The IACOB project: XVII. Nitrogen abundances in Galactic O-type stars: further hints for separating binary-interaction products from effectively single stars

C. Mart\'inez-Sebasti\'an; F. Martins; G. Holgado; J. Puls; S. Sim\'on-D\'iaz

arxiv: 2604.26606 · v2 · submitted 2026-04-29 · 🌌 astro-ph.SR · astro-ph.GA

The IACOB project: XVII. Nitrogen abundances in Galactic O-type stars: further hints for separating binary-interaction products from effectively single stars

C. Mart\'inez-Sebasti\'an , G. Holgado , S. Sim\'on-D\'iaz , F. Martins , J. Puls This is my paper

Pith reviewed 2026-05-07 12:45 UTC · model grok-4.3

classification 🌌 astro-ph.SR astro-ph.GA

keywords O-type starsnitrogen abundanceshelium abundancesbinary evolutionrotational mixingmassive starsstellar evolutionGalactic stars

0 comments

The pith

Helium-rich O-type stars show nitrogen abundances outside single-star model predictions, indicating binary origins.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper measures nitrogen abundances for 117 Galactic O-type stars with projected rotations below 150 km/s and luminosity classes V to III. Stars are grouped by helium content into low, normal, and rich categories, and their nitrogen distributions are compared against single-star evolution models that vary in mixing prescriptions. Helium-normal stars display nitrogen levels that roughly match models with efficient mixing for luminosity classes IV and V, yet the same models overpredict nitrogen for class III stars. Helium-rich stars instead follow a bimodal nitrogen pattern with peaks near 8.1 and 8.5 dex that no single-star model can reproduce. These patterns lead to the claim that helium-rich stars arise from binary interactions while rotational mixing alone cannot account for all observed nitrogen in helium-normal stars.

Core claim

Differences in the N abundance distributions are identified across three He abundance regimes. For the He-normal group the distribution peaks slightly above the birth value and reaches up to 8.4 dex, showing broad agreement with single-star models that include efficient internal mixing and moderate-to-low initial rotation. In contrast the He-rich group exhibits a bimodal N distribution with peaks near 8.1 and 8.5 dex; none of these stars match single-star predictions. He-rich stars are therefore argued to be binary products. The mismatch between models and observations for luminosity class III stars among the He-normal group further shows that rotational mixing cannot explain the full N-abun

What carries the argument

Separation of O-type stars into helium abundance regimes (He-low, He-normal, He-rich) used as a proxy to distinguish binary-interaction products from effectively single stars, with nitrogen abundance acting as the diagnostic of internal mixing history.

If this is right

Helium-rich O stars must be treated as binary products when constructing samples for single-star evolutionary studies.
Single-star models require reduced mixing efficiency to avoid overpredicting nitrogen in luminosity class III O stars.
Binary interaction channels are required to reproduce the full range of observed nitrogen enrichments in massive stars.
The nitrogen distribution among helium-normal stars provides an observational constraint on the efficiency of rotational mixing.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Many apparently single O stars may carry chemical signatures of past binary mass transfer or merger events.
Helium-nitrogen correlations could be applied in other galaxies to estimate the fraction of massive stars that experienced binary interactions.
Additional mixing processes beyond rotation, such as internal gravity waves or magnetic fields, may be needed to explain the nitrogen abundances of evolved O stars.

Load-bearing premise

Helium abundance cleanly separates binary interaction products from single stars without significant misclassification, and the chosen single-star models with varying mixing prescriptions cover the full range of outcomes possible for non-interacting stars.

What would settle it

Detection of helium-rich O-type stars whose nitrogen abundances fall inside the range predicted by single-star models at the stars' measured temperatures and luminosities would undermine the binary-product interpretation.

Figures

Figures reproduced from arXiv: 2604.26606 by C. Mart\'inez-Sebasti\'an, F. Martins, G. Holgado, J. Puls, S. Sim\'on-D\'iaz.

