The interactions of winds from massive young stellar objects : X-ray emission, dynamics and cavity evolution
Two-dimensional axis-symmetric hydrodynamical simulations are presented which explore the interaction of stellar and disc winds with surrounding infalling cloud material. The star and its accompanying disc blow winds inside a cavity cleared out by an earlier jet. The collision of the winds with their surroundings generates shock-heated plasma which reaches temperatures up to ∼10 K. Attenuated X-ray spectra are calculated from solving the equation of radiative transfer along lines of sight. This process is repeated at various epochs throughout the simulations to examine the evolution of the intrinsic and attenuated fluxes. We find that the dynamic nature of the wind-cavity interaction fuels intrinsic variability in the observed emission on time-scales of several hundred years. This is principally due to variations in the position of the reverse shock which is influenced by changes in the shape of the cavity wall. The collision of the winds with the cavity wall can cause clumps of cloud material to be stripped away. Mixing of these clumps into the winds mass-loads the flow and enhances the X-ray emission measure. The position and shape of the reverse shock play a key role in determining the strength and hardness of the X-ray emission. In some models the reverse shock is oblique to much of the stellar and disc outflows, whereas in others it is closely normal over a wide range of polar angles. For reasonable stellar and disc wind parameters, the integrated count rate and spatial extent of the intensity peak for X-ray emission agree with Chandra observations of the deeply embedded massive young stellar objects (MYSOs) S106 IRS 4, Mon R2 IRS 3A and AFGL 2591. The evolution of the cavity is heavily dependent on the ratio of the inflow to outflow ram pressures. The cavity closes up if the inflow is too strong and rapidly widens if the outflowing winds are too strong. The velocity shear between the respective flows creates Kelvin-Helmholtz instabilities which corrugate the surface of the cavity. Rayleigh-Taylor-like instabilities also occur when the cavity wall is pushed forcefully backwards by strong outflows. The opening angle of the cavity plays a significant role and we find that for collimation factors in agreement with those observed for bipolar jets around MYSOs, a reverse shock is established within ≲500 au of the star.