Cyanopolyyne line survey towards high-mass star-forming regions with TMRT

Abstract

Context. Cyanopolyynes (HC 2n+1 N, n = 1,2,3), which are the linear carbon chain molecules, are precursors for the prebiotic synthesis of simple amino acids. They are important for understanding prebiotic chemistry and may be good tracers of the star formation sequence. Aims. We aim to search for cyanopolyynes in high-mass star-forming regions (HMSFRs) at possibly different evolutionary stages, investigate the evolution of HC 3 N and its relation with shock tracers, and detect the existence of HC 5 N and HC 7 N in HMSFRs with a formed protostar. Methods. We carried out a cyanopolyyne line survey towards a large sample of HMSFRs using the Shanghai Tian Ma 65m Radio Telescope (TMRT). Our sample consisted of 123 targets taken from the TMRT C band line survey. It included three kinds of sources, namely those with detection of the 6.7 GHz CH 3 OH maser alone, with detection of the radio recombination line (RRL) alone, and with detection of both (hereafter referred to as Maser-only, RRL-only, and Maser-RRL sources, respectively). For our sample with detection of cyanopolyynes, their column densities were derived using the rotational temperature measured from the NH 3 lines. We constructed and fitted the far-infrared (FIR) spectral energy distributions (SED; obtained from the Herschel FIR data and the Atacama Pathfinder Experiment data at 870 µm) of our HC 3 N sources. Moreover, by analysing the relation between HC 3 N and other shock tracers, we also investigate whether HC 3 N is a good tracer of shocks. Results. We detected HC 3 N in 38 sources, HC 5 N in 11 sources, and HC 7 N in G24.790+0.084, with the highest detection rate being found for Maser-RRL sources and a very low detection rate found for RRL-only sources. The mean column density of HC 3 N was found to be (1.75±0.42)×10 13 , (2.84±0.47)×10 13 , and (0.82±0.15)×10 13 cm −2 for Maser-only, Maser-RRL, and RRL-only sources, respectively. Based on a fit of the FIR SED, we derive their dust temperatures, H 2 column densities, and abundances of cyanopolyynes relative to H 2 . The mean relative abundance of HC 3 N was found to be (1.22±0.52)×10 −10 for Maser-only, (5.40±1.45)×10 −10 for Maser-RRL, and (1.65±1.50)×10 −10 for RRL-only sources, respectively. Conclusions. The detection rate, the column density, and the relative abundance of HC 3 N increase from Maser-only to Maser-RRL sources and decrease from Maser-RRL to RRL-only sources. This trend is consistent with the proposed evolutionary trend of HC 3 N under the assumption that our Maser-only, Maser-RRL, and RRL-only sources correspond to massive young stellar objects, ultracompact Hii regions, and normal classical Hii regions, respectively. Our detections enlarge the sample of HC 3 N in HMSFRs and support the idea that unsaturated complex organic molecules can exist in HMSFRs with a formed protostar. Furthermore, a statistical analysis of the integrated line intensity and column density of HC 3 N and shock-tracing molecules (SiO, H 2 CO) enabled us to find positive correlations between them. This suggests th...