The COS/UVES Absorption Survey of the Magellanic Stream: II. Evidence
  for a complex enrichment history of the Stream from the Fairall 9 sightline
  We present a multi-wavelength study of the Magellanic Stream (MS), a massive
gaseous structure in the Local Group that is believed to represent material
stripped from the Magellanic Clouds. We use ultraviolet, optical and radio data
obtained with HST/COS, VLT/UVES, FUSE, GASS, and ATCA to study metal abundances
and physical conditions in the Stream toward the quasar Fairall 9. Line
absorption in the MS from a large number of metal ions and from molecular
hydrogen is detected in up to seven absorption components, indicating the
presence of multi-phase gas. From the analysis of unsaturated SII absorption,
in combination with a detailed photoionization model, we obtain a surprisingly
high alpha abundance in the Stream toward Fairall 9 of [S/H]=-0.30pm0.04 (0.5
solar). This value is 5 times higher than what is found along other MS
sightlines based on similar COS/UVES data sets. In contrast, the measured
nitrogen abundance is found to be substantially lower ([N/H]=-1.15pm0.06),
implying a very low [N/alpha] ratio of -0.85 dex. The substantial differences
in the chemical composition of MS toward Fairall 9 compared to other sightlines
point toward a complex enrichment history of the Stream. We favour a scenario,
in which the gas toward Fairall 9 was locally enriched with alpha elements by
massive stars and then was separated from the Magellanic Clouds before the
delayed nitrogen enrichment from intermediate-mass stars could set in. Our
results support (but do not require) the idea that there is a metal-enriched
filament in the Stream toward Fairall 9 that originates in the LMC.
