Linear carbon-chain molecules like C_nH, HC_nN and C_nS are observed to be abundant in cold dense clouds. These molecules are proposed to be related to the formation and destruction of palyaromatic hydrocarbon (PAH) and considered to be an evolutionary indicator of early stage of star formation. Despite of its importance in astrochemistry, studies of long carbon-chain molecules have been seriously limited by the small detections. To date, abundant long carbon-chain molecules have only been found in two molecular clouds – Taurus Molecular Cloud 1 (TMC-1) and Lupus molecular cloud 1A (Lupus-1A) — and the envelop of a carbon star IRC+10216.

Using Shanghai Tian Ma Radio Telescope (TMRT), a 65 meter diameter fully-steerable radio telescope located in the western suburbs of Shanghai, China, a team led by Prof. Zhiqiang Shen from Shanghai Astronomical Observatory (SHAO) studied the starless molecular cloud Serpens South 1a. They detected several long carbon-chain molecules species including C₃S, HC₅N, HC₇N, HC₉N, C₆H, C₈H, HC₃N and its ¹³C isotopes. Nine transitions and resolved hyperfine components were detected for the first time in the interstellar medium, with high sensitivity and resolution.

On the column density and abundance of long carbon-chain molecules, the targeted cloud Serpens South 1a shows similarities to TMC-1 and Lupus-1A, which may indicate the same formation mechanism of these long carbon-chain molecules. This work has been accepted by Astrophysical Journal. Including Juan Li, an associate researcher from Shanghai Astronomical Observatory, all co-authors are TMRT team members.

“Since TMC-1 was found to be rich in long carbon-chain molecules in the 1970s, it has been a research focus – astronomers proposed different models to explain the long carbon-chain molecules in it. Not until 2010 did they find another similar dense molecular cloud, Lupus-1A.” Says Juan Li.

TMRT team found that, the column density of the long carbon-chain molecules in Serpens South 1a is around 10¹²~10¹³ cm^-2, which is comparable to that of TMC-1 and Lupus-1A. As the second “TMC-1 like” cloud, Serpens South 1a is the third molecular cloud found to contain HC₉N and C₈H.

Abundance ratio of the ¹³C isotope, such as [H¹³CCCN]:[HC¹³CCN]:[HCC¹³CN], is comparable to that of TMC-1, which may indicate a similar formation mechanism of these long carbon-chain molecules. Prof. Zhiqiang Shen concluded, “Comparison of physical and chemical properties of this source with those of TMC-1 and Lupus-1A, will lead us to a better understanding of formation and evolution of the long carbon-chain molecules.”

Left: Spectra line profiles of the ¹³C isotopes. Red arrows show the lines newly detected by TMRT, and the blue arrow shows the newly resolved hyperfine structures by TMRT. Right: Comparison of column densities among Serpens South 1a, TMC-1 and Lupus-1A.