Molecular Lines in Bok Globules and Around Herbig Ae/be Stars

This paper is intended as part of a more extensive molecular line survey in star forming regions along the evolutionary track of a collapsing cloud toward a young stellar object. We have studied a sample of seven small dark clouds (Bok globules) and eight Herbig Ae/Be stars in the J=1->0 transition of HCO${}^{+}$, H${}^{13}$CO${}^{+}$, HCN and H${}^{13}$CN. The choice of these molecules is determined by the simple chemistry and the predicted high abundance of the reactants leading to their formation. The isotopically substituted species (isotopomers), H${}^{13}$CO${}^{+}$ and H${}^{13}$CN, were observed in order to determine, whenever possible, the optical thickness of the main species. The most abundant isotopomers were found in almost all the sources (detection rate 70-90%). Those sources which exhibited the strongest signals were also searched for the ${}^{13}$C isotopomers. H${}^{13}$CO${}^{+}$ was found in one dark cloud and around three Herbig Ae/Be stars, while H${}^{13}$CN around only one star. The column densities for each species and the physical conditions of the objects were derived whenever the observational data allowed it.