Star burst in W 49 N presumably induced by cloud–cloud collision

Abstract

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 × 102 M$_\odot$ to 4.9 × 104 M$_\odot$ with the mean values of 0.94 × 104 M$_\odot$, 0.88 × 104 M$_\odot$, and 2.2 × 104 M$_\odot$ for the CS, H13CO+, and SiO clumps, respectively. The total masses derived from CS, H13CO+, and SiO clumps are 1.0 × 105 M$_\odot$, 0.70 × 105 M$_\odot$, and 1.3 × 105 M$_\odot$, respectively, which agree well with the corresponding virial masses of 0.71 × 105 M$_\odot$, 1.3 × 105 M$_\odot$, and 0.88 × 105 M$_\odot$, respectively. The average molecular hydrogen densities of the clumps are 0.90 × 106 cm−3, 1.4 × 106 cm−3, and 7.6 × 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 × 104 yr, which gives an accretion rate of 3 × 10−3–1 M$_\odot$ 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.