2018A&A...620A.158S

Context. High mass stars form in groups or clusters in dense molecular clumps with sizes of 1pc and masses of 200M_☉. Infrared-dark clumps and the individual cores within them with sizes <0.1pc and masses <100M_☉ are important laboratories for high-mass star formation in order to study the initial conditions.
Aims. We investigate the physical and chemical properties of high-mass clumps in order to better understand the early evolutionary stages and find targets that show star formation signs such as infall motions or outflows.
Methods. We selected the high-mass clumps from ATLASGAL survey that were identified as dark at 8/24µm wavelengths. We used MALT90 Survey data which provides a molecular line set (HCO⁺, HNC, HCN, N₂H⁺, H¹³CO⁺, HN¹³C, SiO) to investigate the physical and chemical conditions in early stages of star formation.
Results. Eleven sources have significant SiO detection (over 3σ) which usually indicates outflow activity. Thirteen sources are found with blue profiles in both or either HCO⁺ and/or HNC lines and clump mass infall rates are estimated to be in the range of 0.2x10^–3M_☉/yr-1.8x10^–2M_☉/yr. The excitation temperature is obtained as <24K for all sources. The column densities for optically thin lines of H¹³CO⁺ and HN¹³C are in the range of 0.4-8.8(x10¹²)cm^–2, and 0.9-11.9(x10¹²)cm^–2, respectively, while it is in the range of 0.1-7.5(x10¹⁴)cm^–2 for HCO⁺ and HNC lines. The column densities for N₂H⁺ were ranging between 4.4-275.7(x10¹²)cm^–2 as expected from cold dense regions. Large line widths of N₂H⁺ might indicate turbulence and large line widths of HCO⁺, HNC, and SiO indicate outflow activities. Mean optical depths are 20.32, and 23.19 for optically thick HCO⁺ and HCN lines, and 0.39 and 0.45 for their optically thin isotopologues H¹³CO⁺ and HN¹³C, respectively.
Conclusions. This study reveals the physical and chemical properties of 30 high-mass IR-dark clumps and the interesting targets among them based on their emission line morphology and kinematics.