AChandraACIS View of the Thermal Composite Supernova Remnant 3C 391

We present a 60 ks Chandra ACIS-S observation of the thermal compositesupernova remnant 3C391. The southeast-northwest elongated morphology issimilar to that previously found in radio and X-ray studies. This observationunveils a highly clumpy structure of the remnant. Detailed spatially resolvedspectral analysis for the small-scale features reveals that the interior gas isgenerally of normal metal abundance and has approached or basically reachedionization equilibrium. The hydrogen column density increases from southeast tonorthwest. Three mechanisms, radiative rim, thermal conduction, and cloudletevaporation, may all play roles in the X-ray appearance of 3C391 as a "thermalcomposite" remnant, but there are difficulties with each of them in explainingsome physical properties. Comparatively, the cloudlet evaporation model isfavored by the main characteristics such as the highly clumpy structure and theuniform temperature and density distribution over most of the remnant. Thedirectly measured postshock temperature also implies a young age, about 4 kyr,for the remnant. The postshock gas pressure derived from the NE and SW rims,which harbor maser spots, is consistent with the estimate for the maserregions. An unresolved X-ray source is observed on the northwest border and itsspectrum is best fitted by a power-law.Comment: aastex, 27 pages (including 4 figures), to appear in the ApJ 1 Dec. 2004, v616 issu