A survey of molecular cores in M 17 SWex

Abstract

Abstract A survey of molecular cores covering the infrared dark cloud known as the M 17 southwest extension (M 17 SWex) has been carried out with the 45 m Nobeyama Radio Telescope. Based on the N2H+ (J = 1–0) data obtained, we have identified 46 individual cores whose masses are in the range from 43 to $3026\, {M}_{\odot }$. We examined the relationship between the physical parameters of the cores and those of young stellar objects (YSOs) associated with the cores found in the literature. The comparison of the virial mass and the core mass indicates that most of the cores can be gravitationally stable if we assume a large external pressure. Among the 46 cores, we found four massive cores with YSOs. They have large masses of $\gtrsim 1000\, M_{\odot }$ and line widths of $\gtrsim 2.5\:$km s−1 which are similar to those of clumps forming high-mass stars. However, previous studies have shown that there is no active massive star formation in this region. Recent measurements of near-infrared polarization imply that the magnetic field around M 17 SWex is likely to be strong enough to support the cores against self-gravity. We therefore suggest that the magnetic field may prevent the cores from collapsing, causing the low level of massive star formation in M 17 SWex.