[Paper Review] Evolution of CNO abundances in the Universe

Nikos Prantzos|arXiv (Cornell University)|Jan 3, 2003

Stellar, planetary, and galactic studies3 citations

TL;DR

This paper reviews the latest stellar yields of CNO elements, emphasizing the impact of rotation and mass loss on nucleosynthesis. It demonstrates that rotating stars produce primary nitrogen, while non-rotating stars yield secondary nitrogen, and shows that observed C/O and N/O ratios in the Milky Way and extragalactic systems require late carbon sources and low star formation efficiencies to explain plateau-like abundance trends at low metallicities.

ABSTRACT

After summarizing the most important features of current stellar yields of CNO elements (including recent results concerning rotating and mass losing stars) I discuss how these yields may help to interpret relevant observations in the local Galaxy, the Milky Way disk and extragalactic systems (extragalactic HII regions and DLAs).

Motivation & Objective

• To synthesize recent advances in stellar yields of CNO elements, particularly focusing on rotating and mass-losing stars.
• To interpret observed CNO abundance patterns in the solar neighborhood, the Milky Way disk, and extragalactic systems such as HII regions and DLAs.
• To assess the role of primary vs. secondary nitrogen production and the need for late carbon sources in galactic chemical evolution.
• To evaluate the implications of observed abundance plateaus in N/O and C/O for star formation efficiency and evolutionary timescales in low-metallicity systems.

Proposed method

• Uses updated stellar yield models from van den Hoek & Grownewegen (1997), Woosley & Weaver (1995), and Meynet & Maeder (2002), including metallicity-dependent yields.
• Compares non-rotating and rotating stellar models to isolate the effects of rotational mixing on nitrogen production.
• Applies galactic chemical evolution (GCE) models with varying star formation efficiencies to simulate observed abundance trends in O/H space.
• Analyzes observational data on C/O and N/O ratios in HII regions, DLAs, and Milky Way stars to test model predictions.
• Evaluates the consistency of observed abundance plateaus with theoretical models of primary nitrogen from intermediate-mass stars and secondary nitrogen from massive stars.
• Uses the observed decline in O/Fe with increasing metallicity to infer the role of Type Ia supernovae as a late iron source in the disk phase.

Experimental results

Research questions

• RQ1 How do rotational mixing and mass loss affect the yields of CNO elements, particularly nitrogen, in stars of different masses and metallicities?
• RQ2 What determines whether nitrogen is produced as a primary or secondary element in stellar populations?
• RQ3 Why do N/O and C/O abundance ratios exhibit plateau-like behavior at low metallicities in extragalactic systems?
• RQ4 Can simple galactic chemical evolution models with varying star formation efficiency reproduce the observed abundance trends in HII regions and DLAs?
• RQ5 What are the implications of the observed C/O and N/O plateaus for the timing and efficiency of star formation in low-metallicity galaxies?

Key findings

• Rotating stars of all masses produce primary nitrogen via rotational mixing, especially prominent at low metallicities and in intermediate-mass stars.
• Non-rotating stars produce nitrogen as secondary, while hot-bottom burning in intermediate-mass stars leads to nearly primary nitrogen yields.
• The observed plateau in N/O at low O/H (log(O/H) < -4) is best explained by systems with low star formation efficiency and sufficient age for intermediate-mass stars to contribute.
• The plateau in C/O at low O/H is consistent with a late source of carbon, but not a secondary behavior, suggesting a delayed or distinct carbon production mechanism.
• The high N/O values in HII regions and some DLAs at low metallicities are attributed to contributions from intermediate-mass stars, while low N/O values in other DLAs indicate younger systems dominated by massive stars.
• The observed decline in O/Fe with increasing metallicity in the solar neighborhood supports the role of Type Ia supernovae as a late iron source, with timescale constraints of ~1 Gyr.