The Wolf-Rayet features and mass-metallicity relation of long-duration
  gamma-ray burst host galaxies
  Aims. We have gathered optical spectra of 8 long-duration GRB host galaxies
selected from the archival data of VLT/FORS2. We investigated whether or not
Wolf-Rayet (WR) stars can be detected in these GRB host galaxies. We also tried
to estimate the physical properties of GRB host galaxies, such as metallicity.
  Methods. We identified the WR features in these spectra by fitting the WR
bumps and WR emission lines in blue and red bumps. We also identified the
subtypes of the WR stars, and estimated the numbers of stars in each subtype,
then calculated the WR/O star ratios. The (O/H) abundances of GRB hosts were
estimated from both the electron temperature (Te) and the metallicity-sensitive
strong-line ratio (R23), for which we have broken the R23 degeneracy. We
compared the environments of long-duration GRB host galaxies with those of
other galaxies in terms of their luminosity (stellar mass)-metallicity
relations (LZ, MZ).
  Results. We detected the presence of WR stars in 5 GRB host galaxies having
spectra with relatively high signal-to-noise ratios (S/N). In the comparison of
LZ, MZ relations, it shows that GRB hosts have lower metallicities than other
samples with comparable luminosity and stellar mass. The presence of WR stars
and the observed high WR/O star ratio, together with low metallicity, support
the core-collapsar model and implie the first stage of star formation in the
hosted regions of GRBs.
