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Astron. Astrophys. 342, 839-853 (1999)

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5. Summary and conclusions

We have applied single-temperature (1T) and two-temperature (2T) emission models to the spectral analysis of two X-ray observations of the North-East of the Vela SNR shell. Given the difficulties to explain the low ionization times derived assuming a 1T NEI model (Paper II), we have reconsidered the success of the 2T CIE model fitting as a direct evidence of ISM inhomogeneities rather than evidence of NEI, and we have associated the two components (the cooler X1 component and the hotter X2 component) to two ISM phases (an higher density "inhomogeneities" phase and a lower density "inter-cloud" phase, respectively). This association is supported by the clumpy X-ray emission morphology, as suggested for instance by Aschenbach (1993). Considering first the cooler X-ray component, we have found that:

  1. the ISM is inhomogeneous in both the observed regions (RP200133 and RP500015). In any spatial bin the volume filling factor of the inhomogeneities is [FORMULA], and in 60% of the spatial bins is [FORMULA], i.e. the inhomogeneities are dominant in volume;

  2. ISM inhomogeneities may create X-ray clumps in two ways. In one case (mostly in RP500015), several inhomogeneities with a typical density of 0.5 cm-3 overlap along the line of sight to yield a filling factor [FORMULA]. In other cases (mostly in RP200133 and in sectors m and n of RP500015, which are adjacent to sectors a and b of RP200133) the bright X-ray clumps are caused by a joint increase of filling factor and density of the inhomogeneities (up to 1.0 cm-3). The latter is the case of the X-ray feature "Filament D", which may be produced by an isolated ISM clouds more dense than the surroundings inhomogeneities;

  3. Fig. 2 shows that the clumps have sizes of few parsecs. The rebinning scheme we have adopted, which represent the best compromise between spatial resolution and photon counting statistics, has allowed us to perform spectral analysis on scales of than [FORMULA] pc;

  4. the X-ray derived parameters do not exclude that the X-ray clumps are caused by thermal evaporation of shocked plasma in isolated clouds. An origin in terms of secondary shocks propagating in non-evaporative clouds is also consistent with the data. A forthcoming reduction of UV, optical photometric and spectrophotometric data on these shell region will help to detect more dense clouds and to shed light on the association between observations, on-going dynamical processes and ISM models.

Pursuing the correspondence between the hotter X-ray component and the inter-cloud medium, to which a Sedov analysis can be applied, we have found that:

  1. the shell expansion occurs in a medium with a density of 0.03 cm-2;

  2. the explosion energy of the supernova event, which gave rise to the SNR, was [FORMULA] erg, lower than previous estimate but in agreement with supernova models;

  3. the derived distance is [FORMULA] pc, in agreement with recent independent estimates;

  4. very slight departures from instantaneous electron-ion energy equipartition are compatible with our data, but very slow equipartition (such as Coulomb collision) is to be excluded. Our results favour a fast equipartition behind the shock.

Finally, we have estimated the influence on our results of neglecting the possible NEI conditions of the shocked plasma. We found under reasonable assumptions the correction factors for the measured and derived parameters, and we argued that the differences do not affect our conclusions. The correction factors are not to be meant as a substitution for a full 2T NEI analysis, which will be feasible with higher spectral resolution detectors (SAX LECS/MECS, AXAF ACIS, XMM EPIC). The NEI analysis will also allow to address the crucial point of the plasma metal abundances, which cannot be afforded with ROSAT PSPC data.

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999