Gas Associated with Heavily Modified Dust

In the 9th year of IUE, we initiated a program to study the strength, width and central wavelength of the 2175 A extinction bump in a sample of stars known from ANS photometric data to have anomalous extinction. A major result from that program (Cardelli and Savage, "A study of the Fundamental characteristics of the 2175 A Extinction Feature ") is the discovery of the line of sight to HD 62542 which exhibits a UV extinction curve which is the most extreme of any so far found in the Milky Way. The HD 62542 extinction curve has a very weak and very broad UV extinction bump which is centered at 2110 A rather than the normal 2175 A (see Figure 1). Both the bump width and position are well outside the range found for any extinction curve yet observed, including those for the LMC and SMC. The far UV portion of the curve is higher than for any Milky Way sight line for which published data exists. HD 62542 exists in a star forming region of densely packed, swept up interstellar as and dust in the region of the Gum nebula near two stars which exhibit strong stellar winds (zeta Pup and gamma2 Vel). The region appears so extreme that the anomalous extinction may be produced by a combination of shock processing of grains, by grain cogulation, and by mantle growth. In any event the dust toward this star has been so severely modified, it is very likely that the gas phase of the ISM is also extremely anomalous. We therefore propose to obtain high dispersion SWP and LWP spectra of HD 62542 to study the gas-dust interaction along this exceedingly interesting line of sight. Some specific questions we will try to answer include: 1. Are the abundances of molecules anomalous? If the anomalous extinction is produced in a dense interstellar environment either by grain coagulation or mantle growth, the conditions may be favorable for molecule formation. The survival of molecules near HD 62542 may be enhanced by the high far UV extinction of the dust. The HD 62542 sight line may represent the best known sight line to pursue UV studies of interstellar chemical processes. 2. Are the elemental depletions along this sight line anomalous? If shocks produced by the winds from zeta Pup and gamma2 Vel have occurred, then significant matter may have returned to the gas phase at some point in the regions formation. If this effect was significant, then radical variation in the relative depletion patterns may lend clues to the nature of the the gas-grain chemistry.