Candidate X-ray-Emitting OB Stars in the MYStIX Massive Star-Forming Regions

TL;DR

This study uses X-ray data from the MYStIX survey to identify and analyze candidate OB stars in star-forming regions, revealing that many such stars remain undiscovered and providing new insights into their properties and distributions.

Contribution

The paper presents a new method combining X-ray and infrared data to identify candidate OB stars, significantly expanding the known census in multiple star-forming regions.

Findings

Identified 98 new candidate OB stars using SED fitting.

Current OB star census is less than 50% complete in studied regions.

Discovered potential massive binaries and luminous O stars.

Abstract

Massive, O and early B-type (OB) stars remain incompletely catalogued in the nearby Galaxy due to high extinction, bright visible and infrared nebular emission in H II regions, and high field star contamination. These difficulties are alleviated by restricting the search to stars with X-ray emission. Using the X-ray point sources from the Massive Young star-forming complex Study in Infrared and X-rays (MYStIX) survey of OB-dominated regions, we identify 98 MYStIX candidate OB (MOBc) stars by fitting their 1-8 micron spectral energy distributions (SEDs) with reddened stellar atmosphere models. We identify 27 additional MOBc stars based on JHK photometry of X-ray stars lacking SED fitting. These candidate OB stars indicate that the current census of stars earlier than B1, taken across the 18 MYStIX regions studied, is less than 50% complete. We also fit the SEDs of 239 previously-published OB stars to measure interstellar extinction and bolometric luminosities, revealing six candidate massive binary systems and five candidate O-type (super)giants. As expected, candidate OB stars have systematically higher extinction than previously-published OB stars. Notable results for individual regions include: identification of the OB population of a recently discovered massive cluster in NGC 6357; an older OB association in the M17 complex; and new massive luminous O stars near the Trifid Nebula. In several relatively poorly-studied regions (RCW 38, NGC 6334, NGC 6357, Trifid, and NGC 3576), the OB populations may increase by factors of >2.