WN/WO Stars: Spectra, Winds, and Evolution

• WN/WO stars are evolved massive stars with distinct surface compositions—nitrogen-rich in WN and oxygen-rich in WO—that drive their unique spectral signatures.
• Spectroscopic diagnostics leverage line morphology and equivalent-width ratios of O VI, N IV, and C IV to classify subtypes and reveal wind properties.
• Their wind mass-loss rates and metallicity dependencies provide practical insights into stellar evolution and the progenitors of gamma-ray bursts.

Wolf–Rayet stars of the nitrogen and oxygen sequences, conventionally denoted WN and WO, are hot, evolved massive stars whose observed subtype is determined by both surface composition and the presence of an optically thick wind. In stripped-envelope evolutionary models, candidate WN stars have nitrogen-rich envelopes, whereas candidate WO stars have oxygen-rich surfaces with $X_O>0.05$ and occur after core-helium burning; whether such objects are observed as Wolf–Rayet stars rather than transparent-wind stripped-envelope stars depends on wind optical depth (Aguilera-Dena et al., 2021). Observationally, WN spectra are dominated by helium and nitrogen emission, while WO spectra are distinguished by very strong O VI emission, especially O VI $\lambda\lambda3811$–34, together with high excitation carbon and oxygen lines. WO stars represent an extremely rare and highly evolved subtype, but the exact phase ordering of WN, WC, and WO remains model-dependent and is now complicated by evidence for direct WN-to-WO transitions in low-metallicity environments (Tramper et al., 2014, Sander et al., 25 Aug 2025).

1. Spectral definition and nomenclature

The WN sequence comprises nitrogen-rich Wolf–Rayet stars, typically with helium-rich atmospheres and, depending on subtype and environment, either little hydrogen or a significant residual hydrogen fraction. In the comprehensive Large Magellanic Cloud analysis, WN stars are classified as WNE (early, WN2–WN5) or WNL (late, WN6–WN11), and among putatively single stars 43% are hydrogen-free while the remainder retain some hydrogen (Hainich et al., 2014). In theoretical stripped-envelope grids, a model is classified as candidate WN when its envelope is nitrogen-rich during core-helium burning, while candidate WO requires an oxygen-rich surface and a post-core-helium-burning state (Aguilera-Dena et al., 2021).

The WO sequence is chemically and spectroscopically distinct from both WN and WC stars. WO atmospheres are enriched in carbon and oxygen, with observed single WO stars showing $X_C=0.46$–$0.62$ and $X_O=0.10$–$0.25$, and they are hotter than typical WC stars, with $T_*=150$–$210$ kK (Tramper et al., 2014). The usual evolutionary shorthand WN $\rightarrow$ WC $\rightarrow$ WO therefore describes one long-standing framework, but not a universally accepted one; later sections summarize the observational and modeling tensions around this scheme (McClelland et al., 2016, Sander et al., 25 Aug 2025).

A separate nomenclatural issue concerns bracket notation. Objects such as [WN] and [WC] central stars of planetary nebulae mimic the spectra of massive Wolf–Rayet stars but are low-mass post-AGB nuclei rather than massive stellar descendants. The bracketed notation is therefore taxonomically essential and should not be conflated with massive WN and WO stars (Miszalski et al., 2012).

2. Spectroscopic diagnostics and atmospheric properties

Classification of WN and WO stars is anchored in line morphology, ionization balance, and width. For WO stars, the standard optical criteria use equivalent-width ratios of O VI $\lambda\lambda3811$0–34 / O V $\lambda\lambda3811$1, O VI $\lambda\lambda3811$2–34 / C IV $\lambda\lambda3811$3–12, and the FWHM of C IV $\lambda\lambda3811$4–12. WO spectra show extremely strong O VI emission, prominent O V and C IV, and very broad emission profiles; CMFGEN models reproduce most features but still under-predict the strength of O VI $\lambda\lambda3811$5–34 (Tramper et al., 2014). For WN stars, the dominant optical diagnostics include He II $\lambda\lambda3811$6, N IV $\lambda\lambda3811$7, N IV $\lambda\lambda3811$8, N V $\lambda\lambda3811$9, and N III $X_C=0.46$0, with the balance of N V, N IV, N III, He I, and He II varying systematically across subtype (Crowther et al., 2023).

