Wednesday, April 17, 2013

1304.4242 (Philipp Richter et al.)

The COS/UVES Absorption Survey of the Magellanic Stream: II. Evidence for a complex enrichment history of the Stream from the Fairall 9 sightline    [PDF]

Philipp Richter, Andrew J. Fox, Bart P. Wakker, Nicolas Lehner, J. Christopher Howk, Joss Bland-Hawthorn, Nadya Ben Bekhti, Cora Fechner
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).
View original: http://arxiv.org/abs/1304.4242

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