AAS 207th Meeting, 8-12 January 2006
Session 81 ISM: From Dark to Blinded
Poster, Tuesday, 9:20am-6:30pm, January 10, 2006, Exhibit Hall

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[81.11] The Interstellar N/O Abundance Ratio: Evidence for Local Infall?

D. C. Knauth, D. M. Meyer, J. T. Lauroesch (Northwestern University)

Sensitive measurements of the interstellar gas-phase oxygen abundance have revealed a slight oxygen deficiency (~ 15%) toward stars within 800 pc of the Sun as compared to more distant sightlines. Recent FUSE observations of the interstellar gas-phase nitrogen abundance indicate larger variations, but no trends with distance were reported due to the significant measurement uncertainties for many sightlines. By considering only the highest quality (> 5 sigma) N/O abundance measurements, we find an intriguing trend in the interstellar N/O ratio with distance. Toward the six stars within ~ 500 pc of the Sun, the weighted mean N/O ratio is 0.230 ± 0.011, while for the nine stars further away the weighted mean value (N/O = 0.129 ± 0.004) is curiously consistent with the current Solar value (N/O = 0.138+0.20-0.18). It is difficult to imagine a scenario invoking environmental variations (e.g., dust depletion, ionization, etc.) alone that explains this abundance anomaly. The recent infall of low-metallicity gas in the Solar neighborhood has previously been suggested as a possible explanation for the local interstellar oxygen deficiency. We revisit this idea in context of a nucleosynthetic origin for the local N/O enhancement.

The author(s) of this abstract have provided an email address for comments about the abstract: d-knauth@northwestern.edu

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