2022ApJ...931...33T

In this study, 36 cores (30 starless and six protostellar) identified in Orion were surveyed to search for inward motions. We used the Nobeyama 45 m radio telescope, and mapped the cores in the J = 1 - 0 transitions of HCO⁺, H¹³CO⁺, N₂H⁺, HNC, and HN¹³C. The asymmetry parameter δV, which was the ratio of the difference between the HCO⁺ and H¹³CO⁺ peak velocities to the H¹³CO⁺ line width, was biased toward negative values, suggesting that inward motions were more dominant than outward motions. Three starless cores (10% of all starless cores surveyed) were identified as cores with blue-skewed line profiles (asymmetric profiles with more intense blueshifted emission), and another two starless cores (7%) were identified as candidate blue-skewed line profiles. The peak velocity difference between HCO⁺ and H¹³CO⁺ of them was up to 0.9 km s^–1, suggesting that some inward motions exceeded the speed of sound for the quiescent gas (∼10-17 K). The mean of δV of the five aforementioned starless cores was derived to be -0.5 ± 0.3. One core, G211.16-19.33North3, observed using the Atacama Compact Array of the Atacama Large Millimeter/submillimeter Array in DCO⁺J = 3 - 2 exhibited blue-skewed features. Velocity offset in the blue-skewed line profile with a dip in the DCO⁺J = 3 - 2 line was larger (∼0.5 km s^–1) than that in HCO⁺J = 1 - 0 (∼0.2 km s^–1), which may represent gravitational acceleration of inward motions. It seems that this core is at the last stage in the starless phase, judging from the chemical evolution factor version 2.0 (CEF2.0).