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

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.

Figures (16)

• Figure 1: Contributions of single (blue) vs binary (red hatched) WR stars toward to the total WR velocity distribution in the LMC. The distributions are overplotted, not stacked.
• Figure 2: Proper motion vectors for all WR subtypes in the 30 Dor region superposed on H$\alpha$ image of the LMC from Smith2005, color-coded as shown. Vector lengths are scaled as $v_\perp^{0.5}$. The bases of the vectors are plotted at the WR-star positions. WR+WR binary systems are indicated with stars on the vector arrow heads, color-coded according to the companion subtype, which is WNL in both cases. The three large star symbols indicate the locations of the dominant star-forming regions (east to west): 30 Dor A, B, and C (LH 100, 99, and 90, respectively) LeMarne1968Lucke1970. The cyan boundary indicates the general region within which stars are considered to be associated with 30 Dor by Hung2021.
• Figure 3: Proper motion vectors in the entire LMC for WNh, O If*/WN, and WNL stars. Panel ($a$) shows all stars with these subtypes, and panel ($b$) shows only the VMS stars, but additionally includes binary WNE stars, a class which also has high luminosities. Plotting conventions are as in Figure \ref{['fig:vectors_30Dor']}, including for the two WNL binary systems.
• Figure 3: Figure \ref{['fig:vectors_WN']}b
• Figure 4: Left column: Normalized differential transverse velocity distributions for WN stars with $L/L_\odot\leq 5.9$ (blue) and $L/L_\odot > 5.9$ (red). The black outlines show the distributions for all the subtype stars, including those for which no $L$ estimate is available. WNh, O If*/WN, and WNL stars are shown in the top, middle, and bottom rows, respectively. Right panel: Corresponding velocity survival functions $1 - f$, where $f$ is the cumulative distribution function, for the same samples. The figures omit stars with no $v_\perp$ data: 6, 3, and 7 stars in the WNh, O If*/WN, and WNL samples, respectively.