X-ray and optical spectroscopy of the massive young open cluster IC1805

TL;DR

This study uses X-ray and optical spectroscopy to analyze the properties of massive stars in the young open cluster IC1805, testing models of stellar evolution and X-ray emission in early-type stars.

Contribution

It provides new optical spectroscopic data, revisits orbital solutions, and assesses X-ray luminosities, offering insights into the X-ray properties and binarity of stars in IC1805.

Findings

X-ray fluxes are consistent over time, indicating stable emission.

HD15570's X-ray underluminosity aligns with theoretical models.

Evidence of binarity in BD+60° 498 was discovered.

Abstract

Very young open clusters are ideal places to study the X-ray properties of a homogeneous population of early-type stars. In this respect, the IC1805 open cluster is very interesting as it hosts the O4If$^+$ star HD15570 thought to be in an evolutionary stage intermediate between a normal O-star and a Wolf-Rayet star. Such a star could provide a test for theoretical models aiming at explaining the empirical scaling relation between the X-ray and bolometric luminosities of O-type stars. We have observed IC1805 with XMM-Newton and further collected optical spectroscopy of some of the O-star members of the cluster. The optical spectra allow us to revisit the orbital solutions of BD+60$^{\circ}$ 497 and HD15558, and provide the first evidence of binarity for BD+60$^{\circ}$ 498. X-ray emission from colliding winds does not appear to play an important role among the O-stars of IC1805. Notably, the X-ray fluxes do not vary significantly between archival X-ray observations and our XMM-Newton pointing. The very fast rotator BD+60$^{\circ}$ 513, and to a lesser extent the O4If$^+$ star HD15570 appear somewhat underluminous. Whilst the underluminosity of HD15570 is only marginally significant, its amplitude is found to be compatible with theoretical expectations based on its stellar and wind properties. A number of other X-ray sources are detected in the field, and the brightest objects, many of which are likely low-mass pre-main sequence stars, are analyzed in detail.