2022PASJ...74..128M

We present high-resolution observations of CS (J = 1-0), H¹³CO⁺ (J = 1-0), and SiO (v = 0: J = 1-0) lines, together with the 49 GHz and 86 GHz continuum emissions, toward W 49 N carried out with the Nobeyama Millimeter Array. We identified 11 CS, eight H¹³CO⁺, and six SiO clumps with radii of 0.1-0.5 pc. The CS and H¹³CO⁺ clumps are mainly divided into two velocity components, one at 4 km s^–1 and the other at 12 km s^–1, while the SiO clumps have velocities between the two components. The SiO emission is distributed toward the ultracompact H II (UCHII) ring, where the 4 km s^–1 component clumps of CS and H¹³CO⁺ also exist. The 12 km s^–1 component clumps of CS are detected at the east and west of the UCHII ring with an apparent hole toward the ring. The clump masses vary from 4.4 x 10² M_☉ to 4.9 x 10⁴ M_☉ with the mean values of 0.94 x 10⁴ M_☉, 0.88 x 10⁴ M_☉, and 2.2 x 10⁴ M_☉ for the CS, H¹³CO⁺, and SiO clumps, respectively. The total masses derived from CS, H¹³CO⁺, and SiO clumps are 1.0 x 10⁵ M_☉, 0.70 x 10⁵ M_☉, and 1.3 x 10⁵ M_☉, respectively, which agree well with the corresponding virial masses of 0.71 x 10⁵ M_☉, 1.3 x 10⁵ M_☉, and 0.88 x 10⁵ M_☉, respectively. The average molecular hydrogen densities of the clumps are 0.90 x 10⁶ cm^–3, 1.4 x 10⁶ cm^–3, and 7.6 x 10⁶ cm^–3 for the CS, H¹³CO⁺, and SiO clumps, respectively. The density derived from the SiO clumps seems significantly higher than those from the others, probably because the SiO emission is produced in high-density shocked regions. The free-fall time scale of the clumps is estimated to be ∼3 x 10⁴ yr, which gives an accretion rate of 3 x 10^–3-1 M_☉ yr^–1 on to a stellar core. The observed clumps are, if they are undergoing free-fall, capable of producing dozens of massive stars in the next 10⁵ yr. We propose a view that two pre-existing clouds with radial velocities of 4 km s^–1 and 12 km s^–1 collided with each other almost face-on to produce the observed clumps with intermediate velocities and triggered the burst of massive star formation in W 49 N.