**Figure 1.** Figure 1: Distribution in a spectroscopic HR diagram of the sample of Galactic O-type stars considered in this work (large filled symbols). For reference purposes, the full analyzed IACOB sample of O-type stars included as small size circles; red and yellow symbols for LC I and II. GENEC evolutionary tracks during the Main Sequence are plotted for vini/vcrit = 0 with solid lines. Dotted lines for gravities decreasin… view at source ↗

**Figure 2.** Figure 2: Example of the mapping of N abundance as a function of microturbulence. Colored lines show the N abundance that minimizes Ml as a function of microturbulence for each of the analyzed lines in HD 206 183. The horizontal dashed line marks the ξ value determined with the N iii multiplet. The colored X marker corresponds to the abundance of each line at this microturbulence. The dotted line indicates the ave… view at source ↗

**Figure 3.** Figure 3: Example of the diagnostic plot used to correct possible systematic effects in the abundance determination. It shows abundances of the different analyzed lines in HD 206 183 for a fixed microturbulence velocity. Green and red symbols correspond to N ii and N iii lines, respectively. Solid lines in the same colors indicate the average abundance for each ion, with the shaded areas being their 1-σ confidenc… view at source ↗

**Figure 4.** Figure 4: Example of the evaluation of the N abundance fit in selected spectral regions. Orange vertical lines denote the nitrogen transitions employed in the abundance determination; dotted gray vertical lines indicate other N lines present in the model but excluded from the analysis. Note that at the temperature of this object, N iii 4642 is dominated by an overlapping O ii line (Rivero González et al. 2011). 7.5 … view at source ↗

**Figure 5.** Figure 5: Comparison of N abundances derived in this work with values from previous studies. Different panels for different abundance analysis methods and atmosphere codes (see text). Transparent symbols correspond to stars with discrepant comparisons (beyond uncertainties). Vertical and horizontal dotted lines indicate the CAS values at the solar position (Nieva & Przybilla 2012). The solid and dashed diagonal line… view at source ↗

**Figure 6.** Figure 6: Distribution of the sample in the N against He abundance plane (cf view at source ↗

**Figure 7.** Figure 7: Observational histograms (steps) and probability density estimates (solid lines) of ϵN for the sample, separated –from top to bottom– into He-low, He-normal, and He-rich (red, green, and blue, respectively). The CAS ϵN value from Nieva & Przybilla (2012) is indicated by the shaded vertical band. Vertical dotted lines mark the threshold separating N-low stars. Black dashed bins highlight ON stars. overplo… view at source ↗

**Figure 8.** Figure 8: Top: Surface N abundance versus projected rotational velocity. From left to right, He-low, normal, and rich sample. For reference pupposes, we include evolutionary tracks for vini/vcrit = 0.2 and 0.4 from GENEC (Ekström et al. 2012, and priv. comm.; dotted and dashed lines; Mini = 20, 25, 32, and 40 M⊙) and MESA models adopting the Mix1 prescription from Keszthelyi et al. (2022) (equivalent to Brott et al.… view at source ↗

**Figure 9.** Figure 9: sHRD of the analyzed sample, with color and marker size scaled to N abundance. Solid black squares highlight stars with YHe >0.12 and ϵN > 8.45 dex. As a reference, GENEC tracks with vini/vcrit = 0, 0.2, and 0.4 (solid, dotted and dashed lines, respectively.) Blue segments correspond to phases with ϵN ≥ 8.45 dex. 2025). Moreover, the fact that they are far from critical rotation would point to case A mass … view at source ↗

