Hubble TEMPOS Survey reveals weak stellar winds in low-metallicity galaxies

A University of Utah-led survey using the Hubble Space Telescope has delivered the largest ultraviolet (UV) dataset of massive, low-metallicity stars to date

Published in The Astrophysical Journal Supplement Series, the Treasury of Extremely Metal-Poor O Stars (TEMPOS) program observed 29 massive O-type stars across six nearby dwarf galaxies that serve as local analogues for the universe’s earliest galaxies.

The findings reveal that stars in low-metallicity environments exhibit unexpectedly weak stellar winds and significant variations in iron abundance, offering critical physical parameters to help interpret early-universe galaxy observations from the James Webb Space Telescope (JWST).

Observational methodology and sample architecture

Massive stars, weighing more than ten times the mass of the Sun, act as major drivers of galactic evolution by emitting intense ionising radiation, driving material into interstellar space via stellar winds, and ultimately exploding as supernovae.

To model how these processes operated in the early universe, TEMPOS used the Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope to gather high-resolution UV spectra from stars in galaxies with metallicities below one-fifth of solar abundance.

• Sample composition:

  • The survey compiled 29 massive O-type stars across six dwarf galaxies, combining 12 new target observations with archived datasets. Given the high observational cost (requiring up to 35 hours of Hubble time per star), TEMPOS replaces small-sample studies with a statistically robust baseline.

• Target environment:

  • Because nearby dwarf galaxies share the low chemical enrichment (metallicity) characteristic of early cosmic epochs, they allow astronomers to resolve individual stars that would be impossible to isolate at cosmological distances.

Key astrophysical findings

Abrupt Drop in Stellar Wind Velocity:

Stellar winds are driven by light interacting with metal ions in a star’s atmosphere. While astronomers expected wind speeds to decrease gradually with lower metallicity, TEMPOS data revealed an abrupt, sharp decline in wind velocities for stars with metallicities below 10% of solar abundance, falling far below theoretical predictions.

Impact on Mass Loss and Stellar Lifetimes:

Because extremely metal-poor stars experience dramatically weaker winds, they lose significantly less mass over their operational lifetimes. Retaining mass alters their evolutionary tracks, internal structure, and eventual supernova energy output, which in turn alters how gas is heated and recycled into subsequent generations of stars.

Decoupling of Iron and Oxygen Abundances:

Iron plays a pivotal role in driving stellar winds and triggering core-collapse supernovae, but its spectral absorption signatures are notoriously difficult to detect. Astronomers previously used oxygen emission as a proxy for iron content.

TEMPOS directly measured iron absorption features in UV spectra, revealing that iron abundance varies widely among stars with similar oxygen levels.

Research implications and next steps

The TEMPOS survey provides foundational physical parameters for synthetic stellar population models. By pairing Hubble’s UV spectra with optical observations from the Keck Observatory in Hawaii, researchers aim to refine mass-loss equations and chemical abundance maps.

The survey’s science-ready spectra will be hosted on the Mikulski Archive for Space Telescopes (MAST) to support ongoing cross-analysis with JWST observations of distant, high-redshift galaxies.