A binary supernova OB-runaway candidate inside Berkeley 97

Astrophysikalisches Institut und Universitäts-Sternwarte Jena, 07745 Jena, Germany

^⋆ Corresponding author; baha.dincel@uni-jena.de

Received: 19 February 2024
Accepted: 27 August 2024

Abstract

Aims. OB-runaway stars ejected by the binary supernova mechanism can be found near young open star clusters. In this paper, we present an OB-runaway candidate as a pre-SN binary companion to the progenitor of the pulsar PSR J2238+5903 inside the young open star cluster Berkeley 97.

Methods. We tried to find a kinematic outlier based on Gaia DR3 proper motions and parallaxes to be the pre-supernova binary companion to the progenitor of the pulsar. We took the spectra of two bright early B-type stars of the cluster, determined their effective temperature and surface gravity, and updated the parameters of the cluster. Through isochrone fitting of the color-magnitude diagram of the star cluster, we identified the members and determined the stellar parameters of the runaway star.

Results. Two bright members of the cluster, HD 240015 and HD 240016, are massive stars with spectral types of B0.5II and B1.5II and effective temperatures of T_eff = 21 000 ± 1000 K and T_eff = 24 000 ± 2000 K, respectively, as well as surface gravities of log(g[cm/s²]) = 3.0 ± 0.2. We find that Berkeley 97 is a star cluster with an age of log(age[yr]) = 7.1, an uncertainty of < 0.1 dex, and an interstellar extinction of A_V = 3.1 ± 0.1 mag. The runaway star has an effective temperature of T_eff = 12 250 ± 1750 K with a surface gravity of log(g[cm/s²]) = 4.38 ± 0.2 (B8V type star). By tracing back the proper motion of the runaway star, the explosion center was found for different possible pulsar ages of 10, 20, and 26.6 kyr. The pulsar moving out from the 20 kyr position must have a space velocity of ∼340 km s⁻¹, which is consistent with the general pulsar velocity distribution. This supports the idea that the pulsar originated from the cluster as a result of a binary supernova. Despite its young age, τ < 26.6 kyr, the supernova remnant is not visible.