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

Abstract

Sensitive measurements of the interstellar gas-phase oxygen abundance have revealed a slight oxygen deficiency (~15%) toward stars within 500 pc of the Sun as compared to more distant sight lines. 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 sight lines. By considering only the highest quality (>=5 σ) N/O abundance measurements, we find an intriguing trend in the interstellar N/O ratio with distance. Toward the seven stars within ~500 pc of the Sun, the weighted mean N/O ratio is 0.217 +/- 0.011, while for the six stars farther away the weighted mean value (N/O = 0.142 +/- 0.008) 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 (e.g., dust depletion or ionization) variations alone that explains this abundance anomaly. Is the enhanced nitrogen abundance localized to the solar neighborhood or evidence of a more widespread phenomenon? If it is localized, then recent infall of low-metallicity gas in the solar neighborhood may be the best explanation. Otherwise, the N/O variations may be best explained by large-scale differences in the interstellar mixing processes for AGB stars and Type II supernovae.

Based on data obtained by the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE) mission operated by the Johns Hopkins University. Financial support to US participants has been provided by NASA contract NAS5-32985.