The COS/UVES Absorption Survey of the Magellanic Stream: II. Evidence for a complex enrichment history of the Stream from the Fairall 9 sightline. (arXiv:1304.4242v1 [astro-ph.GA])

The COS/UVES Absorption Survey of the Magellanic Stream: II. Evidence for a complex enrichment history of the Stream from the Fairall 9 sightline. (arXiv:1304.4242v1 [astro-ph.GA]):
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 and GASS to study metal abundances and
physical conditions in the Stream toward the quasar Fairall 9. Line absorption
in the MS is detected from a large number of metal ions and from molecular
hydrogen. 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 [alpha/N] ratio of
-0.85 dex. The substantial differences in the chemical composition of the
Magellanic Stream toward Fairall 9 compared to other lines of sight, together
with the presence of dust and molecular gas, point toward a complex enrichment
history of the Magellanic Stream. We favour a scenario, in which the gas was
locally enriched with alpha elements by massive stars in a star-forming region
in the SMC or LMC and then was expelled and/or stripped from the stellar disk
before the delayed nitrogen enrichment from intermediate-mass stars could set
in (abridged).