2021ApJ...922..144Y

The study of infall motion helps us to understand the initial stages of star formation. In this paper, we use the IRAM 30 m telescope to make mapping observations of 24 infall sources confirmed in previous work. The lines we use to track gas infall motions are HCO⁺ (1-0) and H¹³CO⁺ (1-0). All 24 sources show HCO⁺ emissions, while 18 sources show H¹³CO⁺ emissions. The HCO⁺ integrated intensity maps of 17 sources show clear clumpy structures; for the H¹³CO⁺ line, 15 sources show clumpy structures. We estimated the column density of HCO⁺ and H¹³CO⁺ using the RADEX radiation transfer code, and the obtained [HCO⁺]/[H₂] and [H¹³CO⁺]/[HCO⁺] of these sources are about 10^–11-10^–7 and 10^–3-1, respectively. Based on the asymmetry of the line profile of the HCO⁺, we distinguish these sources: 19 sources show blue asymmetric profiles, and the other sources show red profiles or symmetric peak profiles. For eight sources that have double-peaked blue line profiles and signal-to-noise ratios greater than 10, the RATRAN model is used to fit their HCO⁺ (1-0) lines, and to estimate their infall parameters. The mean V_in of these sources is 0.3-1.3 km s^–1, and the {dot}M_in is about 10^–3-10^–4 M_☉ yr^–1, which is consistent with the results of intermediate or massive star formation in previous studies. The V_in estimated from the Myers model is 0.1-1.6 km s^–1, and the {dot}M_in is within 10^–3-10^–5 M_☉ yr^–1. In addition, some identified infall sources show other star-forming activities, such as outflows and maser emissions. Especially for those sources with a double-peaked blue asymmetric profile, most of them have both infall and outflow evidence.