Escape from the Bermuda cluster: Orphanization by multiple stellar ejections

¹ Centro de Astrobiología, CSIC-INTA. Campus ESAC. C. bajo del castillo s/n, 28 692 Villanueva de la Cañada, Madrid, Spain
e-mail: jmaiz@cab.inta-csic.es
² Departamento de Astrofísica y Física de la Atmósfera, Universidad Complutense de Madrid, 28 040 Madrid, Spain
³ Departamento de Astronomía, Universidad de La Serena, Av. Cisternas 1200 Norte, La Serena, Chile
⁴ Departament de Física Quàntica i Astrofísica, Institut de Ciències del Cosmos (ICCUB). Universitat de Barcelona (IEEC-UB), Martí i Franquès 1, 08 028 Barcelona, Spain

Received: 4 October 2021
Accepted: 11 November 2021

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 and runaway stars in the vicinity of the North America and Pelican nebulae. The candidates are analyzed with a combination of 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, dubbed the Bajamar, Toronto, and HD 201 795 events, which expelled (a minimum of) five, two, and two systems, respectively, or six, three, and three stars if we count the individual components in spectroscopic and eclipsing binaries. The events took place 1.611 ± 0.011 Ma, 1.496 ± 0.044 Ma, and 1.905 ± 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; their orientation agrees with that of their relative proper motions with respect to the cluster. Combining the three events, the Bermuda cluster has lost over 200 M_⊙, including its three most massive stars, so it can be rightfully considered an orphan cluster. One consequence is that the present-day mass function of the cluster has been radically altered from its top-heavy initial value to one compatible with a Kroupa-like function. Another is that the cluster is currently expanding with a dynamical timescale 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.