The Medusa fibres subtend 1.̋2 on the sky which, at the distance of the LMC (50 kpc), is equivalent to 0.29 pc. Some of the spectra could therefore be composites of both wide binaries and/or chance line-of-sight alignments. High-quality optical imaging from the wide-field F775W mosaic of 30 Dor taken with the Hubble Space Telescope (HST) in programme GO-12499 (PI: Lennon; see Sabbi et al. 2013) includes 300 of the 403 stars (74%) with RV estimates.
We checked the available shallow and deep images for companions within 1′′. Two of us (PLD and SdM) visually checked each of our targets for nearby companions which could have significantly contaminated the LR02 spectroscopy; notable companions/features in this context are summarised in Table A.1. Although subjective, the two independent evaluations were in excellent agreement, ensuring that important companions, as resolved by HST imaging, are noted.
We detect visual companions that could contribute to the observed spectra for thirty-five of the B-star targets (i.e. 11.7% of those with imaging available). In the context of searching for spectroscopic binaries such visual companions sample a very different range of physical separations (see e.g. Fig. 1 from Sana & Evans 2011) and they will not influence the results for the spectroscopic binary sample analysed in this paper. Even if they
B-type targets that may have a significant contribution to their FLAMES spectra from nearby companions.
are physically bound, the RV amplitudes for these visual-binary systems will be below our detection threshold, and they will therefore be categorised as single using the spectroscopic criteria described in Sect. 3 (in the absence of other sources of RV variability). Such wide systems are unlikely to interact at any point in their evolution, and so will have no influence on our conclusions in this respect. However, to the extent that the observed spectra will be composite to some (generally small) degree, this contamination will need to be taken into account in any future atmospheric analyses of these spectra.
As in Paper VIII, we estimated the accuracy of the method by applying it to synthetic data which shared the same properties as the sample of early-B dwarfs analysed in this paper. We adopted different parent distributions to generate the synthetic observations and investigated how well the method behaved in different parts of parameter space, including the region where the adopted parameters led to the best representation of the merit function. The results from these tests are reported in Table B.1.
Overview of test results from synthetic datasets.
© ESO, 2015