•Title: Hypervelocity Star Candidates in the Segue G & K Dwarf Sample
•Authors: Lauren E. Palladino, Katharine J. Schlesinger, Kelley Holley-Bockelmann, Carlos Allende Prieto, Timothy C. Beers, Young Sun Lee, & Donald P. Schneider
•First Author’s Institution: Vanderbilt University, Nashville, Tennessee
The authors measure each star’s tangential and radial velocity and select for their sample those stars having a total velocity vector larger than 600 kilometers/second, fast enough to escape the galaxy. This sample contains 13 targets. It is worth noting that in many of these cases, the (more robustly measured) radial velocities are quite small, while the tangential velocities are very large. If these stars have had their distances significantly overestimated, then their tangential velocities are correspondingly smaller and the stars may not actually be HVSs. Moreover, the proper motions are small (around 0.05 arcseconds per year for these stars), and the measurement uncertainties are large (around 0.01 arcseconds per year), so out of a sample of 70,000 stars, a few systems would be expected to have significantly overestimated proper motions simply by chance alone. For each star the authors provide a probability that the star is an “interloper” with a much smaller true proper motion; these values range from 0.004 to 0.6 (0.4 to 60 %). The authors note that this sample is simply a list of candidates, and more follow-up is needed to separate the true HVSs from false positive interlopers.
Now that we have a sample of potential hypervelocity stars, where do they come from? The authors first look at the metallicity distribution of the stars and compare it to samples of other stars in the galaxy (above right). They find the HVS G and K dwarf candidates are similar to the distribution of G and K stars in the galactic disk. They find Milky Way bulge stars are much more metal rich than this sample, while stars in globular clusters are much more metal poor. This suggests that these stars originate in the disk, not the bulge near the galactic center or the galactic halo. The authors then use the positions and velocities of each star and integrate their orbits backward in time to determine their origins (at left). They find the stars do not all originate from the same section of the galaxy and none come from the galactic center. Therefore, while the origins of the stars are unclear, their metallicity distribution and velocities are consistent with these objects not originating from near the central black hole.More followup work will be needed to study the origin of these stars and determine which are true HVSs (by improving our estimate of the distances to the stars and their proper motions) to better understand their origins and history, but for now it seems like the galactic center hypothesis is incomplete.