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

Abstract

We extend the survey for organics in the southern hemisphere 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. 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 HC${}_3$N, HC${}_5$N, C${}_4$H, CCS, C${}_3$S, NH${}_3$, and c-C${}_3$H${}_2$. A longer cyanopolyyne, HC${}_7$N, 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-C${}_3$H${}_2$, which is enhanced by a factor of \about 25 with respect to cyanopolyynes in the Chamaeleon cores, and ammonia, which is enhanced by a factor of ~ 125. The deuterated species c-C${}_3$HD is detected in both cores, with a high D/H ratio of ~0.23 in c-C${}_3$H${}_2$. A rare isotopologue of ammonia, ${}^{15}$NH${}_3$, is also detected in Cha-MMS1 suggesting a high ${}^{14}$N/${}^{15}$N ratio of ~ 690 in ammonia. However, this ratio may be artificially enhanced due to the high optical depth of the ${}^{14}$NH${}_3$ (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 is not detected in either Chamaeleon core. The $3\sigma$ upper limits for its column density are a factor of 2 higher than the value derived for TMC-1 and the upper limits for its relative abundance with respect to HC${}_5$N are a factor of 3 higher than the TMC-1 value.

arXiv commentAccepted for publication in Astronomy & Astrophysics, 15 pages, 10 figures

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