[Paper Review] Nitrogen line spectroscopy in O-stars -- II. Surface nitrogen abundances for O-stars in the Large Magellanic Cloud

J. G. Rivero González, J. Puls+2 more|arXiv (Cornell University)|Oct 24, 2011

Stellar, planetary, and galactic studies80 references62 citations

TL;DR

This study presents nitrogen abundance measurements for 20 O-stars and 5 B-stars in the Large Magellanic Cloud using N iv λ4058 line spectroscopy and NLTE atmosphere modeling. It finds strong nitrogen enrichment across most O-stars, with a clear correlation to helium abundance and evidence of efficient mixing even at low rotation, challenging standard rotational mixing models and supporting early-phase mixing in massive stars.

ABSTRACT

This is the second paper in a series aiming at the analysis of nitrogen abundances in O-type stars, to enable further constraints on the early evolution of massive stars. We investigate the NIV lambda4058 emission line formation, provide nitrogen abundances for a substantial O-star sample in the LMC, and compare our findings with recent predictions from stellar evolutionary models. Stellar and wind parameters are determined by line profile fitting of hydrogen, helium and nitrogen lines, based on synthetic spectra calculated by FASTWIND. We derive nitrogen abundances for 20 O- and 5 B-stars, by analyzing all nitrogen lines present in the available optical spectra. The dominating process responsible for emission at NIV lambda4058 in O-stars is the strong depopulation of the lower level of the transition, which increases as a function of Mdot. Unlike the NIII triplet emission, resonance lines do not play a role for typical mass-loss rates and below. The bulk of our sample O-stars seems to be strongly nitrogen-enriched, and a clear correlation of nitrogen and helium enrichment is found. By comparing the nitrogen abundances as a function of vsini ('Hunter-plot') with tailored evolutionary calculations, we identify a considerable number of highly enriched objects at low rotation. Due to the low initial abundance, the detection of strong Nitrogen enrichment in the bulk of O-stars indicates that efficient mixing takes place already during the very early phases of stellar evolution of LMC O-stars.

Motivation & Objective

• To measure nitrogen surface abundances in O-stars in the Large Magellanic Cloud (LMC), where baseline nitrogen is low, enabling clearer detection of enrichment.
• To investigate the formation mechanisms of the N iv λ4058 emission line, particularly the role of lower-level depopulation due to wind effects.
• To test predictions of rotational mixing in massive star evolution by comparing nitrogen abundances with stellar rotation (v sin i) and wind parameters.
• To extend the findings from B-star studies in the VLT-FLAMES survey to O-stars, which have shorter lifetimes and thus constrain early evolutionary mixing more tightly.
• To assess the consistency of nitrogen abundance determinations across different spectral types and analysis methods, especially in the O/B star transition regime.

Proposed method

• Stellar and wind parameters (T_eff, log g, v sin i, v_mic, v_mac, dot{M}) were derived via 'by eye' fitting of hydrogen, helium, and nitrogen line profiles, using ionization equilibrium constraints.
• NLTE synthetic spectra were computed using the fastwind code with a newly developed nitrogen model atom to accurately simulate line formation in hot, massive stars.
• Nitrogen abundances were determined by adjusting the N abundance to match all observable nitrogen lines (N iii, N iv, N v) in the optical spectra, with curve-of-growth analysis used in select cases.
• The N iv λ4058 emission line formation was analyzed, revealing that its strength is primarily driven by depopulation of the lower level, which increases with mass-loss rate (dot{M}).
• A comparison was made between observed nitrogen abundances and tailored evolutionary models, particularly in v sin i vs. [N] diagrams ('Hunter-plots').
• Consistency checks were performed against previous studies (e.g., Hunter et al. 2007, Mok07), with adjustments to stellar parameters to ensure robust abundance derivation.

Experimental results

Research questions

• RQ1 What is the dominant physical mechanism responsible for the formation of the N iv λ4058 emission line in O-stars?
• RQ2 To what extent are O-stars in the Large Magellanic Cloud nitrogen-enriched, and how does this enrichment correlate with rotation and helium abundance?
• RQ3 How do the derived nitrogen abundances compare with predictions from rotational mixing models, particularly in the context of low-rotation, highly enriched stars?
• RQ4 Can consistent nitrogen abundances be derived across different spectral types (O and B stars) using the same analysis framework?
• RQ5 What do the observed nitrogen abundances imply about the timing and efficiency of mixing processes in the early evolutionary phases of massive stars?

Key findings

• The N iv λ4058 emission line is primarily formed through strong depopulation of its lower level, which increases with mass-loss rate (dot{M}), rather than through resonance line pumping.
• The majority of the analyzed O-stars (20 objects) show strong nitrogen enrichment, with [N] ranging from 7.5 to 8.1, and five objects exhibiting extreme enrichment at [N] = 8.17 to 8.5.
• A clear correlation exists between nitrogen and helium surface abundances, indicating co-evolving surface enrichment processes.
• A significant number of highly nitrogen-enriched stars are found at low rotational velocities (v sin i), challenging standard rotational mixing models that predict minimal enrichment without fast rotation.
• The observed nitrogen abundances are consistent with previous studies in overlapping B-star domains, with differences of only 0.1–0.3 dex, primarily due to improved stellar parameters.
• The low initial nitrogen abundance in the LMC (baseline [N] = 6.9) makes the observed strong enrichment in most O-stars a strong indicator of efficient mixing during the very early phases of stellar evolution.