FUSE Observations of Atomic Abundances and Molecular Hydrogen in the Leading Arm of the Magellanic Stream

We present Far Ultraviolet Spectroscopic Explorer observations of the atomic and molecular absorption in high velocity cloud HVC287.5+22.5+240, which lies in front of the ultraviolet-bright nucleus of the Seyfert 1 galaxy NGC3783. We detect H2, N I, N II, Si II, and Fe II and set limits on the amount of absorption due to P III, Ar I, and Fe III. We extend the earlier metallicity and dust-depletion measurements made by Lu and collaborators by examining the relative gas-phase abundances of Si, P, S, and Fe. Corrections to the derived gas-phase abundances due to ionized gas in the HVC are small (. 15%). The HVC has a metallicity of 0.2–0.4 solar, similar to that of the Small Magellanic Cloud. The relative abundance pattern for the elements studied resembles that of warm gas in the SMC, which supports the idea that this HVC is part of the tidally stripped Leading Arm of the Magellanic Stream. The abundance pattern implies that the HVC contains dust grains that have been processed significantly; it is likely that the grain mantles have been modified or stripped back to expose the grain cores. We have identified more than 30 lines of H2 arising in the HVC from rotational levels J= 0 to J=3. Synthetic spectra and a curve-of-growth fit to these lines with b = 12 km s indicate that logN(H2) = 16.80±0.10 and fH2 =2N(H2)/[N(H I)+2N(H2)] = 1.6 × 10. A two-component temperature distribution is necessary to explain the observed populations of the H2 rotational levels. We find T01 = 133± 37 21 K, and T23 = 241± 17 K, indicating that the conditions in the molecular gas are more similar to those found for diffuse molecular clouds in the Galactic halo than to those for molecular clouds in the Galactic disk. From an analysis of the J = 2 and J = 3 populations, we find an absorption rate (at 1000 Å) of βuv < 0.1 times the average value in the solar neighborhood. The presence of molecular gas in the HVC requires that either the H2 formed in situ or that molecules formed within the SMC survived tidal stripping. We favor the latter possibility because of the long H2 formation time (∼ 10 8 years) derived for this HVC. Department of Physics & Astronomy, The Johns Hopkins University, Baltimore, MD 21218 Department of Astronomy, University of Wisconsin, Madison, WI 53706 CASA and JILA, Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309