Magellanic cloud-type interstellar dust along low-density sight lines in the Galaxy

Geoffrey C. Clayton, Louisiana State University
Karl D. Gordon, The University of Arizona
Michael J. Wolff, Space Science Institute


We have studied the UV extinction properties along 30 Galactic sight lines using data from the International Ultraviolet Explorer archive that have never been previously examined. These distant (d > 1 kpc) sight lines were selected to investigate the distribution and physical conditions of gas located in low-density regions of the Galactic disk and halo. The average densities along these sight lines are extremely low. It is likely that they are dominated by the warm intercloud medium and have little contribution from the cold cloud medium. We find that a subsample of these sight lines has extinction curves with weak bumps and very steep far-UV extinction reminiscent of the Magellanic Clouds. These sight lines all lie in the region bounded by 325° ≤ l ≤ 0° and -5°≥ b ≥ -11°. The gas along these sight lines shows forbidden velocities, which may indicate that the dust has been subject to shocks. This type of low-density sight line may mimic the environments found in the Magellanic Clouds. Large values of N(Ca II)/N(Na I) indicating low depletion are associated with steep far-UV extinction. A possible correlation exists between decreasing bump strength and increasing far-UV steepness for extinction curves in the Galaxy and the Magellanic Clouds.