**Figure 10.** Figure 10: Left: N abundance as a function of surface gravity for the He-normal subsample. The dotted and dashed lines correspond to GENEC evolutionary tracks with vini/vcrit = 0.2 and 0.4, respectively (the latter only including tracks for Mini = 20 M⊙ and Mini = 40 M⊙). The blue segments indicate regions where models reach YHe≥ 0.12. Middle: Same targets as left but using Mix1 MESA models with vini/vcrit = 0.2 and… view at source ↗

read the original abstract

Context. Growing evidence is revealing the crucial role of binarity in massive star evolution. This affects evolution models and demands a refinement of the available observational constraints. Aims. To investigate the possible evolutionary origins of a sample of 117 Galactic O-type stars with luminosity classes V to III and projected rotational velocities below 150 ${\rm km}\,{\rm s}^{-1}$. Methods: We extend previous quantitative spectroscopic analyses performed within the framework of the IACOB project and obtain N abundance estimates. We investigate correlations between these abundances and other stellar parameters. As a reference, we use predictions from single-star evolution models computed using different physical prescriptions. Results. We identify differences in the N abundance distributions corresponding to three He abundance regimes (He-low, He-normal, and He-rich). For the He-normal group, the N abundance distribution peaks slightly above the expected birth value and extends up to $\epsilon_{\rm N}$=8.4 dex. For these stars, we find overall agreement with single-star evolutionary models that include efficient internal mixing and assume moderate-to-low initial rotation. In contrast, the He-rich group exhibits a bimodal N abundance distribution, with one peak at $\sim$8.1 dex and a second more enriched peak around $\sim$8.5 dex; none of these stars are consistent with predictions from single-star evolutionary models. We argue that He-rich stars are most plausibly explained as binary products. Furthermore, despite N abundance in He-normal stars with LC IV and V are reproduced by single stellar evolutionary models with efficient mixing, the same models predict a higher N abundance than observed for stars with LC III. This indicates that rotational mixing alone is unable to explain the observed distribution of N abundances among stars with normal He abundances.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

New N abundances for 117 O stars flag He-rich objects as probable binary products and show LC III stars with normal He have less nitrogen than single-star models predict.

read the letter

The paper adds nitrogen abundance measurements to the IACOB sample of Galactic O stars and uses them to separate likely binary products from the rest. He-rich stars sit outside every single-star track tested, while He-normal stars with luminosity classes IV and V line up with models that include efficient mixing and moderate initial rotation. The LC III subset with normal helium, however, shows lower N than those same models produce. The authors conclude rotational mixing alone cannot account for the full distribution in the normal-He group. That is the main new result: an observational split backed by 117 stars and direct model comparisons. The work does what the IACOB series has done before—steady quantitative spectroscopy with consistent analysis—but now ties the abundances explicitly to helium content and luminosity class. The distributional plots and the clean mismatch for the He-rich group are the strongest parts. The argument is straightforward and the sample size is respectable for this type of study. The soft spot is the reliance on existing single-star grids without a detailed check that those grids actually cover the observed range of masses, effective temperatures, and surface gravities for the LC III stars. If the models are systematically too evolved or have the wrong initial conditions for that subsample, the over-prediction of N could be an artifact rather than proof that mixing is insufficient. The abstract does not spell out the full error budget or how the model parameters were chosen to match the data, so those sections will need close reading. This paper is aimed at people who model massive-star populations or who need clean samples of single O stars. Anyone running population synthesis or testing binary channels will want to see the numbers. It is solid enough and specific enough to deserve referee time; the claims are testable and the data are new. I would send it out for review rather than desk-reject.

Referee Report

2 major / 2 minor

Summary. The paper analyzes N abundances in 117 Galactic O-type stars (v sin i < 150 km/s, LC V-III) from the IACOB project, identifying three He-abundance regimes. He-normal stars show N distributions peaking slightly above birth values and extending to 8.4 dex, broadly consistent with single-star models that include efficient mixing and moderate-to-low initial rotation for LC IV/V but overpredicted for LC III. He-rich stars exhibit a bimodal N distribution (peaks ~8.1 and ~8.5 dex) incompatible with all tested single-star prescriptions, leading to the claim that they are binary-interaction products and that rotational mixing alone cannot explain the full N distribution in He-normal stars.

