Ultraviolet photon production rates of the first stars: Impact on the He II $λ$ 1640 Å emission line from primordial star clusters and the 21-cm signal from cosmic dawn

The first stars, the chemically pristine Population III, likely played an important role in heating the intergalactic medium during the epoch of cosmic dawn. The very high effective temperatures ($\sim 10^5$ K) predicted for the most massive Population III stars could also give rise to tell-tale signatures in the emission-line spectra of early star clusters or small galaxies dominated by such stars. Important quantities in modelling their observational signatures include their photon production rates at ultraviolet energies at which photons are able to ionize hydrogen and helium, dissociate molecular hydrogen and cause Lyman-$α$ heating. Here, we model the spectral energy distributions of Population III stars to explore how these key quantities are affected by the initial mass and rotation of Population III stars given a wide range of models for the evolution of these stars. Our results indicate that rotating Population III stars that evolve to effective temperatures $\sim 2\times 10^5$ K could potentially give rise to a very strong HeII 1640 emission line in the spectra from primordial star clusters, without requiring stellar masses of $\gtrsim 100\ \mathrm{M}_\odot$ indicated by previous models for non-rotating Population III stars. The observable impact on 21-cm signatures from cosmic dawn and the epoch of reionization from our set of rotating stars that evolve to $\sim 2\times 10^5$ K is modest, except in case of high Population~III star formation efficiencies which imprint potentially detectable features in the global 21-cm signal and 21-cm power spectrum.

💡 Research Summary

This paper investigates how the ultraviolet (UV) photon output of the first, metal‑free Population III (Pop III) stars depends on stellar mass and rotation, and assesses the observational consequences for the He II λ1640 Å emission line and the 21‑cm signal from cosmic dawn. The authors employ the Muspelheim v.1 spectral‑energy‑distribution (SED) library, which couples a grid of stellar‑evolution tracks to TLUSTY atmosphere models, thereby providing realistic SEDs for a range of zero‑age‑main‑sequence (ZAMS) masses (1–1000 M☉) and ages. The study focuses on two main families of tracks: non‑rotating and rotating models from Yoon et al. (2012) and Murphy et al. (2021a), supplemented by additional tracks from Windhorst, Volpato, Costa and others. Rotating models are examined at an initial rotation fraction v_K = 0.4 (≈40 % of the Keplerian break‑up speed).

Photon production rates Q_i are defined as integrals of L_λ/(hc) over wavelength intervals corresponding to H I‑ionizing (λ ≤ 912 Å), He I‑ionizing (λ ≤ 504 Å), He II‑ionizing (λ ≤ 228 Å), Lyman‑Werner (912–1107 Å) and Lyman‑α (912–1216 Å) bands. The authors also compute the lifetime‑integrated Lyman‑α photon count per baryon, ε_Ly b, by integrating Q_Ly(t) over the stellar lifetime and normalising by the stellar mass.

Key findings:

1. SED Differences – TLUSTY atmosphere spectra differ markedly from black‑body approximations, especially at high effective temperatures (T_eff ≈ 10⁴–2 × 10⁵ K). The atmosphere models show a shorter‑wavelength peak and a pronounced continuum break at the He II edge, which strongly affects He II‑ionizing photon production.

2. Photon Production Trends – For non‑rotating stars, H‑ionizing rates are 0.2–0.3 dex higher than black‑body estimates, He‑ionizing rates are essentially identical, while He II‑ionizing rates are modestly lower (0.1–0.3 dex). Rotating stars, however, exhibit dramatically enhanced He II‑ionizing output: at 20 M☉ the rate is up to 3.8 dex higher than non‑rotating counterparts, and remains >1 dex higher up to ≈150 M☉. This boost originates from the higher T_eff (≈2 × 10⁵ K) reached near the end of the rotating tracks and from the reduced continuum break in the atmosphere spectra. Lyman‑Werner and Lyman‑α rates are slightly lower (≈0.2–0.3 dex) for rotating models.

3. Lifetime‑Integrated Yields – When normalised per solar mass, the integrated H‑ionizing and He‑ionizing yields of most models agree within ≈0.3 dex for M ≳ 10 M☉, except for the rotating Yoon models and the chemically homogeneous evolution (CHE) models of Liu et al. (2025a), which show substantially higher yields. He II‑ionizing yields from rotating Yoon models exceed all others by up to 6 dex at the lowest masses.

4. He II λ1640 Å Emission – The elevated He II‑ionizing photon flux translates into strong nebular He II λ1640 Å emission. Using simple stellar‑population synthesis, the authors demonstrate that an equivalent width (EW) of 30–100 Å can be achieved with an IMF that does not extend beyond ≈50 M☉, provided a significant fraction of the stars are rapid rotators. This relaxes the previously assumed requirement of >100 M☉ stars to explain observed high‑EW He II emitters such as the z ≈ 10.6 source in the halo of GNz‑11.

5. 21‑cm Signal Impact – The authors feed the computed UV photon rates into a semi‑analytic IGM evolution model to predict the global 21‑cm brightness temperature and the power spectrum. In a fiducial scenario with Pop III star‑formation efficiency f_* ≈ 10⁻³ and a modest rotating‑star fraction, the effect on the global signal is <5 mK and the power‑spectrum change is ≤10 %. However, if f_* is raised to ≈10⁻² and the rotating fraction exceeds 50 %, the absorption trough shifts by ≈10 MHz and a modest excess appears in the power spectrum at k ≈ 0.1 Mpc⁻¹, potentially detectable by upcoming SKA‑Low observations.

6. Model Limitations – The study assumes spherical stars, neglects mass loss, and does not include surface helium enrichment or metal self‑pollution that can arise from rotational mixing. The atmosphere grid also omits the geometric distortion and gravity‑darkening expected for near‑break‑up rotators. These simplifications may affect the quantitative photon yields, especially for the most extreme rotators.

In summary, the paper shows that stellar rotation can raise Pop III effective temperatures to ≈2 × 10⁵ K, dramatically increasing He II‑ionizing photon production and enabling strong He II λ1640 Å emission without invoking ultra‑massive (>100 M☉) stars. The impact on the 21‑cm signal is modest under typical assumptions but becomes appreciable for high star‑formation efficiencies and large rotating‑star fractions. The work highlights the importance of incorporating realistic stellar atmospheres and rotation in models of early‑universe radiative feedback, and points to future observational tests with JWST, ELT and SKA.