Slow Massive-Star Formation and the O-Star ZAMS Deficit
Abstract
The scarcity of optically visible O stars near the theoretical zero-age main sequence (ZAMS) has puzzled astronomers for over four decades. We argue that it reflects prolonged accretion. If massive stars assemble over Myr timescales, as the inertial-inflow model predicts, core-hydrogen burning begins before accretion ends, and the stars become optically visible at the end of accretion on a birthline displaced from the standard ZAMS. We test this interpretation by comparing the hot-side envelope of Galactic O stars in the Kiel diagram with accreting stellar-evolution models, adopting a growth law tform=τ0(mf/60 M⊙)α with formation time tform and final mass mf. The resulting birthlines reproduce both the displacement and the high-mass turnover of the observed envelope, with best-fitting parameters (α,τ0)=(0.52,4.3 Myr), consistent with independent constraints from the luminosity functions of compact H II regions and OB stars. The two constraints have orthogonal degeneracy directions, so combining them narrows the allowed parameter space, yielding projected 90% confidence intervals 0.43<α<0.60 and 3.9<τ0/Myr<5.0. The inferred growth law agrees with cloud-scale simulations and the inertial-inflow model, and implies that mass assembly and main-sequence evolution overlap substantially in massive stars, with broader implications for stellar evolution and feedback.
Related
- Probing massive star formation with intrinsic multiplicity constraints from TESS eclipsing binaries
- Low Star Formation Efficiency in M31: Evidence for a Deficit of Bound Gas
- Low Star Formation Efficiency in M31: Evidence for a Deficit of Bound Gas
- Reigniting the FUSE I: Star Formation in Massive Elliptical Galaxies
- Spinning down neutron-star merger remnants with the Tayler-Spruit dynamo: Global simulations reveal the formation of massive disks and neutron-rich ejecta
BibTeX
@misc{padoan2026slow,
title = {{Slow Massive-Star Formation and the O-Star ZAMS Deficit}},
author = {Paolo Padoan and Samuel Shipkoff and Mark Gieles and Fabrice Martins},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.31482},
eprint = {2610.31482},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.31482},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Slow Massive-Star Formation and the O-Star ZAMS Deficit
AU - Paolo Padoan
AU - Samuel Shipkoff
AU - Mark Gieles
AU - Fabrice Martins
PY - 2026
DA - 2026/10/22
PB - arXiv
DO - 10.48550/arxiv.2610.31482
UR - https://arxiv.org/abs/2610.31482
AB - The scarcity of optically visible O stars near the theoretical zero-age main sequence (ZAMS) has puzzled astronomers for over four decades. We argue that it reflects prolonged accretion. If massive stars assemble over Myr timescales, as the inertial-inflow model predicts, core-hydrogen burning begins before accretion ends, and the stars become optically visible at the end of accretion on a birthline displaced from the standard ZAMS. We test this interpretation by comparing the hot-side envelope of Galactic O stars in the Kiel diagram with accreting stellar-evolution models, adopting a growth law t_form=τ_0(m_f/60 M_⊙)^α with formation time t_form and final mass m_f. The resulting birthlines reproduce both the displacement and the high-mass turnover of the observed envelope, with best-fitting parameters (α,τ_0)=(0.52,4.3 Myr), consistent with independent constraints from the luminosity functions of compact H II regions and OB stars. The two constraints have orthogonal degeneracy directions, so combining them narrows the allowed parameter space, yielding projected 90% confidence intervals 0.43<α<0.60 and 3.9<τ_0/Myr<5.0. The inferred growth law agrees with cloud-scale simulations and the inertial-inflow model, and implies that mass assembly and main-sequence evolution overlap substantially in massive stars, with broader implications for stellar evolution and feedback.
ER -