Significance. If the model-observation comparisons hold after verifying parameter matching, the work strengthens evidence for binarity's role in massive-star evolution and highlights deficiencies in current rotational-mixing prescriptions for evolved O stars. It supplies a concrete observational diagnostic (He + N) for separating binary products from single stars and motivates targeted updates to evolutionary grids.

major comments (2)

[model comparisons] § on model comparisons (model grid and N predictions): the central claim that single-star models with efficient mixing overpredict N for He-normal LC III stars (while matching LC IV/V) requires explicit verification that the chosen tracks pass through the observed (T_eff, log g, mass) loci of the LC III subsample. Without showing the distribution of model N values at the precise observed parameters (including any systematic offsets in overshooting or initial rotation), the discrepancy could arise from incomplete coverage of the model parameter space rather than missing physics.
[Results] Results section on He-rich subsample: the assertion that none of the He-rich stars are consistent with single-star models depends on the completeness of the tested prescriptions. The paper should quantify how the full range of initial masses, rotations, and mixing efficiencies in the grid maps onto the observed He-rich parameter space; if the grid does not encompass plausible single-star outcomes at the observed T_eff and log g, the binary-product interpretation rests on an untested assumption.

minor comments (2)

[Abstract] Abstract: the sentence 'despite N abundance in He-normal stars with LC IV and V are reproduced...' contains a grammatical error (subject-verb agreement); rephrase for clarity.
[Methods] The paper should include a table or figure summarizing the error budget on N abundances and the criteria used to assign He regimes, to allow readers to assess robustness of the distributional differences.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and detailed comments, which have prompted us to clarify and strengthen key aspects of our analysis. We address each major comment point by point below.

read point-by-point responses

Referee: [model comparisons] § on model comparisons (model grid and N predictions): the central claim that single-star models with efficient mixing overpredict N for He-normal LC III stars (while matching LC IV/V) requires explicit verification that the chosen tracks pass through the observed (T_eff, log g, mass) loci of the LC III subsample. Without showing the distribution of model N values at the precise observed parameters (including any systematic offsets in overshooting or initial rotation), the discrepancy could arise from incomplete coverage of the model parameter space rather than missing physics.

Authors: We agree that explicit verification of model coverage at the observed parameters is essential to support our interpretation. Our original analysis selected tracks from multiple grids that encompass the T_eff, log g, and mass ranges of the full sample, including the He-normal LC III stars. To address the referee's concern directly, the revised manuscript will include a new figure overlaying the relevant evolutionary tracks on the observed (T_eff, log g) diagram for the LC III subsample, together with the interpolated N-abundance distributions at those precise loci (accounting for the range of overshooting and initial rotation in the grids). This addition will demonstrate that the overprediction for LC III stars persists even when parameter matching is enforced, while the models remain consistent with the LC IV/V observations. revision: yes
Referee: [Results] Results section on He-rich subsample: the assertion that none of the He-rich stars are consistent with single-star models depends on the completeness of the tested prescriptions. The paper should quantify how the full range of initial masses, rotations, and mixing efficiencies in the grid maps onto the observed He-rich parameter space; if the grid does not encompass plausible single-star outcomes at the observed T_eff and log g, the binary-product interpretation rests on an untested assumption.

Authors: We appreciate the referee's emphasis on demonstrating grid completeness for the He-rich stars. The prescriptions we employed already span a wide parameter space (initial masses 15–60 M⊙, initial rotations 0–300 km s⁻¹, and varying mixing efficiencies), as detailed in the methods. In the revision we will add a quantitative mapping—via an additional table or supplementary plot—of the maximum N enrichment attainable across the full grid at the exact observed T_eff and log g values of the He-rich subsample. This will confirm that no single-star combination reproduces the observed bimodal N distribution while remaining consistent with the measured He abundances and other constraints, thereby supporting the binary-interaction interpretation. revision: yes

