INTERSTELLAR CLOUD FORMATION THROUGH AGGREGATION OF COLD BLOBS IN A TWO-PHASE GAS MIXTURE

摘要

We propose a new formation scenario for interstellar clouds through the aggregation of dense cold blobs (phase Ⅱ [PⅡ]), which drift in a diffuse warm medium (phase Ⅰ [PⅠ]). We examine how important it is that there exist numerous PⅡ blobs when the properties of such a two-phase flow are studied. First, we solve a one-dimensional shock-tube problem and find that the shock wave in the mixture is considerably damped because of the drag force between the two phases. This is because the PⅡ blobs are left behind the shock front, since their inertia is larger than that of PⅠ, thus suppressing large spatial variations of PⅠ gas via the drag force. The PⅡ blobs thus play the role of anchors. Therefore, mass aggregation by shocks may be ineffective in a two-phase medium. However, the PⅡ blobs can still aggregate through a kind of fluid dynamical instability. We next suppose that the PⅠ gas is accelerated upward by shocks against downward gravity, while the PⅡ blobs are at rest because of balance between the drag force due to PⅠ and gravity. If we put a positive perturbation in the number density of PⅡ blobs, the upward PⅠ flow above the perturbation is decelerated by the enhanced drag force, and the velocity difference between PⅠ and PⅡ is thereby reduced. Then the PⅡ blobs above the perturbation are accelerated downward, since the gravity on PⅡ now dominates the reduced drag force. As a result, the blobs will fall onto this perturbed region, and this region becomes denser and denser. This is the mechanism of the instability. Therefore, we expect efficient cloud formation by this instability in spiral arms, even when galactic shocks are extremely damped.