Evolution of Low-Mass Population III Stars: Convection, Mass Loss, Nucleosynthesis, and Neutrinos

Abstract

The first stars likely formed from pristine clouds, marking a transformative epoch after the dark ages by initiating reionisation and synthesising the first heavy elements. Among these, low-mass Population III stars are of particular interest, as their long lifespans raise the possibility that some may survive to the present day in the Milky Way's stellar halo or satellite dwarfs. As the first paper in a series, we present hydrodynamic evolutionary models for 0.7 - 1 MSun stars evolved up to the white dwarf phase, utilising the MESA software instrument. We systematically vary mass-loss efficiencies, convective transport, and overshooting prescriptions, thereby mapping how uncertain physics influences nucleosynthetic yields; surface enrichment, including nitrogen-rich post-main sequence stars arising from convective shell mergers; remnant properties, such as low-mass helium or carbon-oxygen white dwarfs (M_WD ~ 0.45-0.55 MSun) and transient UV-bright phases; and potential observational signatures, including neutrino emission during shell mergers and helium flashes. These models establish a predictive framework for identifying surviving Pop III stars and their descendants, providing both evolutionary and observational constraints that were previously unexplored.

Figures (18)

• Figure 1: Main sequence lifetimes ($\Gamma_{\rm MS}$) of low-mass Pop III models follow a simple power-law $\propto M^{-3.2}$, steeper than the canonical Population I relation ($\propto M^{-2.5}$; green dashed line). Similarly, the terminal age of central helium burning (TAHeB; defined here as the point when $X_{^{4}{\rm He}} < 0.01$) obeys a comparable scaling, $\propto M^{-3.25}$ (blue solid line). The horizontal dashed line marks the Hubble time for $H_0 = 70.39~K$m s$^{-1}$ Mpc$^{-1}$ following 2025ApJ...985..203F.
• Figure 2: Evolutionary tracks for Pop III stellar models with ZAMS masses ranging from $0.7$ to $1.0~M_\odot$. Models with $0.7-0.75~M_\odot$ do not leave the MS within the Hubble time. Dashed lines indicate constant stellar radii (0.02, 0.2, 2, 20, 200 $R_\odot$, from left to right). Background colours indicate spectral types. Filled circles mark the point at which the central hydrogen mass fraction drops below 0.01 ($X_c < 0.01$), whilst inverted triangles indicate when the central helium mass fraction falls below 0.01 ($Y_c < 0.01$) during helium burning. The main evolutionary phases---Main Sequence (MS), Subgiant (SG), Red Giant Branch (RGB), Tip of the RGB (TRGB), Double Core Flash (DCF), Horizontal Branch (HB), Planetary Nebula (PN), Extended Horizontal Branch (EHB), and White Dwarf (WD) cooling sequence---are indicated.
• Figure 3: Stellar evolution tracks for $0.85~M_\odot$ models across different metallicities from Pop III ($Z = 0$) to metal-poor Pop II stars ($Z = 10^{-3}$). Coloured dots indicate the MSTO.
• Figure 4: Kippenhahn diagrams showing the internal structural evolution of $0.85~M_\odot$ stellar models as a function of age for $Z = 0$ (first panel) and $Z = 10^{-3}$ (second panel). Each set is presented across three evolutionary intervals: from the zero-age main sequence (ZAMS) to the point where the star attains $10~L_\odot$ (left), from this stage to the tip of the DCF (centre), and from there through to the WD phase (right). Radiative zones are depicted using a colour scale, whilst convective regions (where the Brunt-Väisälä frequency satisfies $N^2 < 0$) are shown in grey. Nuclear burning regions are marked using distinct hatching: leftward diagonal lines for the pp-chain, dots for the CNO cycle, and rightward diagonal lines for the $3\alpha$ process.
• Figure 5: H–R diagram for $0.85~M_\odot$ Pop III models with varied Reimers mass-loss efficiencies $\eta_{\rm R} = 0.1$, 0.5, 0.7, 0.9, evolved from pre-MS to WD cooling. For $\eta_{\rm R} \leq 0.1$, stars retain most of their envelopes, following extended RGB–DCF tracks and ending as relatively cool post-DCF objects ($T_{\rm eff} \lesssim 30~000~K$). For $\eta_{\rm R} \geq 0.5$, pre-helium-ignition envelope stripping produces EHB-like morphologies ($T_{\rm eff} \gtrsim 40~000~K$) with truncated luminosity excursions. The bifurcation is sharp and highly $\eta_{\rm R}-$dependent, whilst pre-dredge-up metallicity remains negligible.