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 ), 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, R135, 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 ≥ 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 = 1.0 keV) are higher than counterparts in Carina (𝑘𝑇 𝑚 = 0.5 keV).

The Tarantula Massive Binary Monitoring

“…Errors represent 1σ (68% confidence interval). , (i) X-ray detection with the Chandra Visionary programme T-ReX (Crowther et al 2022). Targets with no entry were excluded from the X-ray analysis, either because they have log L < 5.0 [erg s −1 ], or due to complex diffuse background contamination.…”

“…Finally, Table 1 provides constraints on intrinsic X-ray luminosities obtained with the Chandra Visionary programme T-ReX Crowther et al (2022). Evidently, only four targets (VFTS 333,404,532,603) have finite detections, all of which smaller than 10 33 erg s −1 and lying below or close to the canonical value L x /L ≈ −7 (Pallavicini et al 1981;Sana et al 2006).…”