Explaining the morphology of centrally brightened Supernova Remnants (SNRs): the case of G18.95-1.1 & G272.2-3.2.

We present the analysis of two centrally brightened Supernova Remnants (SNRs), G18.95-1.1 and G272.2-3.2. These two remnants present the characteristics of both thermally-dominated and composite remnants: their spectrum is that of the former but they present a centrally-brightened X-ray morphology. G18.95-1.1 was observed by the ROSAT PSPC and presents a spectrum which cannot be fitted by a single temperature collisional-plasma model. We have analyzed the Advanced Satellite for Cosmology and Astrophysics (ASCA) data of this remnant which presents no conclusive proof of a high-energy component. The spectrum is dominated by thermal emission and we examine different theories which could explain G18.95-1.1 centrally peaked morphology. G272.2-3.2 can then be best modeled by a non-equilibrium ionization (NEI) model with a temperature around 1 keV, an ionization timescale around 3200 cm(-3) yr and a relatively high column density (N_H ~ 10(22) atoms cm(-2) ). We found that we can explain the morphology and the temperature profile using a cloud evaporation model. This model provides us with a consistent picture of the remnant. We argue that G272.2-3.2 is less than 6000 years old and probably in the adiabatic phase of its evolution.