X-Shooting ULLYSES: Massive stars at low metallicity VI. Atmosphere and mass-loss properties of O-type giants in the Small Magellanic Cloud

• Main Authors: Backs, Frank, Brands, S. A, de Koter, A, Kaper, L, Vink, J. S, Puls, J, Sundqvist, J, Tramper, F, Sana, H, Bernini-Peron, M, Bestenlehner, J. M, Crowther, P. A, Hawcroft, C, Ignace, R, Kuiper, R, van Loon, J. Th, Mahy, L, Marcolino, W, Najarro, F, Oskinova, L. M, Pauli, D, Ramachandran, V, Sander, A. A. C, Verhamme, O
• Format: Journal Article
• Language: English
• Published: 11-11-2024
• Subjects: Physics - Astrophysics of Galaxies; Physics - Solar and Stellar Astrophysics
• DOI: 10.48550/arxiv.2411.06884

Mass loss through a stellar wind is an important physical process that steers
the evolution of massive stars and controls the properties of their end-of-life
products, such as the supernova type and the mass of compact remnants. For an
accurate mass loss determination, the inhomogeneities in the wind, known as
clumping, needs to be taking into account. We aim to improve empirical
estimates of mass loss and wind clumping for hot main-sequence massive stars,
study the dependence of both properties on the metallicity, and compare the
theoretical predictions to our findings. We analyzed the optical and UV spectra
of 13 O-type stars in the Small Magellanic Cloud galaxy, which has a
metallicity of $\sim 0.2\,Z_\odot$. We quantified the stellar atmosphere,
outflow, and wind-clumping properties. To probe the role of metallicity, we
compared our findings to studies of Galactic and Large Magellanic Cloud samples
that were analyzed with similar methods. We find significant variations in the
wind-clumping properties of the target stars, with clumping starting at flow
velocities $0.01 - 0.23$ of the terminal wind velocity and reaching clumping
factors $f_{\rm cl} = 2 - 30$. In the luminosity ($\log L / L_{\odot} = 5.0 -
6.0$) and metallicity ($Z/Z_{\odot} = 0.2 - 1$) range we considered, we find
that the scaling of the mass loss $\dot{M}$ with metallicity $Z$ varies with
luminosity. At $\log L/L_{\odot} = 5.75$, we find $\dot{M} \propto Z^m$ with $m
= 1.02 \pm 0.30$, in agreement with pioneering work in the field. For lower
luminosities, however, we obtain a significantly steeper scaling of $m > 2$.
The monotonically decreasing $m(L)$ behavior adds a complexity to the
functional description of the mass-loss rate of hot massive stars. Although the
trend is present in the predictions, it is much weaker than we found here.