The Tarantula Massive Binary Monitoring: III. Atmosphere analysis of
  double-lined spectroscopic systems
  Accurate stellar parameters of individual objects in binary systems are
essential to constrain the effects of binarity on stellar evolution. These
parameters serve as a prerequisite to probing existing and future theoretical
evolutionary models. We aim to derive the atmospheric parameters of the 31 SB2s
in the TMBM sample. This sample, composed of detached, semi-detached and
contact systems with at least one of the components classified as an O star, is
an excellent test-bed to study how binarity can impact our knowledge of the
evolution of massive stars. 32 epochs of FLAMES/GIRAFFE spectra are analysed
using spectral disentangling to construct the individual spectra of 62
components. We apply the CMFGEN atmosphere code to determine their stellar
parameters and their He, C and N surface abundances. From these properties, we
show that the effects of tides on chemical mixing are limited. Components on
longer-period orbits show higher nitrogen enrichment at their surface than
those on shorter-period orbits, in contrast to expectations of rotational or
tidal mixing, implying that other mechanisms play a role in this process.
Components filling their Roche lobe are mass donors. They exhibit higher
nitrogen content at their surface and rotate more slowly than their companions.
By accreting new material, their companions spin faster and are rejuvenated.
Their locations in the N-vsini diagram tend to show that binary products are
good candidates to populate the two groups of stars (slowly rotating,
nitrogen-enriched and rapidly rotating non-enriched) that cannot be reproduced
through single-star population synthesis. This sample is the largest sample of
binaries to be studied in such a homogeneous way. The study of these objects
gives us strong observational constraints to test theoretical binary
evolutionary tracks.
