A Chandra ACIS view of the thermal composite supernova remnant 3C 391

We present a 60 ks Chandra ACIS-S observation of the thermal composite supernova remnant 3C 391. The southeast-northwest elongated morphology is similar to that previously found in radio and X-ray studies. This observation unveils a highly clumpy structure of the remnant. Detailed spatially resolved spectral analysis of the small-scale features reveals that the interior gas is generally of normal metal abundance and has approached or basically reached ionization equilibrium. The hydrogen column density increases from southeast to northwest. Three mechanisms, radiative rim, thermal conduction, and cloudlet evaporation, may all play roles in the X-ray appearance of 3C 391 as a "thermal composite" remnant, but there are difficulties with each of them in explaining some physical properties. Comparatively, the cloudlet evaporation model is favored by the main characteristics such as the highly clumpy structure and the uniform temperature and density distribution over most of the remnant. The directly measured postshock temperature also implies a young age, similar to4 x 10(3) yr, for the remnant. The postshock gas pressure derived from the northeast and southwest rims, which harbor maser spots, is consistent with the estimate for the maser regions. An unresolved X-ray source is observed on the northwest border, and its spectrum is best fitted by a power law.