high-dipole moment species are better determined; N(HCN)/N(HCO+) approximate to 1, N(CS)IFN(HCO+) < 1.