ALMA Observations Reveal Wind Structure and Mass Loss in Arches Cluster Massive Stars
Astronomers have used the ALMA telescope to make the first millimeter-wavelength continuum observations of 23 massive stars in the Arches cluster, measuring their mass-loss rates and detecting structured clumping in their stellar winds. The Arches cluster, located near the Galactic Centre, is one of the youngest and most massive star clusters in the Milky Way, hosting extreme Wolf-Rayet and O-type stars. The findings provide new constraints on how massive stars shed material and inject energy into their surroundings, with implications for understanding stellar feedback in dense cluster environments.
A new study accepted for publication in Monthly Notices of the Royal Astronomical Society presents the first ALMA Band 3 (100 GHz) and Band 6 (243 GHz) continuum observations of the Arches cluster, detecting millimeter emission from 23 massive stars including Wolf-Rayet and O-type supergiants and hypergiants. By combining these ALMA measurements with archival Very Large Array radio data spanning 5–22.5 GHz, the researchers derived broadband spectral indices that reveal the physical nature of each star's wind. Most Wolf-Rayet stars show spectral indices near α ≈ 0.7–0.8, consistent with thermal free-free emission from dense, partially optically thick winds, while several O-type stars display flat or negative indices suggesting non-thermal synchrotron emission likely from colliding-wind binary systems. Clumping-scaled mass-loss rates were derived spanning roughly log(Ṁ) ≈ −4.1 to −4.9 solar masses per year for WN-type Wolf-Rayet stars and −4.9 to −5.4 for O super- and hypergiants, consistent with theoretical expectations for luminous stars in the Galactic Centre environment. Crucially, the study finds significant evidence that wind clumping is structured and generally decreases with increasing distance from the stellar surface, supporting models in which strong inhomogeneities are concentrated in the inner wind. These results highlight the power of combined radio-millimeter observations for probing wind structure and underscore the role of massive star feedback in shaping extreme cluster environments.
What's missing
The study derives clumping-scaled mass-loss rates, meaning the absolute mass-loss values depend on assumed clumping factors that are not independently constrained; the degree to which clumping corrections affect the derived rates and their uncertainties is an open question. The paper does not fully resolve whether the non-thermal emission in O-type stars definitively confirms binary status, as other mechanisms could contribute. Distance and extinction uncertainties toward the Galactic Centre may also introduce systematic errors in the derived stellar luminosities and mass-loss rates.
What different sources said
- arXiv astro-phCenter
ALMA measurements of mass loss and wind clumping in the massive stars of the Arches cluster
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