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Analysis of late O-type stars observed in the Large Magellanic Cloud (LMC) bythe VLT-FLAMES Tarantula Survey (VFTS) revealed a discrepancy between thephysical properties estimated from model-atmosphere analysis and those expectedfrom their morphological classifications. Here we revisit the analysis of 32 ofthese puzzling objects using new hydrogen-helium-silicon FASTWIND models and adifferent fitting approach to re-evaluate their physical properties. Our newanalysis confirms that these stars indeed have properties that are typical oflate O-type dwarfs. We also present the first estimates of silicon abundancesfor O-type stars in the 30 Dor clusters NGC 2060 and NGC 2070, with a weightedmean abundance for our sample of 7.05 +/- 0.03. Our values are about 0.20 dexlower than those previously derived for B-type stars in the LMC clusters N 11and NGC 2004 using TLUSTY models. Various possibilities (e.g. differences inthe analysis methods, effects of microturbulence, and real differences betweenstars in different clusters) were considered to account for these results. Wealso used our grid of FASTWIND models to reassess the impact of using theGalactic classification criteria for late O-type stars in the LMC byscrutinising their sensitivity to different stellar properties. At the cooledge of the O star regime the HeII 4686/HeI 4713 ratio used to assignluminosity class for Galactic stars can mimic giants or bright giants in theLMC, even for objects with high gravities (log_g > 4.0 dex). We argue that thisline ratio is not a reliable luminosity diagnostic for late O-type stars in theLMC, and that the SiIV 4989/HeI4026 ratio is more robust for these types.

The VLT-FLAMES Tarantula Survey XXXII. Low-luminosity late O-type stars -- classification, main physical parameters, and silicon abundances