5 Conclusions

Considering 2-D or 3-D models of SNRs it is necessary to take into account the effects of the projection. Once projected onto the plane of the sky, such an object changes its appearance depending on the actual density contrast across the remnant and on the angle between the density gradient and the direction towards the observer. If the ambient density gradient does not lie in the plane of the projection and is strong enough, and if the magnetic field is nearly aligned with the line of sight, then the visible thermal X-ray morphology of SNR will be centrally-filled, while the radio morphology will remain limb-brightened. The projection effect is maximum when the density gradient is oriented along the line of sight. Only a scale-height of order $h<10\ {\rm pc}$ in ambient medium density distribution can cause projected composites.

All theoretical properties of projected composites correspond to observational properties of thermal X-ray composites. Thus, the circumstances should exist when SNRs are projected as centrally-filled X-ray objects.

Majority of the members of a mixed-morphology class are really located near molecular clouds. Therefore, at least a part of them may be the result of a simple projection effect of the adiabatic SNR evolved in a nonuniform medium, e.g., at the edge of a molecular cloud.

If the prediction that a part of barrel-like radio SNRs have a centrally-filled thermal X-ray morphology is confirmed, it will provide additional support for the proposed model. The model suggests that TXCs may be prospective sources of proton origin $\gamma$-rays.