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 ($\sim$ 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 ($\ge$ 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${}_{-0.18}^{+0.20}$). 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.

accepted for publication in the Astrophysical Journal Letters

Cited by in corpus (12)

• A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
• The Nitrogen and Oxygen abundances in the neutral gas at high redshift
• Connecting the primordial and Galactic deuterium abundances
• Interstellar Krypton Abundances: The Detection of Kiloparsec-scale Differences in Galactic Nucleosynthetic History
• The Galactic Deuterium Abundance and Dust Depletion: Insights From an Expanded Ti/H Sample
• Interstellar Extinction and Elemental Abundances
• The Distribution of Metallicities in the Local Galactic Interstellar Medium
• Suzaku and XMM-Newton Observations of the North Polar Spur: Charge Exchange or ISM Absorption?
• Origin and evolution of the light nuclides
• Interstellar Extinction and Elemental Abundances: Individual Sight Lines
• Si and Fe depletion in Galactic star-forming regions observed by the Spitzer Space Telescope
• Nitrogen X-ray absorption in the local ISM