An 18 – 25 GHz spectroscopic survey of dense cores in the Chamaeleon I molecular cloud

The presence of over 300 molecules in the interstellar medium, the majority of them organic compounds, raises the question of the extent to which protostellar chemistry is responsible for organic molecules in solar system bodies (e.g., comets, asteroids, planets). The majority of systematic surveys for organic molecules in cold cores have focused on the TMC-1 core in the Taurus complex, along with lesser surveys of other protostellar cores in the northern hemisphere facilitated by the presence of several telescopes available for surveys below 45 GHz, where most organic molecules have relatively strong emission under conditions in cold cores. A few southern hemisphere sources have been surveyed at wavelengths between 7 and 1 mm. Here we extend the survey for organics in the southern hemisphere to 1.3 cm by observing two cores in the Chamaeleon complex using NASA’s Deep Space Network 70-m antenna in Canberra, Australia, over the frequency range of 18 to 25 GHz. In the Chamaeleon complex we surveyed the class 0 protostar Cha-MMS1 and the prestellar core Cha-C2, which represent two stages in the evolution of dense cores. We detect several molecules including HC3N, HC5N, C4H, CCS, C3S, NH3, and c-C3H2. A longer cyanopolyyne, HC7N, is detected with high confidence via spectral stacking analysis. While molecular column densities in the two Chamaeleon cores are typically an order of magnitude lower compared to the cynaopolyyne peak in TMC-1, the molecular abundance ratios are in general agreement with the TMC-1 values. The two exceptions are c-C3H2, which is enhanced by a factor of ∼25 with respect to cyanopolyynes in the Chamaeleon cores, and ammonia, which is enhanced by a factor of ∼125. The deuterated species c-C3HD is detected in both cores, with a high D/H ratio of ∼0.23 in c-C3H2. A rare isotopologue of ammonia, 15NH3, is also detected in Cha-MMS1 suggesting a high 14N/15N ratio of ∼690 in ammonia. However, this ratio may be artificially enhanced due to the high optical depth of the 14NH3 (1,1) line, which increases the effective source size. We use the detections of ammonia, cyanopolyynes, and far-infrared dust continuum to characterize the density and temperature in the Chamaeleon cores and calculate the molecular column densities and their relative ratios. The ring molecule benzonitrile, a tracer for the non-polar molecule benzene, is not detected in either Chamaeleon core. The 3σ upper limits for the benzonitrile column density achieved are a factor of 2 higher than the value derived for TMC-1 and the corresponding upper limits for the relative abundance of benzonitrile with respect to HC5N are a factor of 3 higher than the TMC-1 value.