Escape from the Bermuda cluster: orphanization by multiple stellar ejections

Abstract

CONTEXT. Dynamical interactions in young stellar clusters can eject massive stars early in their lives and significantly alter their mass functions. If all of the most massive stars are lost, we are left with an orphan cluster. AIMS. We study the Bermuda cluster (Villafranca O-014 NW), the most significant young stellar group in the North America and Pelican nebulae, and the massive stars that may have been ejected from it to test if it has been orphaned. METHODS. We use Gaia EDR3 parallaxes and proper motions to search for walkaway/runaway stars in the vicinity of the North America and Pelican nebulae. The candidates are analyzed with spectroscopy and photometry to assess their nature and their trajectories are traced back in time to determine at what time they left the Bermuda cluster. RESULTS. We detect three ejection events (Bajamar, Toronto, and HD 201 795 events) that expelled 5, 2, and 2 systems, respectively, or 6, 3, and 3 stars if we count the individual components in spectroscopic/eclipsing binaries. The events took place 1.611$\pm$0.011 Ma, 1.496$\pm$0.044 Ma, and 1.905$\pm$0.037 Ma ago, respectively, but our analysis is marginally consistent with the first two being simultaneous. We detect bow shocks in WISE images associated with four of the ejected systems. Combining the three events, the Bermuda cluster has lost >200 M_Sol, including its three most massive stars, so it can be considered an orphan cluster. One consequence is that the PDMF of the cluster has been radically altered from its top-heavy initial value to one compatible with a Kroupa-like function. Another one is that the cluster is currently expanding with a dynamical time scale consistent with the cause being the ejection events. A scenario in which the Bermuda cluster was formed in a conveyor belt fashion over several hundreds of ka or even 1 Ma is consistent with all the observables. [ABRIDGED]

arXiv commentAccepted for publication in A&A. New version after acceptance with some added content suggested by referee

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