About 10% of the stars in the solar neighborhood belong to a large group that exhibits an oddly distinctive collective motion. Known as the Hercules stream, the group includes stars that are moving away from the center of the galaxy while falling behind the galaxy’s general rotation. The cause of that motion is the subject of a new study by Angeles Pérez-Villegas and her colleagues at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.
New stars typically condense with other new stars out of the same dense cool region of gas and dust. Because stars are born more or less around the same time, they inherit the motion around the galaxy of their birthplace. The Hercules stream is different. Spectroscopic observations have revealed that it’s made up of stars of widely different ages. Whatever is causing the streaming motion operates across large scales of time and space.
One candidate is the dense central bar of stars from which two of the galaxy’s spiral arms sprout (see accompanying figure). The solar neighborhood lies beyond the orbit of the ends of the bar, but the Sun and the stars of the Hercules stream are close enough to feel the bar’s gravitational influence. Generally speaking, as stars orbit the galaxy, they oscillate toward and away from the galactic center. If a star’s radial oscillation resonates with the bar’s rotation, the star can end up with an extra kick away from the galactic center.
That resonance, the outer Lindblad resonance (OLR), has been invoked to explain the Hercules stream, but new observations have identified a flaw in the explanation. The bar rotates more slowly than originally thought. Consequently, the OLR is too far away from the solar neighborhood to be the stream’s prime mover.
Pérez-Villegas and her colleagues used those new observations to test a detailed dynamical model of the stars in and around the bar. The model not only re-created the positions and velocities of the Hercules stream, it also revealed what causes its unusual motion: The stream’s stars are orbiting two local maxima—the L4 and L5 Lagrange points—in the bar’s effective gravitational potential. The maxima are situated on the bar’s perpendicular bisector close to the distance where stars corotate with the bar. In the model, most of the stars in the Hercules stream originate from the inner part of the galaxy, where the stars tend to be older than the Sun. Their looping orbits around L4 and L5 take them all the way into the solar neighborhood, but not much beyond it. (A. Pérez-Villegas et al., Astrophys. J. Lett. 840, L2, 2017.)
