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

Abstract

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 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% exceed $10^6{L}_{\odot }$), we find an average $\log L_X/L_{Bol}=-6.90\pm 0.65$. 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, R135, Mk 53) and 10 multiple sources within the spatially crowded core of R136a, $\log L_X/L_{Bol}=-7.00\pm 0.49$, 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 $L_{Bol}/L_{\odot }\ge 5.0$. 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 ($\overline{k{T}_m}=1.0$ keV) are higher than counterparts in Carina ($\overline{k{T}_m}=0.5$ keV).

14 pages, 8 figures, accepted for MNRAS plus Appendices providing X-ray plasma fits (A1), properties of early-type stars (A2), LMC baseline abundances (B1), plus X-ray upper limits for other luminous early-type stars (S1)

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