NGC 663 as a laboratory for massive star evolution

TL;DR

NGC 663 is a rich, massive young cluster in the Perseus arm, serving as an excellent natural laboratory for studying the evolution of high-mass stars through comprehensive astrometric, photometric, and spectroscopic analysis.

Contribution

This study provides the first detailed characterization of NGC 663, including its stellar population, mass, age, and evolutionary features, highlighting its potential for advancing massive star evolution research.

Findings

NGC 663 has over 300 B-type members, possibly making it the most massive cluster in the Perseus arm.

Spectral analysis indicates solar metallicity and a turn-off mass of about 8.5 M_sun.

Presence of blue stragglers and peculiar supergiant masses suggests complex formation and evolution processes.

Abstract

Massive young clusters with rich populations of high-mass stars are ideal laboratories to explore their evolutionary paths. Despite being the most prominent cluster in the Perseus-arm Cas OB8 association, NGC 663 remains comparatively little studied. We present a comprehensive investigation of its properties, integrating astrometric, photometric and spectroscopic data for the cluster and its surroundings, including accurate spectral classification for over 150 members. Gaia astrometry indicates over 300 B-type members, possibly rendering NGC 663 the most massive cluster in the Perseus arm, with initial mass likely exceeding 10000 M_\sun . This large population makes NGC 663 an excellent laboratory for studying massive star evolution. Spectral analysis of the earliest members reveals approximately solar metallicity and a turn-off mass of approximate 8.5 M_\sun, consistent with the photometric age of 23 Ma. We identify five spectroscopic blue stragglers, including the Be/X-ray binary RX J0146.9$+$6121. We outline its evolutionary history and compare its properties with other Be stars. Although the cluster contains many Be stars, their relative fraction is not particularly high. Intriguingly, four of the six blue supergiant members appear to have significantly higher masses than the brightest giants near the Hertzsprung gap. These observations suggest that most mid-B supergiants may form via mergers, unless stars of 10-12 M_\sun born as primaries in binaries rarely undergo supernova explosions. Similarly, if Be stars form through the binary channel, then either most are produced through case A evolution or supernovae are uncommon among primaries in this mass range.