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

TL;DR

This study uses multi-wavelength data to analyze the Magellanic Stream near Fairall 9, revealing a complex chemical enrichment history with higher alpha-element abundance and lower nitrogen levels than other sightlines, suggesting localized enrichment processes.

Contribution

It provides the first detailed chemical abundance analysis of the Magellanic Stream toward Fairall 9, highlighting a unique enrichment history with high alpha-elements and low nitrogen.

Findings

High alpha abundance of [S/H]=-0.30 in the Stream toward Fairall 9.

Significantly lower nitrogen abundance of [N/H]=-1.15 compared to other sightlines.

Evidence for a locally enriched, metal-rich filament originating from the LMC.

Abstract

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.