Stellar Paternity Tests: Matching High-Latitude B Stars to the Open Clusters of their Birth

Abstract

OB stars generally form in open clusters within the Milky Way's thin disk, so when they are found at high Galactic latitudes, it is thought that they were ejected from their birth clusters during the past few tens of millions of years. Using Gaia Data Release 3 (hereafter DR3) data, we traced the kinematic trajectories of 39 high-latitude B-type stars and 447 Galactic open clusters with high-quality astrometry to search for moments of past intersection. In cases where we found matching trajectories, we also considered the clusters' HR diagrams to confirm parent-orphan pairs have matching ages. Further analysis of the clusters' core environments allowed us to determine a probable ejection mechanism. Through these paternity tests, we have identified possible origins for five of these orphaned B-type stars. Here we present the likely travel times, ejection velocities, and a discussion of the runaway mechanism for each case. We also identify one star whose trajectory did not bring it near the disk during the time period of our analysis, and we discuss its possible origins as well.

Figures (7)

• Figure 1: Our values for cluster $r$ and $\log$(Age) vs. the values found by HR24. The red points represent the clusters listed in our final results, while the blue points represent the remaining sample. In the left panel, the errorbars are all consistent with or smaller than the blue point sizes. In the right panel, the errorbars are shown for the red points only, but are typical in size for the larger sample. The gray line illustrates the identity function for reference.
• Figure 2: The edge-on view of the trajectories for our 39 remaining target stars. Their current positions are marked by the star symbols, and the trajectories are extended to 50 Myr in the past for illustrative purposes.
• Figure 3: The trajectories of the star HIP 55051 (red) and the cluster NGC 2362 (blue) in Galactocentric coordinates are shown as an example of a candidate match with $T = 23.92$%. The star and square markers indicate their current positions. While these trajectories extend to 50 Myr in the past for illustrative purposes, the intersection points for this pair occur $\sim5.7\pm.4$ Myr ago.
• Figure 4: The HR diagrams for all candidate matches. Three of these cases were identified as potential blue stragglers: EC 03462-5813 with Ruprecht 111, HIP 55051 with NGC 2437, and HIP 55051 with Trumpler 7. All other orphan stars had ages consistent with their parent clusters.
• Figure 5: We present the trajectories of the star HIP 1241 with its five possible origin clusters in a similar format to Figure \ref{['fig:trajectory_good']}. With the assumption of circular motion, at the time of ejection, they were located within 500 pc of one another.