Title: The Kelvin-Helmholtz instability in the Orion nebula: The effect of radiation pressure,
Abstract: The recent observations of rippled structures on the surface of the Orion
molecular cloud (Berné et al. 2010), have been attributed to the
Kelvin-Helmholtz (KH) instability. The wavelike structures which have mainly
seen near star-forming regions taking place at the interface between the hot
diffuse gas, which is ionized by massive stars, and the cold dense molecular
clouds. The radiation pressure of massive stars and stellar clusters is one of
the important issues that has been considered frequently in the dynamics of
clouds. Here, we investigate the influence of radiation pressure, from
well-known Trapezium cluster in the Orion nebula, on the evolution of KH
instability. The stability of the interface between HII region and molecular
clouds in the presence of the radiation pressure, has been studied using the
linear perturbation analysis for the certain range of the wavelengths. The
linear analysis show that consideration of the radiation pressure intensifies
the growth rate of KH modes and consequently decreases the e-fold time-scale of
the instability. On the other hand the domain of the instability is extended
and includes the more wavelengths, consisting of smaller ones rather than the
case when the effect of the radiation pressure is not considered. Our results
shows that for $\lambda_{\rm KH}>0.15\rm pc$, the growth rate of KH instability
dose not depend to the radiation pressure. Based on our results, the radiation
pressure is a triggering mechanism in development of the KH instability and
subsequently formation of turbulent sub-structures in the molecular clouds near
massive stars. The role of magnetic fields in the presence of the radiation
pressure is also investigated and it is resulted that the magnetic field
suppresses the effects induced by the radiation pressure.