## 3. SNR shapes in non-uniform mediaWe now consider the role of the surrounding ISM on the evolution and the X-ray emission of adiabatic SNR on the examples of media with flat exponential and spherically-symmetrical power-law density distributions. ## 3.1. Shapes of SNRs in a flat exponential mediumThe exponential law distribution is frequently encountered in
nature, especially in galactic disks. We consider the evolution of the
shape of the shock front from a point explosion in a medium with
exponential density distribution Eq. (11). The morphological evolution
of such SNR are presented in Fig. 4. We can see from this figure the
remarkable insensitivity of the
The apparent center of the non-spherical SNR does not coincide with real progenitor position. This may be important for localizing a possible compact stellar remnant (pulsar or black hole). The evolution of some shock characteristics is presented in Fig. 5. The main result is that the average visible morphological characteristics of the non-spherical SNRs are usually close to those for Sedov SNRs with the same initial parameters.
## 3.2. Shapes of SNRs in a power-law mediumAnother widely-used density distribution is the power-law one, created by stellar winds, previous SN explosions etc.: We consider the evolution of the shock from a point explosion in a
medium with a spherically-symmetrical power-law density distribution
when the explosion point is displaced by a distance
from the center of symmetry
(wind source etc.). Therefore the
density distribution as a function of the distance Here is the initial density in the point of explosion. Hereafter, we take and . The shape evolution of the SNR in this density distribution is shown in Fig. 6. It is worthy to note that visible shapes of such SNRs may be elongated transverse to the density gradient.
As one can see from Fig. 6b like to the previous case (Fig. 4b), the projection of the SNR on the plane of the sky can cause a spherization of the visible SNR shape. So, even a visible spherical shape of SNR does not guarantee the uniformity of ISM and isotropy of explosion. ## 3.3. DiscussionFrom the above results, it follows that: 1. The non-uniformity of the surrounding medium causes asphericity of SNRs. The visible shapes may be elongated not only along (Fig. 4) but also transverse to (Fig. 6) the density gradient, depending on the type of density distribution. 2. SNR may have an apparent shape close to spherical even in cases of essential anisotropy of the real form and essential gradient of density distribution along the surface. 3. The observed anisotropy of the shape is smaller than the real anisotropy as result of projection. The visible shape remains, generally, close to spherical. 4. The non-uniformity of the surrounding medium results in differences of shock characteristics along the SNR surface. If the initial density varies along the shock surface, the maximal contrasts are expected in the X-ray surface brightness (), they are smaller for the temperature distribution () and minimal in shock and postshock gas velocities () (Figs. 4-6). Therefore to determine the real conditions inside and around SNR, it is necessary to use additional information about SNR. We consider further the X-ray observations as an effective tool for SNR diagnostics. © European Southern Observatory (ESO) 1999 Online publication: March 10, 1999 |