On the Structure and Oscillations of the First Stars

On the Structure and Oscillations of the First Stars Thiago Ferreira1, Earl P. Bellinger1, Ebraheem Farag1, and Christopher J. Lindsay1 1Yale University | Contact:[email protected] Low-mass Population III (Pop III) stars are expected to survive in the Milky Way halo and/or its dwarf satellites, however, their identification remains elusive [1-4]. Surface abundances from internal mixing (e.g., element diffusion and dredge-up) can mimic secondgeneration chemical signatures, obscuring their primordial origin. As these stars differ fundamentally in opacity, convective structure, and evolutionary pathways due to their metalfree composition, asteroseismology may offer a promising, independent probe of their interiors. In this study, we assess whether their oscillation spectra reveal unique seismic signatures that could distinguish Pop III survivors from metal-rich stars of similar mass and evolutionary stage. EVOLUTIONARY TRACKS. Pop III stellar models are hotter, more compact, and luminous than metalrich counterparts. Models with M≲0.75 M⊙ remain on the MS today, while more massive ones evolve to WDs. For M≳0.85 M⊙, deep dredge-up and shell mergers during the asymptotic giant branch bring to the surface CNO elements, producing C/N-enhanced ultra metalpoor stars by 10 Gyr. Extreme horizontal branchlike phases emerge from off-centre He ignition under degenerate conditions, enabled by mass loss (up to 10−7M⊙yr−1). These helium-burning phases are UV-bright and short-lived (∼20 −60 Myr) and may leave fossil signatures in old white dwarf populations. OSCILLATION SPECTRA were computed using GYRE [7] focusing on a 0.85 M⊙case study star at the sub-giant (SG) and red giant branches (RGB). Oscillation modes reveal distinct seismic fingerprints driven by Pop III stars zero-metallicity interiors: (a) The absence of metals drastically reduces opacity, steepening internal temperature gradients and producing more compact, denser cores, with higher sound speeds; (b) Pop III stars show larger ∆νthan metal-poor stars at a fixed model mass, reflecting compact structure and higher mean densities; (c) Elevated ratios r02 =δν02/∆νindicate sharper sound-speed contrasts between core and envelope, a hallmark of primordial composition and efficient radiative energy transport; (d) Gravity modes in Pop III stars exhibit higher normalized inertias, indicating strong trapping in the compact, stratified core and lower surface amplitudes. This contrasts with metal-poor stars where metals smooth gradients and weaken mode trapping; (e) Buoyancy frequency profiles reveal pronounced peaks in Pop III stars, evidencing strong stratification and efficient gravity-mode cavities, in contrast to the smoother profiles in metal-poor models; (f) Pop III stars have higher ψ≡∆ν/∆Π1at fixed ∆Π1during the RGB due to slower core evolution and more modest chemical gradients, distinguishing them from metal-poor counterparts (e.g., [9, 10]). 5000100002000050000100000 Effective Temperature, Teff / K 0.01 0.1 1 10 100 1000 10000 Luminosity, L / L 0.7 M 0.75 M 0.8 M 0.85 M 0.9 M 0.95 M 1 M MS SG RGB TRGB Shell Mergers AGB HB LTP EHB PN WD Cooling Seq. O B A F G K M 0.000 0.004 1.406 8.925 9.759 10.035 10.050 Stellar Age, / Gyr 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Fractional Mass, m / M | 0.85 M Radiative Zones Convective Zones pp-chain CNO cycles 3 proc. PMS MS SG TRGB TRGB Shell Mergers To AGB 6 5 4 3 2 1 0 Buoyancy Frequency, log N / Hz Figure 1. HR diagram for low-mass Pop III stars with stellar radii (dashed), spectral type regions (background shading), and evolutionary phases (right). Kippenhahn diagram for a 0.85 M⊙model, showing convective boundaries, core helium burning, and structural transitions (left). 100 120 140 160 180 200 / Hz 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 r 02 02/ 0.85 M models Varying Metallicities Z = 0 Z = 10 3 Z = 10 2 Z = 2 × 10 2 0.1 0.2 0.3 0.4 0.5 0.6 Xc= 0.7 Figure 2. Top left: Evolution of surface chemical abundances (εX= log10(X/H) + 12) for several elements in a 0.85 M⊙star as a function of stellar age, with shell merger phases highlighted. The shaded grey region represents the approximate lower limit of surface abundances observed in extremely metal-poor stars (e.g., SMSS J031300.36670839.3 with [Fe/H] ≈ −7.3[11]). Top right: Evolution of the small frequency separation ratio r02 ≡δν02/∆νvs. ∆ν for 0.85 M⊙models at varying metallicities. Bottom left: Kiel diagram for 0.85 M⊙models at different metallicities. Crosses represent 1140 observed Kepler RGB stars from [10]. Bottom right: Seismic spacing ratio ψ≡∆ν/∆Π1vs. ∆Π1for varying metallicities. Lower-metallicity models show decreased ψduring late subgiant phases. Pop III stars, in particular, consistently exhibit higher ψvalues throughout the RGB, reflecting weaker coupling between pand g-mode cavities. References: [1] Abel et al. (2000; ApJ 540, 39), [2] Komiya et al. (2016; ApJ 820, 59), [3] Hirano & Bromm (2017; MNRAS 470, 898), [4] Chandra & Schlaufman (2021; AJ 161, 197), [5] Paxton et al. (2011-2019; ApJS, 192, 220, 223, 234, 243), [6] Jermyn et al. (2023; ApJ 913, 72), [7] Townsend & Teitler (2013; MNRAS 435, 3406), [8] Mosser et al. (2012; A&A 540A, 143), [9] Buysschaert et al. (2016; A&A 588A, 82), [10] Gehan et al. (2018; A&A 616A, 24), [11] Keller et al. (2014; Nature 506, 463). TASC9/KASC16 WORKSHOP – JULY 2025 – AUSTRIA