Far-Ultraviolet Spectroscopic Explorer Observations of the Magellanic Bridge Gas toward Two Early-Type Stars: Molecules, Physical Conditions, and Relative Abundances

We discuss Far Ultraviolet Spectroscopic Explorer (FUSE) observations of two early-type stars, DI 1388 (l=291.3d, b=-41.1d) and DGIK 975 (l=287.3d, b=-36.0d), in the low-density and low-metallicity (Z~0.08 Z_solar) gas of the Magellanic Bridge (MB). The data have a spectral resolution of about 15,000, and signal-to-noise ratios range between 10 and 30 per resolution element in the spectra of DI 1388 and between 7 and 11 in the spectra of DGIK 975. DI 1388 is situated near the SMC, while DGIK 975 is closer to the LMC, allowing us to probe the MB gas in a widely different locations. Toward DI 1388, the FUSE observations show molecular hydrogen, O VI, and numerous other atomic or ionic transitions in absorption, implying the presence of multiple gas phases in a complex arrangement. The relative abundance (with respect to S II) pattern in the MB along the DI 1388 sight line is attributed to varying degrees of depletion onto dust similar to that of halo clouds. The N/O ratio is near solar, much higher than N/O in damped Lyα systems, implying subsequent stellar processing to explain the origin of nitrogen in the MB. The diffuse molecular cloud in this direction has a low column density and low molecular fraction [logN(H₂)~15.43 dex; f_H2~10^-5to10^-4], yet two excitation temperatures (T₀₁=94⁺⁵³_-27 K and T₂₃=341⁺¹⁷²_-81 K) are needed to fit the distribution of the different rotational levels. Although this is not typically seen in the Galaxy, we show that this is not uncommon in the Magellanic Clouds. H₂ is observed in both the Magellanic Stream and the MB, yet massive stars form only in the MB, implying significantly different physical processes between them. In the MB some of the H₂ could have been pulled out from the SMC via tidal interaction, but some also could have formed in situ in dense clouds where star formation might have taken place. Toward DGIK 975, the presence of neutral and weakly and highly ionized species suggests that this sight line has also several complex gas phases. The highly ionized species of O VI, C IV, and Si IV toward both stars have very broad features, indicating that multiple components of hot gas at different velocities are present. C IV/O VI varies within the MB, but C IV/Si IV is relatively constant for both sight lines. Several sources (a combination of turbulent mixing layer, conductive heating, and cooling flows) may be contributing to the production of the highly ionized gas in the MB. Finally, this study has confirmed previous results that the high-velocity cloud HVC 291.5-41.2+80 is mainly ionized, composed of weakly and highly ions. The high ion ratios are consistent with a radiatively cooling gas in a fountain flow model.