Circularity Check

0 steps flagged

No significant circularity: observations compared directly to independent external model grids

full rationale

The paper's central claims rest on comparing measured N abundances (and He regimes) against predictions from pre-existing single-star evolutionary models that incorporate varying mixing prescriptions. These models are invoked as an external reference rather than being fitted, tuned, or derived within the present work. No load-bearing self-citations, self-definitional loops, or renaming of fitted quantities as predictions appear in the derivation chain. The reported discrepancies for He-normal LC III stars and the incompatibility of He-rich stars therefore constitute an independent test against external benchmarks, consistent with a score of 0.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the accuracy of published single-star evolution grids and on the assumption that helium abundance cleanly separates evolutionary channels; no new free parameters or entities are introduced in this work.

axioms (1)

domain assumption Single-star evolutionary models with efficient internal mixing and moderate-to-low initial rotation provide a reliable reference for the expected nitrogen enrichment in effectively single O stars.
Invoked when stating agreement or disagreement with model predictions for the He-normal and He-rich groups.

pith-pipeline@v0.9.0 · 5654 in / 1277 out tokens · 45325 ms · 2026-05-07T12:45:33.448946+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

6 extracted references · 6 canonical work pages

[1]

Anderson, T. W. & Darling, D. A. 1954, Journal of the American statistical as- sociation, 49, 765 Aschenbrenner, P., Przybilla, N., & Butler, K. 2023, A&A, 671, A36 Asplund, M., Grevesse, N., Sauval, A. J., & Scott, P. 2009, ARA&A, 47, 481 Barbá, R. H., Gamen, R., Arias, J. I., et al. 2010, in Revista Mexicana de As- tronomia y Astrofisica Conference Seri...

work page arXiv 1954
[2]

The results of both analyses, together with the original model parameters, are summarized in Table C.1

To estimate the effect the fun- damental parameters determination itself, we repeat the abun- dance analysis with the original model parameters. The results of both analyses, together with the original model parameters, are summarized in Table C.1. The second column lists the original parameters used to generate the synthetic spec- trum. The third column ...

work page 2025
[3]

To assess this effect, we performed a consistency check by reanalyzing our sample with IACOB-GBAT (Simón-Díaz et al

50 YHe 0.10 0.10±0.02 0.10 ξ[km/s] 11 8 12 ϵN [dex] 8.1 8.15±0.11 8.09±0.08 tially affecting the conclusions of this work. To assess this effect, we performed a consistency check by reanalyzing our sample with IACOB-GBAT (Simón-Díaz et al. 2011; Sabín-Sanjulián et al. 2014; Holgado et al. 2018), fixing the microturbulence to the values derived in this stu...

work page 2011
[4]

We find that the differences between groups 0.05 0.1 0.15 0.2 0.05 0.1 0.15 0.2 0.17 0.29 0.37 0.44 Y (ξHe) 0.17 0.29 0.37 0.44 Y (ξN) YHe(ξHe) YHe(ξN) Fig

To quantify possible dif- ferences, we compare the number of stars in each helium group, as well as the fraction of SB1 systems, using bothY He(ξHe) and YHe(ξN) (Table C.3). We find that the differences between groups 0.05 0.1 0.15 0.2 0.05 0.1 0.15 0.2 0.17 0.29 0.37 0.44 Y (ξHe) 0.17 0.29 0.37 0.44 Y (ξN) YHe(ξHe) YHe(ξN) Fig. C.2.Comparison of the He a...

work page 2026
[5]

due to the self consistency of the methodology. Appendix C.3: Impact of microturbulence on the estimation of nitrogen abundances We have shown that the impact of a microturbulence uncertainty of∼3 km s −1 is smaller than the typical abundance uncertainty (App. C.1). This effect may become relevant only when the mi- croturbulence error is large or when it ...

work page 2012
[6]

