Kinematics of Luminous Blue Variables in the Large Magellanic Cloud

TL;DR

This study measures the velocities of LBVs in the LMC, revealing they are more often runaways than other massive stars, and identifies potential binary companions and recent outbursts among them.

Contribution

It provides the first detailed kinematic analysis of LBVs in the LMC, highlighting their higher velocity dispersion and runaway fraction compared to RSGs, and suggests binary interactions influence their dynamics.

Findings

LBVs have a velocity dispersion of 40 km/s, higher than RSGs' 16.5 km/s.

33% of LBVs have radial velocities >25 km/s, compared to 9% of RSGs.

LBVs are more likely to be runaways, possibly due to supernova kicks from companions.

Abstract

We study the kinematics of luminous blue variables (LBVs) in the Large Magellanic Cloud (LMC). Using high-resolution spectra, we measure the systemic radial velocities for a sample of 16 LBVs and LBV candidates. In order to measure the net motion of LBVs compared to their local environments, we subtract the projected line-of-sight velocity at the same location derived from the rotation curve model of the LMC. Using nebular and wind emission lines, we infer a velocity dispersion for LBVs of $40.0^{+9.9}_{-6.6}$ km s$^{-1}$. To put LBVs in context with other evolved massive stars, we compare this to red supergiants (RSGs) in the LMC, which have a significantly smaller velocity dispersion of $16.5^{+0.4}_{-0.6}$ km s$^{-1}$. Moreover, 33% of LBVs have radial velocities of more than 25 km s$^{-1}$, while only 9% of RSG have such high velocities. This suggests that LBVs include more runaways than the population of stars that evolves to become RSGs, indicating that LBVs are preferentially kicked by a companion's supernova explosion as compared to other evolved massive stars. Our investigation reveals other interesting clues about LBVs in the LMC as well. We find that radial velocities and widths of emission lines for each target remain constant over several epochs, whereas measured absorption lines exhibit highly variable radial velocities for R110, R81, S Dor, Sk-69$^\circ$142a, and Sk-69$^\circ$279. These five LBVs probably have a binary companion. Additionally, we find that Sk-69$^\circ$142a experienced its second outburst in 2019 September, shifting its status from candidate to confirmed LBV.