X-ray properties of early-type stars in the Tarantula Nebula from T-ReX

We reassess the historical L-X/L-Bol relation for early-type stars from a comparison between T-ReX, the Chandra ACIS X-ray survey of the Tarantula Nebula in the Large Magellanic Cloud (LMC), and contemporary spectroscopic analysis of massive stars obtained primarily from VLT/FLAMES, VLT/MUSE, and HST/STIS surveys. For 107 sources in common (some host to multiple stars), the majority of which are bolometrically luminous (40 per cent exceed 10(6)L(circle dot)), we find an average log L-X/L-Bol = -6.90 +/- 0.65. Excluding extreme systems Mk 34 (WN5h+WN5h), R140a (WC4+WN6+), and VFTS 399 (O9 IIIn+?), plus four WR sources with anomalously hard X-ray components (R130, R134, 8135, Mk 53) and 10 multiple sources within the spatially crowded core of R136a, log L-X/L-Bol = -7.00 +/- 0.49, in good agreement with Galactic OB stars. No difference is found between single and binary systems, nor between O, Of/WN, and WR stars, although there does appear to be a trend towards harder X-ray emission from O dwarfs, through O (super)giants, Of/WN stars, and WR stars. The majority of known OB stars in the Tarantula are not detected in the T-ReX point source catalogue, so we have derived upper limits for all undetected OB stars for which log L-Bol/L-circle dot >= 5.0. A survival analysis using detected and upper limit log L-X/L-Bol values indicates no significant difference between luminous O stars in the LMC and the Carina Nebula. This analysis suggests that metallicity does not strongly influence L-X/L-Bol. Plasma temperatures for single, luminous O stars in the Tarantula ((kT(m)) over bar = 1.0 key) are higher than counterparts in Carina ((kT(m)) over bar = 0.5 keV).

Automated summary

This study reassesses the L-X/L-Bol relation for early-type stars by comparing spectroscopic analysis and X-ray survey data. The results show that the majority of stars have consistent L-X/L-Bol ratios, but O-type stars have relatively harder X-ray emission.