The effect of a weak shock on interstellar gas toward the Rho Ophiuchi cloud
The effect of a low-velocity shock on depletion in interstellar clouds is studied. High-resolution Copernicus observations of interstellar absorption lines toward four stars in the Rho Ophiuchi cloud complex were used to measure differential depletion in interstellar clouds separated by velocities of 10-15 km/s. Optical observations of CH and CH(+) were used as indicators of shock strength and direction of propagation. Observations indicate the presence of a shock with velocity about 10 km/s expanding away from the sun into the Rho Oph cloud. The existence of this shock is supported by the other observations quoted in the literature. Two distinct regions in the lines of sight have been found. A low-density, less depleted, predominantly atomic region is associated with preshock gas, while postshock gas accounts for a predominantly molecular, more highly depleted region. Apparently a weak shock, has the effect of enhancing grain formation or grain growth as a result of increased density in the postshock gas. It is possible for grain growth by accretion to occur in the shocked gas on time scales short compared with the cloud-crossing time of the shock.