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

¹ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Drove Drive, Pasadena, CA 91109, USA
² Exoplanets and Planetary Formation Group, School of Earth and Planetary Sciences, National Institute of Science Education and Research, Jatni 752050, Odisha, India
³ Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai 400094, India
⁴ Department of Astronomy, University of Wisconsin-Madison, 475 N Charter St, Madison, WI 53706, USA
⁵ Department of Chemistry, University of Wisconsin-Madison, 1101 University Ave, Madison, WI 53706, USA
⁶ CSIRO Space & Astronomy/NASA Canberra Deep Space Communication Complex, PO Box 1035, Tuggeranng ACT 2901, Australia

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 9 March 2026
Accepted: 6 April 2026

Abstract

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 18–25 GHz. Molecular column densities of NH₃, c-C₃H₂, HC₃N, HC₅N, CCS, C₃S, and c-C₃HD 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.