G345.45+1.50: an expanding ring-like structure with massive star formation

摘要

Context. Ring-like structures in the interstellar medium (ISM) are commonly associated with high-mass stars. Kinematic studies of large structures in giant molecular clouds (GMCs) toward these ring-like structures may help us to understand how massive stars form. Aims. The origin and properties of the ring-like structure G345.45+1.50 is investigated through observations of the ~(13) CO(3 ? 2) line. The aim of the observations is to determine the kinematics in the region and to compare physical characteristics estimated from gas emission with those previously determined using dust continuum emission. This area in the sky is well suited for studies like this because the ring is located 1 hbox{$dg$} 5 above the Galactic plane at 1.8?kpc from the Sun, thus molecular structures are rarely superposed on our line of sight, which minimizes confusion effects that might hinder identifying of individual molecular condensations. Methods. The ~(13) CO(3 ? 2) line was mapped toward the whole ring using the Atacama Pathfinder Experiment (APEX) telescope. The observations cover 17′ × 20′ in the sky with a spatial resolution of 0.2?pc and an rms of ~ 1?K at a spectral resolution of 0.1?km?s ~(-1) . Results. The ring is found to be expanding with a velocity of 1.0?km?s ~(-1) , containing a total mass of 6.9 × 10~(3) M _(⊙) , which agrees well with that determined using 1.2?mm dust continuum emission. An expansion timescale of ~3 × 10~(6) ?yr and a total energy of ~7 × 10~(46) ?erg are estimated. The origin of the ring might have been a supernova explosion, since a 35.5?cm source, J165920-400424 , is located at the center of the ring without an infrared counterpart. The ring is fragmented, and 104 clumps were identified with diameters of between 0.3 and 1.6?pc, masses of between 2.3 and 7.5 × 10~(2) M _(⊙) , and densities of between ~10~(2) and ~ 10~(4) ?cm ~(-3) . At least 18% of the clumps are forming stars, as is shown in infrared images. Assuming that the clumps can be modeled as Bonnor-Ebert spheres, 13 clumps are collapsing, and the rest of them are in hydrostatic equilibrium with an external pressure with a median value of 4 × 10~(4) ?K?cm ~(-3) . In the region, the molecular outflow IRAS?16562-3959 is identified, with a velocity range of 38.4 km s ~(-1) , total mass of 13 M _(⊙) , and kinematic energy of 7 × 10~(45) erg. Finally, five filamentary structures were found at the edge of the ring with an average size of 3?pc, a width of 0.6?pc, a mass of 2 × 10~(2) M _(⊙) , and a column density of 6 × 10~(21) ?cm ~(-2) .