An 18 -- 25 GHz spectroscopic survey of southern hemisphere dense cores

We extended the radio K-band spectroscopic survey for organics in southern hemisphere dense cores by observing seven sources using NASA's Deep Space Network 70 m antenna in Canberra, Australia, over the frequency range 18 to 25 GHz. Molecular column densities of NH_3, c-C_3H_2, HC_3N, HC_5N, CCS, C_3S, and c-C_3HD were derived for each source assuming local thermodynamic equilibrium. The resulting column density ratios were compared with predictions of a state-of-the art astrochemical model to constrain the C/O ratio and chemical age of each source. Most cores have similar C/O ratios of 0.5 - 0.7, much different from the best studied TMC-1 dense core, which is characterized by a high C/O ratio of ∼ 1.4. The chemical ages of the cores are also similar and fall between 0.6 and 5 Myr. The less dense cores tend to have the oldest chemical ages, as might be expected given that chemical timescales scale with density. Our results showcase the synergistic approach of combining radio observations using the DSS-43 antenna with state-of-the-art astrochemical models to study the chemical composition of southern hemisphere dense cores, enabling constraints to be placed on their C/O ratios and chemical ages, which remain largely unexplored.