Why Is Vega Metal-Poor and Rapidly Rotating? Four Hypotheses, One Laboratory — A Multi-Hypothesis Framework with Quantitative Predictions

Vega (α Lyrae, HD 172167) is the fifth-brightest star in the night sky and the original photometric calibration standard for optical and near-infrared astronomy. Interferometric observations established in 2006 that Vega rotates at approximately 93% of its breakup velocity (v_eq ≈ 270 km/s), is viewed nearly pole-on (i ≈ 5°), and displays a genuine surface metallicity of [m/H] ≈ −0.5 — with refractory elements (Fe, Mg, Si, Ca) depleted by 3–4× relative to solar and volatile elements (C, O) depleted by only ~2×. JWST/MIRI observations (2024) confirm a smooth, axisymmetric debris disk extending to 170 AU with no planet-scale structure, in striking contrast to the highly structured disk of the otherwise-similar star Fomalhaut. This paper develops and systematically evaluates four physically distinct mechanisms capable of explaining these anomalies: Centrifugal barrier segregation (Hypothesis I): During protostellar collapse, angular momentum sorting at the centrifugal barrier preferentially diverts refractory-bearing dust grains into the disk while the star accretes from metal-depleted polar streamlines. Predicts a refractory-enriched debris disk testable with JWST/MIRI grain spectroscopy. Binary disruption (Hypothesis II): Vega acquired its extreme rotation through orbital angular momentum transfer during the loss of a binary companion ~450 Myr ago. The 10.9 km/s velocity offset from Fomalhaut is interpreted as the disruption recoil. Pressure-relief valve (Hypothesis III): Near-critical rotation is maintained by episodic equatorial mass shedding that regulates angular momentum. The debris disk is the cumulative product of 455 Myr of shedding events. Gravitational settling and radiative levitation (Hypothesis IV): Atmospheric atomic diffusion depletes heavy elements from the photosphere without requiring any special formation history, though this mechanism faces a fundamental tension with Vega's rapid rotation. For each hypothesis we present: order-of-magnitude derivations and toy models; parameter sweeps with figures; element-specific abundance predictions; the Fomalhaut comparison as a control experiment; a ranked set of falsifying observations; and an explicit self-critique section identifying weaknesses and open questions. Key quantitative results include: The centrifugal barrier model requires a dust-gas coupling parameter α_dg ≈ 0.78 (Stokes number St ~ 0.1–0.5) to match the observed [Fe/H] ≈ −0.5 — physically plausible at the barrier shock. The binary disruption parameter space admits companions of 0.5–1.0 M☉ at 0.5–2 AU that simultaneously satisfy the spin and kinematic constraints. The pressure-relief valve predicts ~0.12 M☉ shed over 455 Myr, but the metallicity depletion timescale (~10⁹ yr) exceeds Vega's age by a factor of ~2. The diffusion equilibrium predicts [Fe/H] ≈ −1.28, over-depleting the surface by a factor of ~5 relative to observations. A comparative scoring matrix (seven criteria, four-point scale) identifies the combination of Hypotheses I + II as the most complete current picture: centrifugal barrier sorting explains the metallicity and volatile-refractory contrast, while binary disruption explains the rotation and the Vega–Fomalhaut kinematic offset. The single most decisive pending test is the composition of the Vega debris disk grains (refractory-enriched → confirms Hypothesis I; matching stellar composition → confirms Hypothesis III), measurable with existing JWST/MIRI data. This is the revised version of the manuscript, incorporating all corrections requested in peer review, including: correction of a sign error in the latitude-dependent settling analysis (Section 7.4); resolution of a circular argument in the angular momentum constraint (Section 4.2.3); addition of magnetic field constraints (Lignières et al. 2009); discussion of λ Boötis age statistics (Murphy et al. 2015); and formalization of the scoring methodology. The repository includes the full manuscript PDF, all Python scripts used to generate figures, and the LaTeX source files. Keywords Vega; alpha Lyrae; stellar metallicity; stellar rotation; debris disks; lambda Bootis stars; centrifugal barrier; binary disruption; gravitational settling; radiative levitation; protostellar accretion; chemical segregation; A-type stars; Fomalhaut; JWST; ALMA; asteroseismology Additional notes Related identifiers: Su, K. Y. L. et al. (2024), ApJ, 977, 277 — JWST/MIRI observations of the Vega debris disk Peterson, D. M. et al. (2006), Nature, 440, 896 — Interferometric determination of Vega's rotation Sakai, N. et al. (2014), Nature, 507, 78 — ALMA detection of centrifugal barrier in L1527 License Creative Commons Attribution 4.0 International (CC BY 4.0) Resource type Publication → Preprint Language English Subjects (Zenodo communities) Astronomy Astrophysics Stellar physics AUTHOR: Kriger, Boris — Information Physics Institute, Gosport; Institute of Integrative and Interdisciplinary Research, Toronto https://orcid.org/0009-0001-0034-2903 Zenodo form fields Field Value Upload type Publication Publication type Preprint Title Why Is Vega Metal-Poor and Rapidly Rotating? Four Hypotheses, One Laboratory Publication date 2026-03-05 Creators Kriger, Boris Affiliation Information Physics Institute, Gosport; Institute of Integrative and Interdisciplinary Research, Toronto Access right Open Access License CC-BY-4.0 Language eng Imprint publisher Zenodo Version 2.0 (revised manuscript)