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 atomi c and molecular absorption in high-velocity cloud HVC 287.5+22.5+240, which lies in front of the ultraviolet-bright nucleus of the Seyfert 1 galaxy NG C 3783. We detect H-2, N I N II, Si II, and Fe II and set limits on the amo unt of absorption due to P III, Ar I, and Fe III. We extend the earlier met allicity and dust-depletion measurements made by Lu and collaborators by ex amining 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 (less than or similar to 15%). The HVC has metallicity 0.2-0.4 solar, simi lar to that of the Small Magellanic Cloud. The relative abundance pattern f or the elements studied resembles that of warm gas in the Small Magellanic Cloud (SMC), which supports the idea that this HVC is part of the tidally s tripped 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 H-2 arisin g in the HVC from rotational levels J = 0 to J = 3. Synthetic spectra and a curve of growth fitted to these lines with b = 12 km s(-1) indicate that l og [N(H-2)] = 16.80 +/- 0.10 and f(H2) = 2N(H-2)/[N(H I) + 2N(H-2)] = 1.6 x 10(-3). A two-component temperature distribution is necessary to explain t he observed populations of the H-2 rotational levels. We find T-01 = 133(-2 1)(+37) K, and T-23 = 241(-17)(+20) K, indicating that the conditions in th e molecular gas are more similar to those found for diffuse molecular cloud s in the Galactic halo than to those for molecular clouds in the Galactic d isk. From an analysis of the J = 2 and J = 3 populations, we find an absorp tion rate (at 1000 Angstrom) of beta (uv) < 0.1 times the average value in the solar neighborhood. The presence of molecular lar gas in the HVC requir es that either the formed H-2 in situ or that molecules formed within the S MC 2 survived tidal stripping. We favor the latter possibility because of t he long H-2 formation time (<similar to>10(8) yr) derived for this HVC.