Near-infrared classification is especially important in obscured Galactic sightlines. Refined YHJK criteria use He I $X_C=0.46$1m / He II $X_C=0.46$2m, He I $X_C=0.46$3m / He II $X_C=0.46$4m, Pa$X_C=0.46$5/He II $X_C=0.46$6m, and Br$X_C=0.46$7/He II $X_C=0.46$8m to separate WN subtypes and diagnose hydrogen. WO stars are recognized in the near-IR by strong O VI emission at $X_C=0.46$9, $0.62$0, and $0.62$1m relative to adjacent C IV or He II features (Rosslowe et al., 2017).

Class	Key diagnostics	Representative parameters
Late WN9h	Br$0.62$2, He I, He II; H present	$0.62$3, $0.62$4–35 kK, $0.62$5 up to 0.45, $0.62$6–900 km s$0.62$7, $0.62$8 to $0.62$9 (Liermann et al., 2010)
Single SMC WN	H detectable in all seven stars	$X_O=0.10$0–112 kK, $X_O=0.10$1–6.07, $X_O=0.10$2–0.55, $X_O=0.10$3–2200 km s$X_O=0.10$4 (Hainich et al., 2015)
Single WO	O VI $X_O=0.10$5–34, O V $X_O=0.10$6, C IV $X_O=0.10$7–12	$X_O=0.10$8–210 kK, $X_O=0.10$9–5.68, $0.25$0–0.62, $0.25$1–0.25 (Tramper et al., 2014)
WN3/O3	WN emission plus O3V-like absorption	$0.25$2–105{,}000 K, $0.25$3, $0.25$4 to $0.25$5, $0.25$6 (Neugent et al., 2017)
WN/WO transitional	Strong N V and O VI $0.25$7	$0.25$8–180 kK, $0.25$9–6.0, $T_*=150$0 to $T_*=150$1, $T_*=150$2–5500 km s$T_*=150$3 (Sander et al., 25 Aug 2025)

The atmospheric spread within the WN class is correspondingly large. Galactic-center WN9h stars in the Quintuplet cluster are cool for Wolf–Rayet stars, with $T_*=150$4–35 kK and significant hydrogen by mass, whereas single SMC WN stars are mostly above 75 kK and still hydrogen-rich (Liermann et al., 2010, Hainich et al., 2015). WO stars sit at the opposite thermal extreme, and their high excitation is consistent with extensive exposure of helium-burning products (Tramper et al., 2014).

3. Stellar winds, mass loss, and high-energy emission

Quantitative analysis of WN and WO stars is dominated by non-LTE expanding-atmosphere modeling, especially PoWR and CMFGEN. In the WN literature, a central scaling variable is the transformed radius,

$T_*=150$5

together with the wind-efficiency parameter

$T_*=150$6

which is typically $T_*=150$7 for LMC WNL stars and $T_*=150$8 for LMC WNE stars in the large PoWR survey (Hainich et al., 2014).

For hydrogen-free WC and WO stars, the empirical prescription

$T_*=150$9

fits the calibration sample with residual scatter $210$0 dex. The same functional form can also describe hydrogen-free WN stars if the metallicity term is strengthened to $210$1 (Tramper et al., 2016). This contrast is important: WC/WO winds exhibit only mild sensitivity to initial iron abundance in that calibration, whereas hydrogen-free WN stars require a steeper metallicity dependence.

Observed WN winds span a broad range. In the LMC sample, single WN stars have terminal wind velocities of $210$2–$210$3 km s$210$4 and mass-loss rates approximately $210$5 to $210$6, with the highest values associated with the most luminous and hydrogen-bearing objects (Hainich et al., 2014). In the SMC, WN stars have on average lower mass-loss rates and weaker winds than their counterparts in the Milky Way, M31, and the LMC; the empirical comparison across galaxies yields $210$7, while a multidimensional regression gives a metallicity exponent of $210$8 (Hainich et al., 2015).

WO stars are not weak-wind objects in an absolute sense, but they differ from WN stars in composition and in the luminosity, helium, and iron dependence of their mass-loss law (Tramper et al., 2016). By contrast, the LMC WN3/O3 stars are explicitly anomalous: they have effective temperatures and bolometric luminosities similar to other early-type LMC WNs, but mass-loss rates three to five times lower than expected (Neugent et al., 2017).

Single WN and WO stars are also X-ray sources, though modest ones. Early-type WN stars detected in pointed observations typically show $210$9–$\rightarrow$0 erg s$\rightarrow$1, while WR142, a WO2 star, has $\rightarrow$2 erg s$\rightarrow$3 and a very hard spectrum with $\rightarrow$4 MK. WR X-ray spectra are usually thermal, with plasma temperatures from $\rightarrow$5 MK up to tens of MK, and the most promising currently discussed mechanism for at least some single WN stars is the presence of corotating interaction regions in their winds (Oskinova, 2016).

