Sensitive measurements of the interstellar gas-phase oxygen abundance have revealed a slight oxygen deficiency ( \sim 15 % ) toward stars within 500 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 ( \geq 5 \sigma ) N/O abundance measurements , we find an intriguing trend in the interstellar N/O ratio with distance .
Toward the seven stars within \sim 500 pc of the Sun , the weighted mean N/O ratio is 0.217 \pm 0.011 , while for the six stars further away the weighted mean value ( N/O = 0.142 \pm 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 , ionization , etc . )
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 .