The most massive progenitors of neutron stars: CXO J164710.2-455216

TL;DR

This paper investigates the formation of a neutron star in Westerlund 1, showing that binary evolution can produce neutron stars from very massive progenitors, challenging previous assumptions about stellar mass limits.

Contribution

It demonstrates that binary interactions, especially Roche lobe overflow, allow stars up to 80 solar masses to form neutron stars, expanding the known progenitor mass range.

Findings

Neutron star progenitors in Westerlund 1 likely exceeded 40 Msun.

Binary evolution enables massive stars up to 80 Msun to become neutron stars.

The presence of neutron stars does not strictly constrain progenitor mass due to binary effects.

Abstract

The evolution leading to the formation of a neutron star in the very young Westerlund 1 star cluster is investigated. The turnoff mass has been estimated to be 35 Msun, indicating a cluster age ~ 3-5 Myr. The brightest X-ray source in the cluster, CXO J164710.2-455216, is a slowly spinning (10 s) single neutron star and potentially a magnetar. Since this source was argued to be a member of the cluster, the neutron star progenitor must have been very massive (M_zams > 40 Msun) as noted by Muno et al. (2006). Since such massive stars are generally believed to form black holes (rather than neutron stars), the existence of this object poses a challenge for understanding massive star evolution. We point out while single star progenitors below M_zams < 20 Msun form neutron stars, binary evolution completely changes the progenitor mass range. In particular, we demonstrate that mass loss in Roche lobe overflow enables stars as massive as 50-80 Msun, under favorable conditions, to form neutron stars. If the very high observed binary fraction of massive stars in Westerlund 1 (> 70 percent) is considered, it is natural that CXO J164710.2-455216 was formed in a binary which was disrupted in a supernova explosion such that it is now found as a single neutron star. Hence, the existence of a neutron star in a given stellar population does not necessarily place stringent constraints on progenitor mass when binary interactions are considered. It is concluded that the existence of a neutron star in Westerlund 1 cluster is fully consistent with the generally accepted framework of stellar evolution.