4. Evolutionary channels and phase ordering

The evolutionary interpretation of WN and WO stars is no longer captured by a single consensus sequence. In a coupled Geneva-plus-CMFGEN calculation for a non-rotating $\rightarrow$6 star, the spectroscopic sequence runs from O3 I at the ZAMS through B supergiant, B hypergiant, hot and cool LBV, rapid evolution through late and early WN, early WC, and finally WO from the end of He-core burning until core collapse. The corresponding spectroscopic lifetimes are $\rightarrow$7 yr for WN, $\rightarrow$8 yr for WC, and $\rightarrow$9 yr for WO, with the spectroscopic WNE phase much longer and WNL much shorter than abundance-cut definitions would imply (Groh et al., 2014).

Pure helium-star calculations lead to a different global interpretation. In those models, varying mass-loss rate and envelope clumping changes the extent of envelope inflation and therefore the effective temperature, allowing the observed cool hydrogen-free WN stars to be reproduced. The same study concludes that WN and WO stars arise from more massive stars, whereas WC stars come from lower masses, explicitly contradicting the standard Conti scenario in which WN and WC stars form a simple age sequence (McClelland et al., 2016). A later observationally anchored study goes further by identifying five stars interpreted as direct WN-to-WO transition objects, arguing that some WR stars can evolve directly from the WN to the WO stage in low-metallicity environments with weaker winds (Sander et al., 25 Aug 2025).

Empirical population studies reinforce the diversity of channels. In the LMC WN census, 12% of putatively single stars are more luminous than $\rightarrow$0 and contain significant hydrogen, while 88% occupy $\rightarrow$1–5.8 with little or no hydrogen; the authors conclude that the very luminous group, if single, descended directly from the main sequence at very high initial masses, whereas the bulk likely passed through a red-supergiant phase and originated from initial masses between $\rightarrow$2 and $\rightarrow$3 (Hainich et al., 2014). In M31, late-type WN stars analyzed with PoWR all retain hydrogen and, if single, stem from an initial mass range between $\rightarrow$4 and $\rightarrow$5 (Sander et al., 2014). In the Quintuplet cluster, by contrast, WN9h stars have $\rightarrow$6, ages $\rightarrow$7–$\rightarrow$8 Myr, and may still be central hydrogen-burning objects (Liermann et al., 2010).

Binary evolution adds a further layer of complexity. For the LMC WN binaries, no obvious dichotomy in the locations of apparently single and binary WN stars on the Hertzsprung–Russell diagram is apparent. The orbital and spectroscopic analysis concludes that, according to commonly used stellar evolution models, most apparently single WN stars could not have formed as single stars; either they were stripped by an undetected companion, or pre-WR mass loss and mixing are strongly underestimated in standard models (Shenar et al., 2019).

5. Metallicity, populations, and galaxy-scale diagnostics

Metallicity is a first-order control parameter for both the formation and observability of WN and WO stars. The single SMC WN stars are hotter, more luminous, more hydrogen-rich, and substantially weaker-wind objects than typical WN stars in more metal-rich Local Group galaxies; the absence of any WC star and the presence of only a single WO primary in a binary system in the SMC were interpreted as evidence that strong WR winds are needed to expose the deeper carbon-rich layers (Hainich et al., 2015). In the LMC, the empirical WN mass-loss rates are about a factor of two lower than Galactic counterparts, consistent with $\rightarrow$9, even though the minimum initial masses inferred for many WN stars do not show the strong metallicity shift predicted by standard single-star models (Hainich et al., 2014).

Population synthesis based on stripped helium-star grids formalizes this picture using a wind-optical-depth criterion. The wind optical depth is approximated by

$\lambda\lambda3811$00

and the fitted minimum luminosities for WR visibility are

$\lambda\lambda3811$01

for WN stars and

$\lambda\lambda3811$02

for WC/WO stars. In these models, the number of transparent-wind stripped-envelope stars decreases and the number of Wolf–Rayet stars increases as metallicity rises; WC and WO stars become more common at high metallicity, although WN stars remain the majority (Aguilera-Dena et al., 2021).