C.5.Results of the analysis of synthetic spectra from FASTWIND models

0.06 0.08 0.10 0.12 0.14 0.16 0.18 YHe =He/H 7.6 7.8 8.0 8.2 8.4 8.6 8.8 ϵN = 12 + log(N /H) 0.19 0.24 0.29 0.32 0.36 0.39 0.42 Y = mHe/mtot Fig. C.5.Results of the analysis of synthetic spectra from FASTWIND models. Six parameter combinations:T eff–logg=32 kK–3.5 dex, 35 kK–3.5 dex, and 40 kK–3.9 dex (red, green, and blue symbols, re- spectively), each w...

work page 2012

[1] [1]

Anderson, T. W. & Darling, D. A. 1954, Journal of the American statistical as- sociation, 49, 765 Aschenbrenner, P., Przybilla, N., & Butler, K. 2023, A&A, 671, A36 Asplund, M., Grevesse, N., Sauval, A. J., & Scott, P. 2009, ARA&A, 47, 481 Barbá, R. H., Gamen, R., Arias, J. I., et al. 2010, in Revista Mexicana de As- tronomia y Astrofisica Conference Seri...

work page arXiv 1954

[2] [2]

The results of both analyses, together with the original model parameters, are summarized in Table C.1

To estimate the effect the fun- damental parameters determination itself, we repeat the abun- dance analysis with the original model parameters. The results of both analyses, together with the original model parameters, are summarized in Table C.1. The second column lists the original parameters used to generate the synthetic spec- trum. The third column ...

work page 2025

[3] [3]

To assess this effect, we performed a consistency check by reanalyzing our sample with IACOB-GBAT (Simón-Díaz et al

50 YHe 0.10 0.10±0.02 0.10 ξ[km/s] 11 8 12 ϵN [dex] 8.1 8.15±0.11 8.09±0.08 tially affecting the conclusions of this work. To assess this effect, we performed a consistency check by reanalyzing our sample with IACOB-GBAT (Simón-Díaz et al. 2011; Sabín-Sanjulián et al. 2014; Holgado et al. 2018), fixing the microturbulence to the values derived in this stu...

work page 2011

[4] [4]

We find that the differences between groups 0.05 0.1 0.15 0.2 0.05 0.1 0.15 0.2 0.17 0.29 0.37 0.44 Y (ξHe) 0.17 0.29 0.37 0.44 Y (ξN) YHe(ξHe) YHe(ξN) Fig

To quantify possible dif- ferences, we compare the number of stars in each helium group, as well as the fraction of SB1 systems, using bothY He(ξHe) and YHe(ξN) (Table C.3). We find that the differences between groups 0.05 0.1 0.15 0.2 0.05 0.1 0.15 0.2 0.17 0.29 0.37 0.44 Y (ξHe) 0.17 0.29 0.37 0.44 Y (ξN) YHe(ξHe) YHe(ξN) Fig. C.2.Comparison of the He a...

work page 2026

[5] [5]

due to the self consistency of the methodology. Appendix C.3: Impact of microturbulence on the estimation of nitrogen abundances We have shown that the impact of a microturbulence uncertainty of∼3 km s −1 is smaller than the typical abundance uncertainty (App. C.1). This effect may become relevant only when the mi- croturbulence error is large or when it ...

work page 2012

[6] [6]

C.5.Results of the analysis of synthetic spectra from FASTWIND models

0.06 0.08 0.10 0.12 0.14 0.16 0.18 YHe =He/H 7.6 7.8 8.0 8.2 8.4 8.6 8.8 ϵN = 12 + log(N /H) 0.19 0.24 0.29 0.32 0.36 0.39 0.42 Y = mHe/mtot Fig. C.5.Results of the analysis of synthetic spectra from FASTWIND models. Six parameter combinations:T eff–logg=32 kK–3.5 dex, 35 kK–3.5 dex, and 40 kK–3.9 dex (red, green, and blue symbols, re- spectively), each w...

work page 2012