Stellar Bow Shocks in the Northern Arm of the Galactic Center: More Members and Kinematics of the Massive Star Population

Abstract

We present new 2.2 μm diffraction-limited images from the W. M. Keck 10 m and Gemini 8 m telescopes of the cool Galactic center sources, IRS 1W, 5, 8, 10W, and 21, along with new proper motions for IRS 1W, 10W, and 21. These observations were carried out to test the bow shock hypothesis presented by Tanner et al. as an alternative to a very recent (104 yr) epoch of star formation within the tidal stream of gas and dust known as the northern arm. Resolved asymmetric structure is detected in all the sources, with bow shock morphologies associated with IRS 1W, 5, 8, and 10W. For IRS 1W and 10W, there is an agreement between the position angle of the asymmetry and that of the inferred relative velocity vector of the near-infrared source with respect to the northern arm gas, strengthening the bow shock hypothesis. We therefore conclude that the observed morphology is indeed a bow shock generated by sources plowing through the northern arm. Furthermore, the large extent of the resolved structures (310-1340 AU), along with their luminosities (~104-105 L☉), suggests that their central sources are Wolf-Rayet stars, comparable to the broad He emission-line stars, which have strong winds on the order of 1000 km s-1. The bow shock geometry, along with the proper motion measurements, provide three-dimensional orbital solutions for this enigmatic class of objects. The orientations of the orbital planes of IRS 1W and 10W are consistent with that of the putative clockwise plane which has been proposed as a solution for the He I emission-line stars. While these observations eliminate the need to invoke star formation within the northern arm, they increase by 10% the total known population of massive, young stars with strong winds, whose origin remains unexplained in the context of the nearby supermassive black hole.