Observed line luminosities in the optical follow the same general trend, especially for early WN stars. The mean He II $\lambda\lambda3811$03 luminosity for weak-lined WN2–5 stars declines from $\lambda\lambda3811$04 erg s$\lambda\lambda3811$05 in the Milky Way to $\lambda\lambda3811$06 erg s$\lambda\lambda3811$07 in the LMC and $\lambda\lambda3811$08 erg s$\lambda\lambda3811$09 in the SMC. For WO stars, O VI $\lambda\lambda3811$10 and C IV $\lambda\lambda3811$11 remain the defining extragalactic optical diagnostics, and the average line luminosities of Magellanic Cloud WO stars are higher than those of Galactic WO stars, although the statistics are small (Crowther et al., 2023). These calibrations matter directly for unresolved galaxies: broad He II $\lambda\lambda3811$12 tracks WN populations, while O VI $\lambda\lambda3811$13 is the cleanest indicator of WO stars.

Survey work has also shown that census incompleteness can bias subtype ratios. A modern Magellanic Cloud survey that was only 15% complete nevertheless found nine new WR stars in the LMC, including five or possibly six WN3/O3 stars, implying that the total WR population may have been underestimated by 10–40%. Because eight of the nine new stars were WN stars, the observed WC/WN ratio was shown to be biased high; Geneva rotating models at LMC metallicity predict a WC/WN ratio of 0.09, while the updated observed ratio was 0.23 (Massey et al., 2014).

6. Peculiar subclasses, transitional objects, and analogues

Among the most consequential WN-related discoveries are the LMC WN3/O3 stars. Their spectra show both strong WN3-like emission and O3V-like absorption, but their faint visual magnitudes, $\lambda\lambda3811$14 in the detailed 2017 analysis and $\lambda\lambda3811$15 to $\lambda\lambda3811$16 in the discovery paper, rule out an O3V companion, which would be much brighter. UV spectroscopy shows no C IV $\lambda\lambda3811$17, and single-star CMFGEN models reproduce both emission and absorption features without invoking binarity. These stars have $\lambda\lambda3811$18–105{,}000 K, $\lambda\lambda3811$19, $\lambda\lambda3811$20 to $\lambda\lambda3811$21, He/H by number of $\lambda\lambda3811$22–$\lambda\lambda3811$23, and nitrogen at its CNO-equilibrium value, while quasi-homogeneous evolution is judged highly unlikely because $\lambda\lambda3811$24 km s$\lambda\lambda3811$25 is too low and the mass-loss rate is too small to spin the stars down from the $\lambda\lambda3811$26 km s$\lambda\lambda3811$27 required for efficient QHE (Neugent et al., 2017). The authors instead suggest a possible connection to lower metallicity environments and, via Type Ic-BL progenitor arguments, to long-duration gamma-ray bursts.

A second recent anomaly is the proposed WN/WO transitional class. These stars show both prominent N V and strong O VI $\lambda\lambda3811$28, with weak optical carbon lines, very high temperatures of $\lambda\lambda3811$29–$\lambda\lambda3811$30 kK, low mass-loss rates relative to classical WN/WC/WO stars, and extremely hard ionizing continua. Their He II-ionizing photon production reaches $\lambda\lambda3811$31–49.0 s$\lambda\lambda3811$32, and the analysis argues that such objects trace a direct WN $\lambda\lambda3811$33 WO evolutionary path favored by low metallicity and weaker winds (Sander et al., 25 Aug 2025). A plausible implication is that standard integrated-light WR diagnostics may miss a significant hard-photon source in metal-poor stellar populations.

Not every WN-like spectrum belongs to a massive star. The clearest counterexample is IC4663, identified as the first unambiguous [WN3] central star of a planetary nebula. Its low luminosity nucleus, $\lambda\lambda3811$34 at the reference distance used in the study, $\lambda\lambda3811$35 mag, and surrounding AGB halo establish its PN nature. CMFGEN modeling yields a helium-dominated atmosphere with He $\lambda\lambda3811$36, H $\lambda\lambda3811$37, N $\lambda\lambda3811$38, Ne $\lambda\lambda3811$39, and O $\lambda\lambda3811$40 by mass, leading to the proposal of a second H-deficient evolutionary sequence, $\lambda\lambda3811$41, parallel to the carbon-rich $\lambda\lambda3811$42 sequence (Miszalski et al., 2012). This case clarifies a recurrent misconception: WN-like spectra do not uniquely identify massive Wolf–Rayet stars, and the bracket notation marks a fundamentally different evolutionary context.