SIMBAD references

The dense cloud associated with W40, which is one of the nearby Hii regions, has been studied in millimetre-wave molecular lines and in the 1.2-mm continuum. Moreover, 1280- and 610-MHz interferometric observations have been carried out. The cloud has a complex morphological and kinematical structure, including a clumpy dust ring and an extended dense core. The ring has probably been formed by the `collect and collapse' process as a result of the expansion of the neighbouring Hii region. Nine dust clumps in the ring have been deconvolved. Their sizes, masses and peak hydrogen column densities are ∼ 0.02-0.11 pc, ∼ 0.4-8.1M_☉ and ∼ (2.5-11)x10²²/cm2, respectively. Molecular lines are observed at two different velocities and have different spatial distributions, which implies that there is strong chemical differentiation over the region. The CS abundance is enhanced towards the eastern dust clump 2, while the NH3, N2H+ and H13CO+ abundances are enhanced towards the western clumps. HCN and HCO+ do not correlate with the dust, probably tracing the surrounding gas. The number densities derived towards selected positions are ∼ (0.3-3.2)x10⁶/cm. The two western clumps have kinetic temperatures of 21 and 16K and are close to virial equilibrium. The eastern clumps 2 and 3 are more massive, have a higher extent of turbulence and are probably more evolved than the western clumps. They show asymmetric CS(2-1) line profiles because of infalling motions, which is confirmed by model calculations. An interaction between ionized and neutral material is taking place in the vicinity of the eastern branch of the ring and probably triggers star formation.