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

¹ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Drove Drive, Pasadena, CA 91109, USA
² Haverford College, 370 Lancaster Avenue, Haverford, PA 19041, USA
³ Department of Astronomy, University of Wisconsin-Madison, 475 N Charter St, Madison, WI 53706, USA
⁴ Department of Chemistry, University of Wisconsin-Madison, 1101 University Ave, Madison, WI 53706, USA
⁵ Exoplanets and Planetary Formation Group, School of Earth and Planetary Sciences, National Institute of Science Education and Research, Jatni 752050, Odisha, India
⁶ Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai 400094, India
⁷ Aerospace Corporation, 4745 Lee Road, Chantilly, VA 20151, USA
⁸ CSIRO Space & Astronomy/NASA Canberra Deep Space Communication Complex, PO Box 1035, Tuggeranng ACT 2901, Australia
⁹ Univ. Rennes, CNRS, IPR (Institut de Physique de Rennes) – UMR 6251, 35000 Rennes, France
¹⁰ Nantes Université, CNRS, CEISAM, UMR 6230, 44000 Nantes, France

^★ Corresponding author; dariusz.c.lis@jpl.nasa.gov

Received: 22 January 2025
Accepted: 25 February 2025

Abstract

The presence of over 300 molecules in the interstellar medium, the majority of which are 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 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 18–25 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 detected 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 used 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 two 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 three higher than the TMC-1 value.