# A very massive runaway star from Cygnus OB2

## TL;DR

This study confirms that the massive star BD+43 3654 is a runaway star originating from Cygnus OB2, supported by spectroscopic, astrometric, and bow shock analyses, indicating a dynamical ejection origin.

## Contribution

It provides the first detailed spectroscopic classification and kinematic analysis linking BD+43 3654 to Cygnus OB2, demonstrating its status as one of the most massive runaway stars.

## Key findings

- BD+43 3654 is an O4If star with ~70 solar masses.
- The star's motion aligns with an origin in Cygnus OB2.
- It is one of the three most massive known runaway stars.

## Abstract

Aims: We analyze the available information on the star BD+43 3654 to investigate the possibility that it may have had its origin in the massive OB association Cygnus OB2. Methods: We present new spectroscopic observations allowing a reliable spectral classification of the star, and discuss existing MSX observations of its associated bow shock and astrometric information not previously studied. Results: Our observations reveal that BD+43 3654 is a very early and luminous star of spectral type O4If, with an estimated mass of (70 +/- 15) solar masses and an age of about 1.6 Myr. The high spatial resolution of the MSX observations allows us to determine its direction of motion in the plane of the sky by means of the symmetry axis of the well-defined bow shock, which matches well the orientation expected from the proper motion. Tracing back its path across the sky we find that BD+43 3654 was located near the central, densest region of Cygnus OB2 at a time in the past similar to its estimated age. Conclusions: BD+43 3654 turns out to be one of the three most massive runaway stars known, and it most likely formed in the central region of Cygnus OB2. A runaway formation mechanism by means of dynamical ejection is consistent with our results.