Kinematics of Wolf-Rayet Stars in the LMC: Clues to Subtype Origins

TL;DR

This study measures proper motions of Wolf-Rayet stars in the LMC using Gaia DR3, revealing diverse ejection velocities and suggesting different origins and ejection mechanisms for various subtypes.

Contribution

It provides the first detailed kinematic analysis of LMC Wolf-Rayet stars, linking their velocities to possible formation and ejection processes.

Findings

VMS stars show both slow and fast velocities, indicating ejection timescales of about 1.5 Myr.

Binary WC stars are faster and more luminous than single WC stars, implying different evolutionary paths.

Single and binary WNE stars have distinct velocity distributions, suggesting different ejection mechanisms.

Abstract

We measure transverse proper motion velocities of LMC Wolf-Rayet (WR) stars using Gaia DR3 astrometry. The combined velocity distribution of WNh, O If*/WN, and WNL very massive stars ($>100\ M_\odot$; VMS) shows both slow, unejected objects ($v_\perp < 10$ $\rm km\ s^{-1}$) and stars dominated by fast, runaway velocities ($v_\perp > 24$ $\rm km\ s^{-1}$). This supports expectations that VMS ages are comparable to the dynamical ejection timescale ($\sim1.5$ Myr). These kinematics share similarities with those of lower-luminosity, classical WNh, O If*/WN, and WNL stars, as well as the SMC field OB stars, suggesting that dynamical ejections may also dominate these populations. In contrast, both single and binary WNE stars are ejected populations that show single-peaked velocity distributions, suggesting a different ejection mechanism(s). We speculate that single WNE stars might result from explosive mergers onto the shell-burning layer, thereby stripping the H envelope. Binary WC stars appear to be faster (median $v_\perp = 54$ $\rm km\ s^{-1}$) and have higher luminosities than singles (median $v_\perp = 38$ $\rm km\ s^{-1}$), suggesting that single WC stars are not descendants of the binaries. Thus, the binaries are probably stripped by mass transfer, while the WC singles likely originate from another process. The high velocities of binary WC stars are consistent with some predictions that lower mass clusters generate fast dynamical ejections. Single WC and WN3/O3 stars have ambiguous kinematics, but both show high $v_\perp$ (median $\sim 38$ $\rm km\ s^{-1}$), possibly linked to their lower masses.