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 j) 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 p 0.142 0.008) is curiously consistent with the current solar value (N/O p). It is difficult to imagine a scenario invoking environmental (e.g., dust 0.20 0.138 0.18 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.