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

TL;DR

This study extends a radio spectroscopic survey of southern hemisphere dense cores, deriving molecular abundances to constrain their chemical composition, ages, and C/O ratios using observations and astrochemical models.

Contribution

It provides new measurements of molecular column densities in southern dense cores and compares them with models to estimate their chemical ages and C/O ratios.

Findings

Most cores have C/O ratios of 0.5-0.7, lower than TMC-1.

Chemical ages range from 0.6 to 5 million years.

Less dense cores tend to be older chemically.

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 of 18 to 25 GHz. Molecular column densities of NH$_3$, $c$-C$_3$H$_2$, HC$_3$N, HC$_5$N, CCS, C$_3$S, and $c$-C$_3$HD were derived for each source assuming LTE. 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 characterized by a high C/O ratio of $\sim 1.4$. 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 on their C/O ratios and chemical ages, which remain largely unexplored.