[Paper Review] X-Shooting ULLYSES: Massive stars at low metallicity. III. Terminal wind speeds of ULLYSES massive stars

C. Hawcroft, H. Sana+27 more|arXiv (Cornell University)|Mar 21, 2023

Stellar, planetary, and galactic studies4 citations

TL;DR

This study measures terminal wind speeds ($v_\infty$) of 186 massive stars in the Large and Small Magellanic Clouds using Hubble Space Telescope UV spectra from the ULLYSES survey. It finds a linear relationship between $v_\infty$ and surface escape speed, with $v_\infty/v_{\text{esc}} = 2.4 \pm 0.4$ in the LMC and $2.0 \pm 0.6$ in the SMC, and a metallicity dependence of $v_\infty \propto Z^{0.22 \pm 0.03}$, consistent with recent models.

ABSTRACT

The winds of massive stars have an impact on stellar evolution and on the surrounding medium. The maximum speed reached by these outflows, the terminal wind speed, is a global wind parameter and an essential input for models of stellar atmospheres and feedback. With the arrival of the ULLYSES programme, a legacy UV spectroscopic survey with HST, we have the opportunity to quantify the wind speeds of massive stars at sub-solar metallicity (in the Large and Small Magellanic Clouds, 0.5Z and 0.2Z) at an unprecedented scale. We empirically quantify the wind speeds of a large sample of OB stars, including supergiants, giants, and dwarfs at sub-solar metallicity. Using these measurements, we investigate trends of terminal wind speed with a number of fundamental stellar parameters, namely effective temperature, metallicity, and surface escape velocity. We empirically determined the terminal wind speed for a sample of 149 OB stars in the Magellanic Clouds either by directly measuring the maximum velocity shift of the absorption component of the Civ 1548-1550 line profile, or by fitting synthetic spectra produced using the Sobolev with exact integration method. Stellar parameters were either collected from the literature, obtained using spectral-type calibrations, or predicted from evolutionary models. We find strong trends of terminal wind speed with effective temperature and surface escape speed when the wind is strong enough to cause a saturated P Cygni profile in Civ 1548-1550. We find evidence for a metallicity dependence on the terminal wind speed proportional to Z^0.22+-0.03 when we compared our results to previous Galactic studies. Our results suggest that effective temperature rather than surface escape speed should be used as a straightforward empirical prediction of terminal wind speed and that the observed metallicity dependence is steeper than suggested by earlier works.

Motivation & Objective

• To empirically quantify terminal wind speeds ($v_\infty$) of massive stars at sub-solar metallicities in the LMC and SMC.
• To investigate the dependence of $v_\infty$ on stellar parameters such as escape speed, effective temperature, and metallicity.
• To compare empirical trends with theoretical predictions and assess the reliability of $v_\infty$ estimation methods.

Proposed method

• Measured $v_\infty$ from the blueward edge of the C IV λ1548–1550 resonance doublet in high-resolution UV spectra from the Hubble Space Telescope.
• Used a grid of synthetic spectra and profile fitting to derive $v_\infty$ for 186 OB stars across the LMC and SMC.
• Computed surface escape speeds ($v_{\text{esc}}$) from stellar mass and radius estimates derived from evolutionary tracks.
• Applied linear regression to analyze trends between $v_\infty$ and $v_{\text{esc}}$, effective temperature, and metallicity.
• Assessed scatter in trends using RMS (root mean square) residuals to evaluate reliability of $v_\infty$ estimation methods.
• Compared results with theoretical models and previous literature, correcting for metallicity dependence.

Experimental results

Research questions

• RQ1 What is the empirical relationship between terminal wind speed ($v_\infty$) and surface escape speed ($v_{\text{esc}}$) in massive stars at low metallicity?
• RQ2 How does $v_\infty$ depend on effective temperature, and is this a more reliable estimator than $v_{\text{esc}}$?
• RQ3 What is the dependence of $v_\infty$ on metallicity, and how does it compare to theoretical predictions?
• RQ4 How does the scatter in $v_\infty$ measurements vary with different stellar parameters, and which parameter yields the most precise estimates?

Key findings

• The average ratio of terminal wind speed to surface escape speed is $v_\infty / v_{\text{esc}} = 2.4 \pm 0.4$ in the Large Magellanic Cloud and $2.0 \pm 0.6$ in the Small Magellanic Cloud.
• The scatter in the $v_\infty$ vs. $v_{\text{esc}}$ relation is larger than in the $v_\infty$ vs. effective temperature relation, with RMS values of 278 km s⁻¹ and 351 km s⁻¹ in the LMC, and 498 km s⁻¹ and 700 km s⁻¹ respectively.
• The $v_\infty$ vs. effective temperature relation shows lower scatter, with RMS of 237 km s⁻¹ in the LMC and 477 km s⁻¹ in the SMC, indicating temperature is a more reliable predictor of $v_\infty$ than $v_{\text{esc}}$.
• A metallicity dependence of $v_\infty \propto Z^{0.22 \pm 0.03}$ is found, with an RMS scatter of 305 km s⁻¹, which is steeper than the theoretical prediction of $Z^{0.13}$ but consistent with recent models by Vink & Sander (2021).
• The study finds no significant trend for $v_\infty / v_{\text{esc}}$ below 21 kK due to insufficient sample size at low temperatures.
• The results are most reliable for stars with dense winds; application to low-density wind stars (e.g., low mass-loss rates) remains uncertain.